摘要:

384 QUARTERLY REVIEWS Triterpenes tetracyclic 9 328 Tropolones 5 99 Tryptophan biological degradation of 5 227 Ultrasonic analysis of molecular relaxation processes in liquids 11 134 Ultrasonic waves effects of on electrolytes and electrolytic pro- cesses 7 84 Veratrum alkaloids 12 34 Wool wax constitution of 5 390 A'-Ray crystal analysis modern methods of determination of mole- cular structure by and their ac- curacy 7 335 p-Xylylene chemistry of and of its analogues and polymers 12 301 384 QUARTERLY REVIEWS Triterpenes tetracyclic 9 328 Tropolones 5 99 Tryptophan biological degradation of 5 227 Ultrasonic analysis of molecular relaxation processes in liquids 11 134 Ultrasonic waves effects of on electrolytes and electrolytic pro- cesses 7 84 Veratrum alkaloids 12 34 Wool wax constitution of 5 390 A'-Ray crystal analysis modern methods of determination of mole- cular structure by and their ac- curacy 7 335 p-Xylylene chemistry of and of its analogues and polymers 12 301 QUARTERLY REVIEWS QUARTERLY REVIEWS contains articles by recognised authorities on selected topics from general physical inorganic and organic chemistry.The Journal and Annual Reports interest primarily the research worker Quarterly Reviews is designed for a wider range of readers. It is intended that each review article shall be of interest to chemists generally and not only to workers in the particular field being reviewed. The submission of reviews for publication is welcomed but intending authors are advised to write in the first place to the Editor The Chemical Society Burlington House Piccadilly London W.1. Such preliminary communications should be accompanied by an outline of the ground to be covered rather than by the completed manuscript. QUARTERLY REVIEWS QUARTERLY REVIEWS contains articles by recognised authorities on selected topics from general physical inorganic and organic chemistry. The Journal and Annual Reports interest primarily the research worker Quarterly Reviews is designed for a wider range of readers. It is intended that each review article shall be of interest to chemists generally and not only to workers in the particular field being reviewed. The submission of reviews for publication is welcomed but intending authors are advised to write in the first place to the Editor The Chemical Society Burlington House Piccadilly London W. 1. Such preliminary communications should be accompanied by an outline of the ground to be covered rather than by the completed manuscript.

ISSN:0009-2681    

DOI:10.1039/QR95913FP001

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

SUGAR EPOXIDES By F. H. NEWTH B.Sc. PH.D. (PARTINGTON RESEARCH LABORATORY PETROCHEMICALS LTD. URMSTON MANCHESTER) 1. Introduction AN ethylene oxide group is easily introduced into a sugar molecule by alkaline hydrolysis of an 0-arene(or a1kane)sulphonyl derivative or deoxyhalogeno-compound which has a vicinal hydroxyl group trans to the anionic group. This reaction occurs easily and with few exceptions and the epoxide is formed nearly quantitatively. The interest in sugar epoxides as distinct from other anhydro-derivatives lies in this reactive group. The oxide ring is opened by nucleophilic reagents to give usually two products with the trans-configuration and a substituent group which is derived from the reagent. This reaction has therefore provided a versatile method for the preparation of rare sugars from easily accessible ones and for the selective introduction of groups or atoms such as 0-alkyl amino and halogen into sugar molecules.The reaction is easily carried out but separation and characterisation of the products have often needed pro- longed study. It will be appreciated that much impetus has been given to the chemistry of sugar epoxides by the desire to synthesise naturally occurring sugars and two well-known examples are the synthesis of glucosamine (2-amino-2-deoxy-~-g~ucose) in 1939 and chondrosamine (2-amino-2-deoxy-~-galactose) in 1946. In the years following 1946 much attention was also given to sugar epoxides as intermediates in the prepara- tion of deoxy-sugars. The syntheses of the sugar components of the cardiac glycosides were successfully achieved but the chemistry was not so favour- able for 2-deoxyribose because of the direction of ring opening.Although the preparative aspect of the reactions of the sugar epoxides has been most closely studied there are important features in the field such as different rates of formation of the various oxides and the different proportions in which the products of ring fission are obtained. These have led to a consideration of the influence which other groups in the sugar molecule may have upon the formation and fission of the epoxide ring. In many cases it is possible to assign a particular conformation to the sugar ring with reasonable certainty and the chemistry of the sugar epoxides has now reached a stage where study of their reactions can reveal much about the intramolecular interactions which operate in the sugar molecule.2. Formation 2.1. Fischer’s Epig1ucosamine.-One of the earliest reactions in which a sugar epoxide must have been formed was an attempt to synthesise 30 NEWTH SUGAR EPOXDES 31 glucosamine from glucal.l3 4 6-Tri-O-acetyl-~-glucal 1 2-dibromide was converted into methyl 3 4 6-tri-O-acetyl-2-brorno-2-deoxy-~-~-glucoside (I). When this compound (or its chloro-analogue) was treated with ammonia the product was not 2-amino-2-deoxy-~-g~ucose or mannose but a substance which Fischer called methyl epiglucosaminide. This was later2 shown to be methyl 3-amino-3-deoxy-P-~-altroside (11). Fischer suspected this at the time and suggested that a 2 3-anhydro-ring might have been formed intermediately. CH,-OAc CH,-OH AcO 0”‘ - Epoxide -c HO QMe H2N (II) Br (1) It can of course be recognised now that the intermediate was methyl 2:3-anhydro-P-~-mannoside but it was not until 13 years later that the existence of 2 3-anhydrohexosides was established by concurrent work at Birmingham and St.Andrews. 2.2. 1 2-Anhydro-~-glucopyranose and 5 6-Anhydro-~-glucofuranose,- The first example of a sugar epoxide was given by BrigP in 1921. He found that when tetra-O-acetyl-/3-D-glucose was treated with phosphorus pentachloride 3 4 6-tri-O-acetyl-ZO-trichloroacetyl-~-~-glucosyl chlor- ide (111) was formed. Careful treatment of this compound with ammonia removed only the trichloroacetyl group forming 3 4 6-tri-O-acetyl-/3-~- glucosyl chloride (IV) and on further mild treatment with ammonia this was converted into 3 4 6-tri-O-acetyl-l 2-anhydro- a-D-glucose (V).This compound is a valuable synthetic intermediate. It reacts with methanol to give methyl 3 4 6-tri-O-acetyl-/3-~-glucoside and the 2-0-tosyl derivative (VI) was important in establishing the chemistry of the 2:3-epoxides. (VI) (VII) R=Br (I )o (VIII) R =OTs Ts = p€,H4Me-SO2 Fischer Bergmann and Schotte Ber. 1920,53 509. Haworth Lake and Peat J. 1939 271. Brigl 2. physiol. Chew. 1921 116 1 245. 32 QUARTERLY REVIEWS Before considering this aspect however it is convenient to mention the conversion of 6-bromo-6-deoxy- 1 2-0-isopropylidene- a-~-glucofurano~e~ (VII) and I 2-0-isopropylidene-6-0-tosyl- a-D-glucofuranose6 (VIII) into 5 6-anhydro-1 2-0-isopropylidene- a-D-glucofuranose (IX) since this is a reaction not complicated by Walden inversion.2.3. 2 3- and 3 4Anhydrides.-In 1930 Helferich and Miillera isolated a crystalline methyl anhydro-#bhexoside after treating methyl 2 3 6- t~-~-acety~-4-0-tosy~-~-~-~ucoside (X) with sodium methoxide; this product was shown by Muller' to be methyl 3 4-anhydro-/3-~-galactoside (XI). In 1933 an anhydro-sugar was obtained from methyl 4:6-di-0- methyl-2 3-di-0-tosyl- a-D-glucoside (XII) and the alkaline conditions produced also some methyl 4 6-di-O-methyl- a-D-altroside* (XIII). The following year Haworth and his co-workersg described the formation of a methyl 2 3-anhydro-/3-~-hexoside from methyl 3 4 6-tri-0-acetyl-2-0- tosyl-fl-D-glucoside (VI) and they commented that the change of optical rotation to a high negative value on hydrolysis of the anhydride suggested the formation of an altrose derivative.bMc Q GMe TsO Me0 OMe Me0 OAc OH OTs HO (XI (Xi) (XI I) (XIII) Me PhCH- Me The manner in which these reactions were proceeding was now becom- ing obvious and Robertson and GriffithlO made an important contribution by showing that the products obtained from methyl 3-0-benzoyl-4 6-0- benzylidene-2-0-tosyl-u-~-glucoside (XIV) and methyl 2-0-benzoyl-4 6- O-benzylidene-3-O-tosyl-u-~-glucoside (XV) by treatment with sodium Freudenberg Toepfer and Anderson Ber. 1928 61 1751. Ohle and Vargha Ber. 1929 62 2435. 13 Helferich and Miiller Ber. 1930 63 2142. Miiller Ber. (a) 1934 67 421 ; (b) 1935 68 1094. * Mathers and Robertson J. 1933 1076. Haworth Hirst and Panizzon J. 1934 154. lo Robertson and Griffith J . 1935 1193. NEWTH SUGAR EPOXIDES 33 methoxide are methyl 2 3-anhydro-4 6-0-benzylidene-a-u-mannoside (XVI) and -a-D-alloside (XVII) respectively.In all these reactions the formation of an anhydro-ring has been ac- companied by inversion of configuration at that carbon atom to which the sulphonyloxy-group is attached. In the alkaline medium the anion of the vicinal trans-hydroxyl group can displace the toluene-p-sulphonyloxy- anion to form an epoxide and this can be represented as the intramolecular SN2 process (1) I I ( I I OTs If the vicinal group is cis as in methyl 2 3 6-tri-0-acetyl-4-0-methane- sulphonyl- p-D-galactose (XVIII) then only deacetylation occurs and the methanesulphonyloxy-group is not displaced.ll An interesting reaction examined by Peat and Wiggins12 was the treatment of methyl 3-0-tosyl- p-D-glucoside (XIX) with mild alkali.Methyl 2 3- (XX) (60 %) and methyl 3 4-anhydro-/%~-alloside (XXI) (25 %) were formed since the toluene-p- sulphonyloxy-group has two neighbouring hydroxyl groups which can cause its displacement. In addition to the two epoxides some methyl 3 6- anhydro-p-D-glucoside (XXII) [shown later13 to be the furanoside (XXIII)] was formed. Although at the time it was considered that this was one of the examples of hydrolysis of a toluene-p-sulphonyloxy-group with retention of configuration the 3 6-anhydride is a secondary product formed as Ohle and Wilke l4 pointed out by the anion from the 6-hydroxyl group attacking at position 3 in either (XX) or (XXI).15 ' OAc (XVIII) CH,*OH HO QMe OH O(IX) HO QMe @Me Ho@e H O z e OMe (XXI) (XXI I) (xx I I I) Ms=CH,-SO2 l1 Muller Moricz and Verner Ber.1939 72 745. l2 Peat and Wiggins J. 1938 1088. l3 Haworth Jackson and Smith J. 1940 620. l4 Ohle and Wilke Ber. 1938 71 2316. l5 Peat Adv. Carbohydrate Chern. 1946,2 37. 34 QUARTERLY REVIEWS 2.4. The Stereochemistry of Epoxide Formation.-The examples given so far illustrate the development of the subject and all appear straight- forward; in the light of the accepted views on inversion of configuration at a saturated carbon atom they are what would be expected. This part of carbohydrate chemistry is unfortunately completely lacking in quantita- tive data. In spite of this however inspection shows that although the experimental conditions chosen are often excessive some reactions are more difficult to carry out than others. The Haworth ring formulz admirably show configurational changes but do not help in finding the steric cause of different reactivities or the reason why a reaction follows a particular course.It is not until conformational drawings are made that intramolecular interactions can become apparent and a posteriori explana- tions can be given for such behaviour. In the base-catalysed formation of an epoxide from a trans-1 2-diol mono-0-toluene-p-sulphonate (reaction l) the intramolecular SN2 process in a six-membered ring requires the two groups to be in the diaxial position. The entering and the departing anion and the carbon atoms to which they are attached are then co-planar and this permits maximum participation. This condition is found in 1 6-anhydro-2-0-methane- sulphonyl-fl-D-galactose (XXIV) and 1 6-anhydro-4-0-tosyl-/3-~-mannose (XXV) and they are easily converted into the 2 3 3 (XXVI) and the 3 4- taZo-epoxide17 (XXVII) by mild alkali.The majority of toluene-p-sulphon- ales vicinal to a trans-hydroxyl group are however in the diequatorial 4 H O B HO 0 (XXIV) OMS (XXVI) mH - @ (x x v) (xxvr I) Ts 0 position as for example in the 2- (XIVa) and the 3-0-tosyl derivative and (XVa) which are smoothly converted into the manno- (XVI) and do-epoxide (XVII). The ease with which these compounds react suggests (X I Va) lR James Smith Stacey and Wiggins J . 1946 625. l7 Ham and Hudson J. Amer. Chem. SOC. 1942 64 925 2435. NEWTH SUGAR EPOXIDES 35 structural modification before epoxide formation and indeed in the analogous bimolecular ionic elimination from 1 2-di halides no reaction occurs when both trans-substituents are rigidly held in equatorial posi- tions.l* In a monocyclic system the diequatorial groups can pass into the diaxial position without much difficulty (conformation C1-+1C19) but in the bicyclic compounds (XIVa) and (XVa) the trans-fused ring containing the 4:6-O-benzylidene group confers rigidity on the chair form of the sugar ring and prevents this change.C(2) on the other hand can move downwards to give the boat form (XIVb) without disturbing the point of ring fusion and the two groups at C 0 and C(31 are now co-planar and in a position to react.20 n \ (OH) The hexose epoxides must be considered as analogous to 1 2-epoxycyclo- hexane which has been shown to have the half-chair conformation similar to that in cyclohexane.21 The manno-epoxide (XVI) can therefore be shown as (XVIa).22 It must then be recognised that the epoxide conformation is halfway in the chair-boat conversion and C(,) and C(,) in the boat form would have to move again in the reverse direction to‘become co-planar with C(l) and C(a).For this reason the true boat formmay never be reached. It is convenient however to formulate this extreme condition since in its attainment steric interactions are apparent which explain different activities and permit predictions. It is of interest to consider here the alkaline hydrolysis of methyl 3-O-tosyl-/3-~-glucoside~~ (XIX) to see whether conformational analysis can account for the predominance of the 2 3-ah-epoxide (XX) [allowing for the formation of the 3 6-anhydride] over the 3 4-ah-epoxide (XXI) in the reaction product.The form (XIXa) is the least hindered (all groups equatorial) and will be the “resting position” of the molecule. Form (XIXb) although it has the desired axial relation between the reacting groups must be ignored because of its state of extreme hindrance (all groups axial). Forms (XIXc d and e) represent three points in the cycle of positions possible in the flexible boat form23 in which the 2- 3- and 4-groups are axial (Reeves’s 2B B3 lS Barton and Rosenfelder J. 1951 1048. lS Reeves J. Amer. Chem. SOC. 1949 71 21 5. Newth J. 1956 441. a1 Ottar Acta Chem. Scand, 1947 1 283. Cookson Chem. and Ind. 1954,223 1512. 23 Reeves J. Amer. Chem. SOC. 1957 79 2261. 36 QUARTERLY REVIEWS and 1B conformation^^^). Form (XIXd) appears to be able to give equally the 2 3- and the 3 4-epoxide; form (XIXc) will lead to the 2 3-epoxide and (XIXe) to the 3:4-epoxide.A difference must be found therefore between the last two forms and probably lies in the 1:3-interactions OTs(,)/OMe(, and OTs(,)/CH,.OH( ;). In the alkaline reaction medium the "o- l&Me CHiOH (XIXa) OH HO OH (XIXb) TsO OMe CHiOH HO*H2C & HO.H,C 0 ) q f i HO OMe OH HO OH OH (x I xc) (XIXd) (X I Xc) anion of the 5-hydroxymethyl group will cause more hindrance to the departing toluene-p-sulphonyloxy-anion than the glycosidic methoxyl group and so the more favoured form would be (XIXc) leading to the 2:3-epoxide. It would be interesting now to know the composition of the products from the reaction of methyl 6-0-methyl-3-O-tosy1-~-~-glucoside and methyl 3- O-tosyl-fl-~-xyloside with alkali.There is a very striking difference in the reactivity of the U-tosyl deriva- tives of 1 6-anhydro-fi-~-altrose.~~ It was found that although 1 6- anhydro-3-O-tosyl-~-~-altrose (XXVIII) could be converted into an epoxide 1 6-anhydro-2-0-tosyl-/?-~-altrose (XXIX) and the 3 4-di-0- tosyl derivative (XXX) were quite resistant to alkaline hydrolysis. This behaviour can be explained when steric interactions are considered. In the (xxvr I I) (XXIX) otxx> y 2 - ? y2- ? HO mH OTS HOQ (XXV I I I a) OTS (XXVI I I b) boat form (XXVIIIb) which is the condition suitable for epoxide forma- tion there are interactions between OH( and Ocl) and between OH(, and C(s). On passing from the chair to the boat conformation there will be steric interaction between vicinal groups when one is axial and the other equatorial (cis) since they must move past each other.Thus in (XXVIIT 24 Idem ibid. 1950 72 1499. NEWTH SUGAR EPOXIDES 37 a+b) there will also be a OTS,,)/OH,~) passing interaction. Although the alkaline hydrolysis does not occur with great ease these combined steric factors are not sufficiently great to prevent reaction. In the ester (XXIX) on the other hand the interaction OH(,,/OH(4) will be less but when it is combined with the more severe interactions OTs( 2j0(1) and OTs(,)/C[g the total hindrance must be sufficiently great to prevent reaction. Similarly in the diester (XXX) there will be a very severe OTS(~)/OTS(~) passing inter- action and with the interactions OH(2)/0(1) and OH(2&,) there is enough hindrance again to prevent reaction or even attainment of the boat form.More weight is given to the concept of passing interaction when the alkaline hydrolysis of methyl 4 6-O-benzylidene-2-0-tosyl-a-~-gluco- sidelo (XXXI) and 1 5-anhydro-4 6-U-benzylidene-2-O-tosyl-~-glucit~l~~ (XXXII) is considered. The ester (XXXI) requires the temperature of boiling methanol and compound (XXXII) is hydrolysed easily at 0'. The only difference between the two compounds is the presence or absence of the 1-methoxyl group. If the first postulate that the diaxial condition must be attained by chair-boat transformation is correct the difference in reactivity must be due entirely to the 3fferent passing interactions OTs(,,/ This concept also provides an explanation for the difference in reactivity between methyl 4 6-0-benzylidene-2-0-tosyl-a- and -/hhgalactoside.20s26 OMe(1) and OTs(,/H(,).2.5. Epoxides from Di-Q-sulphonyl Compounds.-In addition to the reactions already described it is also possible to obtain an epoxide by alkaline hydrolysis of di-0-sulphonyl compounds. A well-known example of this is the formation of methyl 2 3-anhydro-4 6-0-benzylidene-a-~- alloside (XVII) from methyl 4 6-0-benzylidene-2 3-di-O-tosyl-a-~- glucoside (XXXIII). This reaction occurs easily with cold sodium methox- ide and the anhydro-glycoside (XVII) is formed quantitatively as the sole p r o d ~ c t . ~ ~ * ~ ~ The preparative value lies in the ease of formation of the di-0-tosyl derivative since only a protecting 0-benzylidene group need be introduced into the glucoside. The reaction is also of considerable theo- retical interest since one of the ester groups must undergo 0-S cleavage without difficulty in contrast to the well-known SN2 reaction (2) of alkyl sulphonates.This displacement at a sulphur atom which is very prevalent 8s Newth XVIth Int. Congr. Pure Appl. Chem. Paris 1957. 26 Wiggins J. 1944 522. 27 Richtmyer and Hudson J. Amer. Chem. SOC. 1941,63 1727. 2 38 QUARTERLY REVIEWS in carbohydrate chemistry has been called SN2S28 and must occur in those “isolated” secondary sulphonates which are hydrolysed with difficulty but with retention of config~ration.~~ Angyal and Gilham30 consider that ApSq0-R 4- R’O‘ - AreSOiO- 4- R’OR . . . . (2) the removal of the first sulphonyl group which will be the more accessible one will be facilitated by the inductive effect of the other sulphonyloxy- group (reaction 3).Ts? I Q- I 0 -5-9- t MeO“ - MaOTs t -7-v- - -<-‘$- + TsO- . . . . . .(3) OTs OTs If this is so the side reaction (4) should occur but the presence of the low- boiling dimethyl ether in the reaction product seems to have eluded investigators TsOMe f MeO’ -W Me20 t TSO- . - - - . (4) It is tempting to combine the reaction sequence (3) and postulate the concerted mechanism (9 but when the di-0-tosyl compound (XXXIIT) is treated with mild alkali for a short time the ah-epoxide (XVII) is formed together with some methyl 4 6-O-benzylidene-3-O-tosyl-a-~- glu~oside.~~ The formation of the mono-0-tosyl derivative supports Angyal and Gilham’s view and clearly hows the 2-sulphonyl group to be the more accessible. Before the discussion of epoxide formation from 2 3-di-0-sulphonyl compounds is continued the alkaline hydrolysis of 1 2-0-isopropylidene-5 6-di-0-tosyl- wD-glucofuranose (XXXIV) should be mentioned.Instead of 5 6-anhydro-1 2-0-isopropylidene-&-~- glucofuranose 3 6-anhydro- 1 2- 0-isopropylidene-5-0-tosyl- a-D-gluco- M a O t Gois sp7 ( 5 ) 0-CH CH,.OTs y 2 Ph CH-0 1 Q*Me TSO*$%.$? ”“*CQ? OTs 0-CMe -CMe (x x x I 1 I> (xxx I v) (XXXV) furanose (XXXV) is formed.32 It is not immediately obvious why the reaction should follow this course and not yield the 5 6-epoxide. 28 Cope and Shen ibid. 1956 78 5912. 2 9 Tipson Adv. Carbohydrate Chem. 1953 8 207. 30 Angyal and Gilham J. 1957 3691. 31 Honeyman and Morgan J. 1955 3660. 32 Ohle and Thiel Ber. 1933 66 525. NEWTH SUGAR EPOXIDES 39 There are several 2:3-di-O-tosyl derivatives and the problem is why should those of glucose and altrose give only one epoxide (allo- and manno-) whereas those of galactose give mixtures of the gulo- and the tab-epoxide.The factors which have been discussed in the preceding section must operate here and although the uncertainty about the exact mode of hydrolysis makes it difficult to be precise it is valuable to make a pre- liminary conformational appraisal. In the following discussion it is assumed that reaction (3) operates. 0-S fission may then occur in either diequatorial or diaxial systems but by the argument already developed the resulting anion must be in or approaching the diaxial position before epoxide formation. The same factors thenshouldaffect this reaction as are believed to influence the reaction of mono-0-toluene-p-sulphonates ; namely non-bonded interactions by axial substituents and passing interaction of two &-groups on change of conformation.It is however the first step in reaction (3) which determines the course of the reaction. It will of course be recognised that there may be an electronic influence from the acetal character of C(l) but in none of the reactions-epoxide formation or fission-has the Reviewer found any consistent indication that this is so. In methyl 4 6-0-benzylidene-2 3-di-O-tosyl-a-~-altroside (XXXVI) the reacting groups are diaxial and the manno-epoxide (XVI) is formed;33 it is unfortunate that the reaction conditions employed were too severe to give any indication of the ease of reaction. It can be inferred that the 2-toluene- O-CH2 P h T 0 / c H 2 Ph*CH-0 TsO QOM.O-OMe (XXXI t la) (xx XVI) Ph t-* Ph t - O P h T O l C H 2 oe" o& o& TsO TsO TsO OMe TsO (XXXVI I) Ts 0 Tso OMe p-sulphonyloxy-group is the more accessible since this must provide the anion to displace the 3-group. In compound (XXXIIIa) and 1 5-anhydro- 4 6-0-benzylidene-2 3-di-O-tosyl-o-glucitol (XXXVII) which has been shown to give also the a l l o - e p ~ x i d e ~ ~ ~ ~ ~ the primary attack must be also at OTs 21. The picture is complicated by the analogous galactose derivatives. (XXXVI I I) (XXXIX) 33 Robertson and Whitehead J. 1940 319. 34 Zissis and Richtmyer J. Amer Chem. Sac. 1955 77 5154, 40 QUARTERLY REVIEWS Methyl 4 6-0-benzylidene-2 3-di-O-tosyl-/3-~-galactoside (XXXVIII) gives only the talo-epo~ide~~ whereas the a-glycoside (XXXIX) gives a mixture of tab- and gulu-epo~ide.~~ In the former the primary attack must be at OTq3) and in the latter at both OTq,) and OTs,,).There does not appear at present to be any obvious reason for this behaviour. 2.6. Epoxides from Amino- and Nitrate Derivatives.-Deamination of aminodeoxy-compounds with nitrous acid occurs when there is a hydroxyl trans to the amino-group and an epoxide is formed. The reaction follows the course shown in (6). Methyl 2-amino-4 6-0-benzylidene-2-deoxy-a- D-altroside (XL) and methyl 3-amino-4 6-0-benzylidene-3-deoxy-a-~- altroside (XLI) are rapidly converted into the epoxides (XVII) and (XVI) by a solution of sodium nitrite in acetic acid.37 Sugar epoxides are equally easily formed in the same way from 4-amino-1 6-anhydro-4- deoxy-/h-mannose (XLII) and 6-amino-6-deoxy-l 2-0-isopropylidene- a-D-glucofuran ose3* (XLIII).During his studies on the sugar nitrates Honeyman has found that certain derivatives yield an epoxide on alkaline hydrolysis. The subject is complex and it can only be pointed out here that the derivatives of methyl 4 6-O-alkylidene-u-~-glucoside which yield the 2 3-allo-epoxide are the 2 3-dinitrate 3-nitrate 2-0-tosyl 3-nitrate and 3-0-tosyl 2-nitrate.39 S o r b and Reichsteh Helv. Chim. Acta 1945 28 1 662. Wiggins Nature 1946 157 300. 36 Gyr and Reichstein ibid. 1945 28 226. 38 Bashford and Wiggins ibid. 1950 165 566. 39Ansell and Honeyman J. 1952 2778; Honeyman and Morgan J. 1955 3660; Honeyman and Stening J. 1957,2278. "TH SUGAR EPOXIDES 41 3. Reactions Peat15 has very clearly described the stereochemistry of ring-opening by nucleophilic reagents (7).It is only necessary to add that with acidic reagents the same reaction occurs but is faster because protonation of the epoxide facilitates the movement of electrons (8). In the early work the reactions were nearly all with alkaline reagents and the products were .(7) Y I Y- *J 1 " -c-y- - ;c-c( ;c-c; 4 -c-c- I 0- 20' '03 6- * . * * ' [X may be HO- RO- NH,,RS- H- etc.; Y may be CI- Br- (RO),PO- etc.] sugar derivatives which could be easily characterised. The isolation of both isomers established the constitution of the epoxide although there appeared to be no reason for the predominance of one isomer over the other. In the last few years epoxide fission has been much discussed and the pattern of reaction is now clear.22~40~41 3.1.Ring-opening in Systems with a Rigid Conf~rmation.-Mills~~ first suggested the applicability to sugar epoxides of Furst and Plattner's rule that steroid epoxides break to give predominantly the axial isomer. The geometry of axial opening is shown at (a) (the small arrows indicate the direction of movement of the oxiran-carbon atoms) and it is obvious that there is a favourable co-planar transition state. Equatorial opening (b) is seen to be a very hindered process. When the conformation of a sugar epoxide is made rigid by a tram- fused 4:6-benzylidene group or a 1:6-anhydro-ring the product of ring scission contains almost exclusively the axial isomer. Thus with nucleo- philic reagents the epoxides shown in the annexed group of formulze (A) 40 Angyal Chem. a d Ind. 1954 1230. 41 Overend ibid.1955 995. 4a Mills cited by Newth and Homer J. 1953 989. 42 QUARTERLY REVIEWS give predominantly the products indicated. The very small amount of the other isomer which is usually formed shows that equatorial opening can occur to a limited extent. PhT0,5~2 ,o 0 ,o 0- OMe OMe“ CH 0 X = OMeI6 ( 4 0 . - N H l6 (Scheme A) x 3.2. Ring-opening in Systems with a Flexible Conformation.-The monocyclic sugar epoxides have a flexible conformation and can exist in two forms (9).22 If it is accepted that axial attack is the rule either *,=* . . . . . . . . (9) form may react and the products can then change into their most stable conformation^.^^ From this point of view it is unnecessary to postulate “exceptions” to Furst and Plattner’s rule. The predominant isomer will have its origin in the more stable form of the epoxide; the proportion of each isomer will reflect the energy difference between the two conforma- tions of the epoxide.It is not possible to predict at present which conforma- tion will be the more stable. Charalambous and P e r ~ i v a l ~ ~ examined the fission of methyl 2 3- and 43 Peat and Wiggins J. 1938 1810. 44 Grob and Prins Helv. Chim. Acta 1945 28 840; Jeanloz Prins and Reichstein 46 Harvey Michalski and Todd f. 1951 2271. 46 Prins J. Amer. Chem. Sac. 1948 70 3955. 47 Myers and Robertson ibid. 1943 65 8; Wiggins f. 1947 18. 48 Charalambous and Percival J. 1954 2443. ibid. 1946 29 371. NEWTH SUGAR EPOXIDES 43 3 4-anhydro-6-deoxy-a-~-taloside and their 2- and 4-0-methyl derivatives by sodium methoxide. The course of the reactions of the two 0-methyl derivatives is shown in the following formula= (B) and the axial opening of the stable conformations of the epoxides (eq’ eq’ ax‘) accounts for the products which were isolated.The unrnethylated OMe ( 7 d O M e O 4 - O M e omMe Me OMe (Scheme B) anhydrides in contrast --c MeoMe Hb bMe gave predominantly the alternative isomers and for comparison the “un- stable” (ax‘ ax‘ eq’) conformations are shown in (C). It is however from these two conformations that the products must originate. The only reasonable explanation must involve the free hydroxyl group and it is suggested that hydrogen-bonding between this and the lactol-oxygen atom makes the conformations in (C) the more stable (cf. 1 6-anhydrides for similar atomic distances). M e ~ T e JMe H o o M e -c H o e e OMc 0 OH H O ‘ 0 H 2 pts M e a H 0’ ocyMe OH Ipt.(Scheme C) c This suggested role of the hydroxyl group adequately explains the persistent formation of xylose derivatives (XLV) from methyl 2 3-anhydro- P-D(and L)-ribopyranoside (XLIV). 3.3. Epoxide Migration.-It has been seen that trans-opening of an epoxide ring occurs by attack of a nucleophilic reagent on one of the oxiran-carbon atoms from the side opposite to the oxygen atom. This reagent may be within the molecule itself as in the formation of methyl 44 QUARTERLY REVIEWS 3 6-anhydro-fl-~-glucoside (XXII) from (XX) or (XX1).12 If there is a hydroxyl group adjacent to the epoxide but trans to the ring an intra- molecular displacement by the hydroxyl anion can also occur with the formation of a second epoxide (Scheme 10).This migration was first a G-> -0- ?J postulated by Lake and Peat5* to explain the formation of methyl 3:4- anhydro-/3-D-altroside (XLVIII) as well as methyl 2 3-anhydro-P-~- mannoside (XLVII) from methyl 2-0-tosyl-P-~-glucoside (XLVI). The reaction was also assumed to occur when 1 6-anhydro-3-O-tosyl-#%~- altrose (XXVIII) was found to give not 1 6-2 3-dianhydro-P-~-mannose but 1 6-2 3-dianhydro-/3-~-altrose~~ (XLIX). CHiOH CH2-OH CH2-OH CH,-0 HO Icsp” - “o&e - o e e o&$ OTs (x LV I) (XLVI I) (XLVI I I) (XLIX) Epoxide migration in the inositol series has recently been demonstrated3* in the conversion of (lS)-l 2-anhydroalloinositol (L) into ( 1 9 - 1 2- anhydroneoinositol (LI) by very mild alkali. At equilibrium there is present 10% of the compound (L) and 90% of its isomer (LI).The latter is more stable by about 1-3 kcal./mole and in its preferred conformation has only one axial hydroxyl group. In Scheme (D) it can be seen that it is an obvious requirement that the attacking hydroxyl anion shall be in an axial position; the displaced anion will then be equatorial. For the reverse reaction there must be a conforma- tional shift and axial attack can again occur. It is necessary to consider the non-bonded interactions in all four forms when attempting to predict the 4B Honeyman J. 1946 990. Mukherjee and Todd J. 1947 969. 61 Baker and Schaub J. Org. Chem. 1954,19 646. 63 Kent Stacey and Wiggins J. 1949 1232. 63 Allerton and Overend J. 1951 1480. 64 Lake and Peat J. 1939 1069. NEWTH SUGAR EPOXIDES 45 direction of equilibrium.It is interesting that the conformational shift cannot occur in 1 6-3 4-dianhydro-fl-~-altrose (XLIXa) and it is very doubtful whether its reconversion into 1 6-2 3-dianhydro-j3-~-mannose is possible. 0-J ‘ 0 JF Jf (X LI Xa) (Scheme D) 3.4. 3:4-AnhydrogaIactose.-In 1935 Oldham and Robertsons5 isolated mono-0-isopropylidene derivatives of galactose and gulose from the reaction of the 3 4-anhydro-derivative of methyl a-D-galactoside (LII). This apparently anomalous cis- and trans-opening of the oxide ring was re-investigated by Labaton and Newths6 who confirmed the earlier work and examined the action of hydrochloric acid on the anhydro-sugar. It was however their assignment of a 3 6-structure to the benzylidene derivative of one of the chlorohydrins believed to be methyl 4-chloro-4-deoxy-a-~- glucoside that stimulated further investigation.Buchanan5’ saw that the syrupy anhydride (LII) could be a mixture of the guto- and galacto- epoxides owing to epoxide migration and that some of the earlier products could be derived from the gulo-epoxide. By treating methyl 2 3-anhydro- 4 6-0-benzyl.idene-a-~-gu~oside~~ with hydrochloric acid he showed that “methyl 3 6-0-benzylidene-4-chloro-4-deoxy-a-~-glucoside”~~ was methyl 4 6-0-benzylidene-2-chloro-2-deoxy-a-~-idoside (LIII) and this con- firmed the presence of the gulo-epoxide in the mixture designated 0.11). There was now an obvious uncertainty about the identity of “methyl 3-chloro-3-deoxy-a-~-guloside”. Buchanan re-examined this compound 66 Oldham and Robertson J. 1935 685. 66 Labaton and Newth J. 1953 992.67 Buchanan J. 1958,995; Chem. and Ind. 1954 1484. 46 QUARTERLY REVIEWS and observed a very slow consumption of 1 mol. of periodate. It was therefore methyl 4-chloro-4-deoxy-a-~-glucoside (LIV). This was con- firmed when authentic methyl 3 4-anhydro-a-~-galactoside was treated with hydrochloric and one of the chlorohydrins was identical with Labaton and Newth's compound; the other by virtue of its stability to periodate was methyl 3-chloro-3-deoxy-~-~-guloside (LV). To explain the formation of 0-acetyl-3 4-isopropylidene-a-~-galacto- side in the Oldham and Robertson reaction acid-catalysed cis-opening of the epoxide by acetone was suggested.56 Buchanan5' showed however that this was not the 2-0-acetate but the 6-0-acetate (LVI) and its forma- tion was explained when he treated pure and authentic methyl 2-0-acetyl- 3 4-anhydro-6-O-trityl-a-~-galactoside (LII) and methyl 4-0-acetyl-2 3- anhydro-6-O-trityl-a-~-guloside (LVII) with anhydrous hydrogen chloride in acetone.58 The former was converted into methyl 0-acetyl-4 6-0- isopropylidene-a-D-guloside (LVIII) and the latter into 6-0-acetyl-3 4- 0-isopropylidene-a-D-galactoside (LVI).This was explained by the directive influence of the neighbouring trans-0-acetyl group on the epoxide fission through the carbonium-type intermediates shown in (LII)+(LVIII) and (LVII)+( LVI) . HO OH (Llll) (LI v) (LV) Me H Ac CMe t &Vlll) Me H 3.5. Reaction with Grignard Reagents.-The epoxides (XVI) and (XVII) have been very completely examined. With alkyl- and phenyl-magnesium halides the products are the halogenohydrins and these are also formed with magnesium halides.Diethylmagnesium and diphenylmagnesium give 68 Buchanan J, 1958 2511. NEWTH SUGAR EPOXIDES 44 C-ethyl and C-phenyl derivatives. The reactions which are shown in the batch of formulze below do not show a consistent pattern and more varied examples are required before the mechanism of reaction can be fully under stood. (X= I Br ,or Cl) 0-CH 0-CH 0-CH Ph *C I H-0 o O M e A X Ph-iH-O&Mrz z P h - l H - O o O M e Et (X=Bror I) (XVO Reagents 1 (a) MgX262 (ii) EtMgBrso; EtMgl PhMgBr. 2 MgMelSS. 3 (a) MeMgP 6s Newth Richards and Wiggins J. 1950 2356. 6o Richards and Wiggins J. 1953 2442. 61 Richards J. 1954 4511. 6a Richards Wiggins and Wise J. 1956 496. 63 Foster Overend Stacey and Vaughan J. 1953 3308. 64 Richards J. 1955 2013. EtMgl PhMgl; (b) MgBP2 Mgl (not MgCI,). 4 MgPh,. 5 MgEt,63.

ISSN:0009-2681    

DOI:10.1039/QR9591300030

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

ASYMMETRY THE NON-CONSERVATION OF PARITY AND OPTICAL ACTIVITY By T. L. V. ULBRICHT B.Sc. PH.D. (DEPARTMENT OF ORGANIC AND INORGANIC CHEMISTRY UNIVERSITY OF CAMBRIDGE) Introduction.-The discovery that parity is not conserved in certain processes has aroused a great deal of interest and within a year of the initial discoveries’ being made Lee and Yang were awarded the Nobel Prize for their work. Unfortunately virtually all the papers and most of the review articles on this subject are only intelligible to those familiar with nuclear theory. It is the aim of this Review to present the underlying principles of the theory and experiments relating to this discovery in a manner which may be understood by scientists not specialised in this field. It is hoped that it may be of particular interest to chemists who are familiar with the problem of asymmetry in a different context.Parity.-The principle of parity states that the laws of Nature are invariant under space reflection i.e. that the mirror-image of a sequence of events is also a possible sequence of events; it also means that the mirror-image of an object is a possible object in Nature (as suggested by Dirac for elementary particles and again recently confirmed by the discovery of the antiproton and antineutron). In the parity operation P the spatial co-ordinates are inverted through the origin x y and z becoming -x -y and -2; a state designated by a wave-function which remains unchanged in sign under the operation P is said to have “even parity” and one which changes sign “odd parity”. To illustrate the meaning of these terms let A screw is an asymmetrical object; that is it Direction 9 * Rotation us consider an example.is not identical with its c‘/ Rotation 4 1 Direction FIG. 1 mirror-image (or one can express this by saying that a left-handed screw and a right-handed screw are non-superimposable). Under the reflection in Fig. 1 the direction is changed. If we imagine the screw to be moving with a momentum p then after reflection it would have momentum -p. 48 ULBRICHT ASYMMETRY 49 Momentum is an example of a polar vector (length + direction); polar vectors change sign on reflection i.e. they have oddparity. On the other hand the sense of rotation of the screw (or its spin if we imagine it to be moving) is not changed by reflection. Spin is an example of an axial vector (surface + sense of rotation); axial vectors are unchanged by reflec- tion and therefore have even parity.Clearly an object (like a screw) which is defined by a polar vector coupled with an axial vector must be asymmetric with respect to the parity operation since one vector changes sign whereas the other does not. A quantity which is the product of a polar vector and an axial vector is called a pseudoscalar and processes involving pseudoscalar quantities will not obey the law of parity. This is only another way of saying that an asymmetrical object or process cannot be described by symmetrical functions. Elementary Particles their Interactions and Conservation Laws.-Of the elementary particles the electron proton and neutron are fairly familiar to chemists. The electron (e-) is a very light particle with a unit negative charge; the proton (p) is nearly 2000 times heavier and has a unit positive charge.The corresponding anti-particles with the opposite charges are the positron (e+) and the antiproton (p). The neutron (n) has almost the same mass as the proton but carries no charge and its anti- particle the antineutron @) differs only in having the opposite magnetic moment. Protons and neutrons (together called nucleons) make up the nuclei of atoms. Before anything can be said about other elementary particles their interactions must be briefly defined. These are of three kinds (a) Nuclear interactions. These involve very strong forces operating only at very small distances (e.g. inside the nucleus) between pairs of nucleons p-p p-n n-n. (b) Electromagnetic interactions.These are the “normal” interactions involving fairly strong forces. For example the fact that charged particles attract or repel each other is explained by supposing that each particle produces an electromagnetic field and that the interaction proceeds by the emission and absorption of photons. (c) Weak interactions. These are so-called because the ratio of the strengths of the three types of interaction (nuclear electromagnetic and weak) is as 1 10-l2. An example of a weak interaction is &decay. After an average life-time of twelve minutes a neutron decays into a proton and an electron n -f p + e - Until recently it was believed that six conservation laws were valid for all types of interaction that is conservation of energy momentum and angular momentum and three symmetry laws (i) Parity (P); (ii) charge 50 QUARTERLY REVIEWS conjugation (C) an operation which changes all particles into their anti- particles (e.g.e- -+ e+) and should not affect the symmetry of any possible physical process; and (iii) time-reversal (T) better defined as reversal of direction of motion; this requires that the reverse of a possible process in Nature should also be a possible process in Nature. It was observed that in p-decay the proton and electron produced could not account for the total energy momentum and angular momentum of the initial system and Pauli suggested that another particle the neutrino was also produced. Since it is believed that the overall number of particles and anti-particles remains balanced it is a neutrino (v) which is emitted together with a positron and an antineutrino (Y) with an electron n 3 i + e - + i Thus the neutrino was postulated to explain an awkward experimental result and although other evidence for its existence was soon forthcoming,l it has always seemed a very odd particle bearing no charge and it seems little or no mass.Another particle which had its origin in theory was the meson postulated by Yukawa to explain the nuclear interactions (it is supposed that mesons are exchanged between nucleons as photons are between particles in electromagnetic interactions). The meson was required to be about 300 times heavier than the electron; this is the n-meson discovered by Powell. It decays to give a p-meson (which has about 200 times the mass of an electron) which itself decays very rapidly to give an electron and two neutrinos Those particles lighter than the n-meson i.e.p e and v are called leptons (light particles). Finally there are the strange particles which are of two kinds (a) Those heavier than nucleons. All decay to give either p or n for example the hyperon (A) .............. T* -t pI + v (or a). (1 ) pf -f e I + v + 5 . . .............. (2) A -+ p f r - (b) Those intermediate in mass between nucleons and 71-mesons e.g. the K-mesons. All weak interactions involve either leptons or strange particles. The 8-r Puzzle.-A few years ago few people questioned the validity of parity conservation or considered devising specific experiments to test it. For example it was held that elementary particles cannot have electric dipole moments since it can be shown that this would violate parity; Purcell and Ramsay2 alone proposed actually to investigate this question.Wick For a recent review on the neutrino see G. Luders Naturwiss. 1958,45,456. * E. M. Purcell and N. F. Ramsay Phys. Rev. 1950,78,807. ULBRICHT ASYMMETRY 51 Wightman and Wigner3 pointed out that it was difficult to justify theo- retically either the operation P or the operation C (charge conjugation) as exact symmetry laws ; the disturbing possibility remained that they were only approximate and that the combined operation CP was the only exact symmetry law. The 6 (=Kn2) and T ( z K n 3 ) mesons have apparently identical masses and lifetimes,* which would normally indicate that they are the same particle but analysis of the decay products K* -f 7Th + no (0 mode) Ki- 4 n* + T+ f n- (T mode) indicates that one decay mode (6) has even parity and the other mode (7) odd parity since the mneson has been assigned odd parity from other experiments.Hence they cannot be different modes of decay of one and the same particle-unless parity is not conserved. It was this problem which led Lee and Yang5 to examine the evidence for parity conservation and to conclude that for weak interactions there was in fact no such evidence and to propose specific experiments designed to decide this question. Parity Non-conservation.-One possibility is to measure the angular distribution of the electrons coming from the /3-decays of oriented nuclei. If 6 is the angle between the orientation of the parent nucleus and the momentum of the electrons an asymmetry of distribution between 6 and (180O-6) would indicate a correlation of the spin (an axial vector) with the P-ray momentum (a polar vector) which can only be understood in terms of parity violation (cf.p. 49). An experiment was carried out along these lines by Wu et u Z . ~ using cobalt-60 6oCo -f 60Ni f e- + v The nuclei of 6oCo were strongly polarised by cooling to 0.01"~ in a strong magnetic field. If parity were conserved the distribution of the emergent electrons should have been symmetrical as shown by the mirror- reflection in Fig. 2. In fact the angular distribution of the electrons was FIG. 2 asymmetrical many more electrons emerging in the direction opposite G. C. Wick A. S. Wightman and E. P. Wigner Phys. Rev. 1952,88,101. R. Dalitz Phil. Mag. 1953 44 1068. T. D. Lee and C. N. Yang Phys. Rev. 1956,104 254. C. S.Wu E. Ambler R. W. Hayward D. D. Hoppes and R. P. Hudson Phys. Rev. 1957 105 1413. 52 QUARTERLY REVIEWS to that of the nuclear spin i.e. the electrons were left-handed as shown in Fig. 3. FIG. 3 The Two-component Neutrino Theory of Lee and Yang.-Even before experimental evidence was available parity non-conservation was ex- plained in terms of a new theory of the neutrino. Lee and Yang7 suggested that for a given mornentump the neutrino has only one spin state the spin always being parallel to p ; the spin of the antineutrino is always anti- parallel to its momentum. The spin and momentum automatically define the sense of the screw the neutrino represents the spiral motion of a right- handed screw and the antineutrino the spiral motion of a left-handed screw. (In four-component antineutrinos may be left- Under space inversion P - V V FIG.4 theories of the neutrino both neutrinos and or right-handed.) one inverts the momentum of a neutrino but not its spin direction. Since these must be parallel inversion leads-to a non-existent state by definition and parity is not conserved. (The inversion is as in Fig. 1.) The operation charge conjugation C changes a particle into its anti- particle but does not change its spin direction or momentum; operation C on the neutrino leads to an antineutrino with its spin and momentum still parallel; this by definition is also a non-existent state. The theory is therefore not invariant under charge conjugation. If the screw-like nature of the neutrino is to be an intrinsic property the neutrino must necessarily have zero rest-mass.This was also the basis of a similar theory of Sitlam.* To see this point let us suppose that we are on our way to the moon and that we are passed by a neutrino which in some miraculous way we are able to see. The neutrino has a velocity of say 0.8~ (c=velocity of light) and is left-handed. We accelerate to 0 . 9 ~ and pass this same neutrino which will now appear to us as right-handed (Le. relative to us we have carried out the parity operation on the T. D. Lee and C. N. Yang Phys. Rev. 1957 105 1671. A- Salam Nuova Cim 1957 5. 299 ULBRICHT ASYMMETRY 53 neutrino-inverted its momentum). If the neutrino had the velocity of light then its handedness would be independent of the velocity of the observer and since any finite rest mass would be infinite at this velocity the neutrino must have zero rest-mass.Landaug suggested that if parity non-conservation implied a fundamental asymmetry of space this might lead to difficulties (however cosmological asymmetry is compatible with Riemannian space-time of general rela- tivitylO). Landau therefore suggested the principle of combined inversion in which space inversion (P) and transformation of a particle into its anti- particle (C) occur simultaneously. Obviously parity does not hold since combined inversion does not change charged particles into themselves. The principle of combined inversipn leads again to the theory of the neutrino in which it is always polarised in its direction of motion (Le. its spin and momentum are parallel). It should be noted that the mirror-image of the neutrino cannot exist in the ordinary world but would exist in the anti-matter world.From this theory it follows that in n-meson decay (I) the p-mesons will be completely polarised in proportion to v/c (i.e. the ratio of their velocity to that of light). Further Experimental Evidence.-The decay processes (1) and (2) had already been considered by Lee and Yangs. If (1) violates parity conserva- tion the p-meson will be polarised in its direction of motion. In (2) the angular distribution problem will then be very similar to that in /3-decay that is the direction of the electrons will depend on the polarisation of the p-mesons. Garwin Lederman and Weinrichll used scintillation counters to identify the mesons entering a block of material and the electrons emerging after a delay of not more than 2 microseconds.There is a large asymmetry for the electrons in (2) indicating that the p-mesons are strongly polarised. As in /3-decay the electrons are left-handed.la (All experiments have shown electrons to be left-handed and positrons to be right-handed.) There have been numerous further experiments on polarisation in ,8-decay,lG17 in which the asymmetry has as predicted been found L. Landau Nuclear Physics 1957,3 127. lo E. C. G. Stueckelberg Phys. Rev. 1957 106 388. l1 R. L. Garwin L. M. Lederman and M. Weinrich Phys. Rev. 1957,105 1415. la J. I. Friedman and V. L. Telegdi Phys. Rev. 1957,105,1681. H. Frauenfelder R. Bobone E. Von Goeler N. Levine H. R. Lewis R. N. Peacock l4 P. E. Cavanagh J. F. Turner C. F. Coleman G. A. Gard and B. W. Ridley l6 E. Ambler R. W. Wayward D. D. Hoppes R.P. Hudson and C. S. Wu Phys. l6 H. Frauenfelder A. 0. Hanson N. Levine A. Rossi and G. de Pasquali Phys. M. Deutsch B. Gittelman R. W. Bauer L. Grodzins and A. W. Sunyar Phys. A. Rossi and G. de Pasquali Phys. Rev. 1957 106 386. Phil. Mag. 1957 2 1105. Rev. 1957 106 1361. Rev. 1957 107 643. Rev. 1957 107 1733. 54 QUARTERLY REVIEWS approximately equal to v/c ; on p-meson decay;1s-21 on the longitudinal polarisation of positrons from 5 8 C ~ 66Ga and 13N 16y22-24 and unpolarised p + - m e s ~ n s . ~ ~ > ~ ~ It has been pointed out that /%particles emitted by randomly oriented nuclei can be longitudinally polarised which could be detected in double ~cattering,~' and this has been observed.28 It was also suggested by Lee and Yang that ,8-decay should leave the nucleus partially polarised with respect to the p-ray momentum and consequently any following y-ray should be circularly polarised to an extent proportional to the cosine of the angle between the direction of the emission and the y-proton.This has been shown to be the case by experi- ments on /3-y polarisation c~rrelation.~~-~l Of particular interest in connection with the question raised on p. 57 is the demonstration that the Bremsstrahlung due to longitudinally polar- ised B-rays is circularly polarised. As the electrons emitted in $-decay slow down they lose some of their energy by emitting y-radiation and this is called Bremsstrahlung (literally brake-radiation). The circular polarisa- tion of the external Bremsstrahlung (that produced after the electron has left the atom) has been ~ a l c u l a t e d ~ ~ - ~ ~ and rneas~red,~~-~' down to quite small energies.38 Current Theory and Experiment on Parity Non-conservation.-All the evidence cited so far relates to the first group of weak interactions (those involving leptons).The asymmetry of these processes can be ascribed to the special properties of the neutrino. However neutrinos are not involved in 1* A. Abashian R. K. Adair R. Cool A. Erwin J. Kopp L. Leipuner T. W. Morris D. C. Rahm A. M. Thorndike W. L. Whittemore and W. J. Willis Phys. Rev. 1957 105 1927. J. M. Cassels T. W. O'Keeffe M. Rigby H. M. Wethrell and J. R. Wormald Proc. Phys. SOC. 1957 A 70 543. 2o M. H. Alston W. H. Evans T. D. N. Morgan R. W. Newport P. R. Williams and A. Kirk Phil. Mag. 1957 2 1143. 21 C. Castagnoli C . Franzinetti and A. Manfredini Nuovo Cirn.1957 5 684. 22 H. Postma W. H. Huiskamp A. R. Miedema M. J. Steenland H. A. Tolhoek and C . J. Gorter Physica 1957 23 259. 23 S. Frankel P. G. Hansen 0. Nathan and G. M. Temmer Phys. Rev. 1957 108 1099. 24 F. Boehm T. B. Novey C. A. Barnes and B. Stretch Phys. Rev. 1957,108 1497. 25 G. Culligan S. G. F. Frank J. R. Holt J. C. Kluyver and T. Massam Nature 26 L. A. Page and M . Heinberg Phys. Rev. 1957 106 1220. 27 L J. Tassie Phys. Rev. 1957 107 1452. 28 A. de-Shalit S. Kuperman H. J. Lipkin and T. Rothem Phys. Rev. 1957 107 H. Schopper Phil. Mag. 1957 2 710. 30 H. Appel and H. Schopper 2. Physik 1957 149 103. 31 F. Boehm and A. H. Wapstra Phys. Rev. 1957,106,1364; 1957,107,1202,1462. 32 K. W. McVoy Phys. Rev. 1957 106 828. 33 C. Fronsdahl and H. Uberall Phys. Rev. 1958 111 580.34 K. W. McVoy Phys. Rev. 1958 111 1484. 35 M. Goldhaber L. Grodzins and A. W. Sunyar Phys. Rev. 1957,106,826. 36 S. Galster and H. Schopper Phys. Rev. Letters 1958 1 330. 1957 180 751. 1459. A. Bisis and L. Zappa Phys. Rev. Letters 1958 1 332. S. Galster and H. Schopper Nuclear Phys. 1958 6 125. ULBRICHT ASYMMETRY 55 strange-particle decay and Lee and Yang's two-component theory apparently leaves the 0-7 puzzle which gave it birth unsolved. Moreover parity is not conserved in hyperon a process also not involving neutrinos. At a time when the situation was rather confused there came the result of a crucial experiment. For reasons that cannot be explained here it follows from Lee and Yang's theory" that the electron and the anti- neutrino which emerge together should have the same helicity (i.e.handedness). Since the electron is always left-handed the antineutrino should be left-handed also and both the positron and the neutrino should be right-handed. It was conclusively in the decay of 152mE~ that the neutrino is left-handed and this result is supported by other experi- ments on electron-neutrino angular correlation.42~43 A new universal theory of weak interactions has been ~ u g g e s t e d ~ ~ - ~ ~ in which parity non-conservation is no longer restricted to processes involving neutrinos and which successfully explains virtually all the experimental results. Although the fundamental asymmetry now no longer resides in the neutrino but in a Hamiltonian the theory still yields a two- component neutrino (but a right-handed one). The new theory makes a number of predictions which are already being tested (i) Weak interactions should be invariant under time-reversal (probable but not yet certain4').(ii) That one in 8000 of n-mesons should decay directly to an electron without going through a p-meson. Such decays have now been f o ~ n d . ~ ~ ~ ~ (iii) That one in 16 x hyperons should undergo @decay A + 'p + e- + v Isolated cases of such decays have recently been o b ~ e r v e d . ~ ~ ~ ~ ~ "For those familiar with the symbols the interaction turned out to be A and V not S and T as was first thought. 39 F. S. Crawford M. Cresti M. L. Good K. Gottstein E. M. Lyman F. T. Solnitz M. L Stevenson and H. K. Ticho Phys. Rev. 1957,108,1102. 40 F. Eisler R. Plano A. Prodell N. Samios M. Schwartz J. Steinberger P. Bassi V. Borelli G.Puppi G. Tanaka P. Woloshek V. Zuboli M. Conversi P. Franzini I. Manell R. Santangelo V. Silvestrini D. A. Glaser C. Graves and M. L. Per1 Phys. Rev. 1957 108 1353. 41 M. Goldhaber L. Grodzins and A. W. Sunyar Phys. Rev. 1958 109 1015. 42 K. H. Lauterjung B. Schimmer and H. Maier-Leibnitz,Z. Physik 1958,150,657. 43 W. B. Herrmannsfeldt R. L. Burman P. Stahelin J. S. Allen and T. A. Braid 44 R. P. Feynman and M. Gell-Mann Phys. Rev. 1958 109 193. 45 E. C. G. Sudarshan and R. E. Marshak Phys. Rev. 1958 109 1860. 46 J. J. Sakurai Nuovo Cim. 1958 7 649. 47 M. A. Clark J. M. Robson and R. Nathans Phys. Rev. Letters 1958 1 100. 48 T. Fazzini G. Fidecaro A. W. Merrison H. Paul and A. V. Tollestrup Phys. 49 G. Impeduglia R. Plano A. Prodell N. Samios M. Schwartz and J. Steinberger 6o F.S. Crawford M. Cresti M. L. Good G. R. Kalbfieisch M. L. Stevenson and 61 P. Nordin J. Orear L. Reed A. H. Rosenfeld F. T. Solnitz H. D. Taft and R. D. Phys. Rev. Letters 1958 1 61. Rev. Letters 1958 1 247. Phys. Rev. Letters 1958 1 249. H. K. Ticho Phys. Rev. Letters 1958 1 377. Tripp Phys. Rev. Letters 1958 1 380. 56 QUARTERLY REVIEWS The Induction of Optical Activity by Physical Agents.-In their Review on asymmetric transformation and induction Turner and confined themselves to chemical effects. Attempts to induce optical activity by physical agents-attempts which go back to the times of Pa~teur~~-are too numerous to be reviewed in full but some of the more important work will be mentioned. Curie54 criticised the view that a magnetic field alone could induce optical activity and suggested that a combination of a magnetic field and an electric field was necessary (i.e.an axial vector and a polar vector). In 1894 van’t H ~ f f ~ ~ stated that the direct formation of asymmetric products might take place in reactions induced by circularly polarised light and this was soon given a practical basis by the discovery of the Cotton effect.56 Much of the early unsuccessful experimental work was discussed by Bredig,57 who pointed out the im- portance of studying a reaction in which the primary reaction centre is actually the carbon atom which becomes asymmetric.t A small rotation (O*OSO) was first obtained by the use of circularly polarised light by Kuhn and Braun in 1929.s8 In the following year59 rotations of - 1 * 0 4 O and +0*78” were obtained by the partial photo- chemicaldecomposition of ethyl a-azidopropionate CH3CH(N3)C02C2H with circularly polarised light of wavelength 2800-3200 A.It should be noted that these and similar successful experiments60v61 do not in fact constitute true asymmetric synthesis there is a net asymmetric synthesis because of asymmetric decomposition. Karaganis and Drikos62 obtained rotations of up to 0.2” by the reaction of unsymmetrical triarylmethyl radicals with chlorine in the presence of circularly polarised light. Later63 they showed that when the racemic triaryl chloride formed in the reaction was irradiated with circularly polarised light of the same wavelength no optical activity was produced and that the chloride did not decompose at this wavelength. Similarly Davies and Heggie64 obtained rotations of 0-04-0-05° in the reaction of trinitrostilbene with bromine or chlorine in the presence of circularly 62 E.E. Turner and M. M. Harris Quart. Rev. 1947,1 299. 63 L. Pasteur Revue Scientifique 1884 7 3. 64 P. Curie J. Physique 1894 3 409. 66 J. H. van’t Hoff “Lagerung der Atome im Raume” Braunschweig 1894. b6 A. Cotton Ann. Chim. Phys. 1896 8 347. 67 G. Bredig 2. angew. Chem. 1923,36,456. toptically active molecules often have axial symmetry but as the word “dissym- metry” (as used by Pasteur and W. H. Mills) is not generally employed now the word “asymmetry” has been retained. The Reviewer thanks a Referee for drawing his attention to this point. W. Kuhn and E. Knopf Naturwiss. 1930 18 183; 2. phys. Chem. 1930 B 7 68 W. Kuhn and E. Braun Naturwiss. 1929 17 227. L7L. 8o S.Mitchell J. 1930 1829. 62 G. Karaganis and G. Drikos Naturwiss. 1933,21,607; Z. phys. Chem. 1934 B 63 G. Karaganis and G. Drikos Praktika 1936,9,177; Chem. Zentr. 1936 I 3298. g* T. L. Davies and R. Heggie J. Amer. Chem. Soc. 1935,57,377 1622. J. A. Berson and E. Brown J. Amer. Chem. SOC. 1955,77,450. 26 428. ULBRICHT ASYMMETRY 57 polarised light. The racemic dibromide could not be made optically active by exposure to circularly polarised light and in the experiments with chlorine the wavelengths used were in a region in which the dichloride does not absorb. All these experiments therefore appear to represent true asymmetric syntheses. (It is not clear whether this also applies to the work of Radulescu and Moga. No explanation has been offered for these results; possibly they involve a metastable intermediate formed by absorption of the circularly polar- ised light which has a slightly preferred configuration (e.g.a triarylmethyl radical which is not planar). Optical Activity and Parity Non-conservation.-The type of fundamental asymmetry suddenly encountered amongst elementary particles inevitably recalls the spatial asymmetry responsible for optical activity. In Fig. 5 we have an example of the simplest type of such asymmetry the central carbon atom in glyceraldehyde having four different substituents arranged spatially as if in the corners of a tetrahedron. Fundamentally this is a very similar situation to that in Fig. 1. A vector has two components; an object will be asymmetric in n-dimensional space if it has (n + 1) “properties”. Thus a triangle (which requires three properties for definition e.g.three lengths two lengths and one angle etc.) is asymmetric in a plane (two dimensions); a screw (a polar vector and an axial vector) and a carbon atom with four different substituents are asymmetric in 3-dimensional space. It is natural to ask whether there is any connection between asymmetry at the molecular level and asymmetry at the level of elementary particles. Could optical activity be produced by polarised #3-radiation ? A dynamic interaction between molecules and high-energy electrons would have to be mediated by secondary effects of lower energy (since the interaction is negligibly small if the energy levels are far apart). We have already seen that polarised p-rays give rise to circularly polarised Bremsstrahlung and that in the energy range required for photochemical asymmetric synthesis esD.Radulescu and V. Moga Bul. SOC. chim. Romania 1939 1 18; Chem. Abs. 1943 37 4070. 58 QUARTERLY REVIEWS measurable asymmetry is still present. One possible pathway is therefore the following Long i t u d i n a I l y polarised ,brays -+ polarised -+ active Circularly Optically light molecules Other secondary effects (e.g. magnetic interaction) might conceivably produce optical activity. However the sum of such effects would be very small in terms of percentage of molecules actually effected; possibly too small to be detected experimentally. One worthwhile experiment would be to see whether there is any difference in the absorption by D- and L- isomers of the circularly polarised Bremsstrahlung from 13-rays.The question arises if some other pathway is possible. Optical isomers are identical in all physical and chemical properties except the transmission of plane-polarised light. That is to say it is merely a matter of probability (Le. entropy) that a 50/50 mixture of the isomers is formed in chemical reactions and to shift this balance to 51/49 or even lOO/O$ does not require any energy in principle (the idea of an entropy exchange in a reaction during irradiation will be considered elsewhereG6). It requires some kind of transmission of information regarding form and this transmission need not be by way of a dynamic interaction. An analogy in physics would be the so-called “exchange forces” which are not forces in the ordinary sense at all. The Pauli exclusion principle introduces a correlation in the behaviour of particles which though its effects are similar to the effects of forces has no explanation in dynamic terms.In other words how does an electron joining an orbital know the spin quantum number of the electron already in that orbital?G7 The difficulty in answering this question shows that an effect cannot be ruled out simply because one cannot suggest an exact mechanism which can be easily visualised-as we have already seen in the case of asymmetric synthesis. That one asymmetry may lead to another is not only philosophically reasonable but in conformity with the second law of thermodynamics ; certainly symmetry by itself cannot give rise to asymmetry. A non-energetic interaction for the induction of optical activity by polarised /3-radiation was first suggested by Vester.G8 The experimental difficulties in the investigation of this problem are numerous.A reaction is required with an intermediate whose lifetime is long enough for it to receive the required information (asymmetric configuration) but not so long that it loses it again before reacting. The reaction should be one whose velocity is increased but whose mechanism is little affected by high-energy ,B-rays. A difficulty here is that ionisations are mainly produced by elec- trons towards the end of their paths when their velocity has been reduced $The entropy of mixing is certainly not more than about 2 kcal./mole. 66 F. Vester and T. L. V. Ulbricht to be published. 67 H. Margenau “The Nature of Physical Reality” McGraw-Hill New York 1950. 68 F. Vester Seminar at Yale University 7th February 1957.ULBRICHT ASYMMETRY 59 and asymmetry may have been reduced by scattering (Coulomb scattering should not affect the polarisation of particles with near-relativistic Experiments have been carried ~ ~ t ~ ~ j ~ with a number of chemical systems including the synthesis of 1-chloroethyl ethyl ether. Unfortunately this has a low specific rotation but the reaction has the advantage of being a simple one with an ionic mechanism little affected by high-energy electrons72 and yielding a liquid product whose rotation could be measured directly. Control experiments were carried out in the absence of radiation and with unpolarised electrons from a linear accelerator. No consistent effect outside the margin of error was observed under a variety of condi- tions with ,&sources (32P Sr-goY 152E~) in the range of 25-3000 mc indicating that an effect cannot be demonstrated in this system.Ideally the optical activity due to the secondary effects discussed (which may be calculated e.g. for BremsstrahlungB6) should be just detectable ; then significantly greater optical activity than this would constitute evidence for a non-energetic effect. If optical activity could be produced by polarised /3-radiation it would be tempting to speculate whether the optical activity that asymmetric radiation (from cosmic rays natural radioactivity etc.) might have produced on Earth was associated with the origin of life. From a thermo- dynamic point of view life represents a strange phenomenon order emerging out of apparent chaos and resisting the otherwise universal tendency of entropy to i n c r e a ~ e .~ ~ ~ ~ ~ In the ordered structure of living systems optical purity plays a very important part,75 and the widespread occurrence of D-amino-acid oxidase is not in the least s~rprising.~~ It has been shown by H a ~ i n g a ~ ~ that a compound which is easily racemised may be spontaneously resolved during crystallisation ; one isomer begins to crystallise first racemisation occurs in the solution now richer in the other isomer and finally unequal quantities of the dextro- and the hvo-isomer may be obtained. This is certainly a suggestive experiment and if we assume that optical activity was required for the origin of life (of course we do not know this) represents the most satis- fying explanation for the origin of optical activity by chance.Other such explanations do not bear close examination; for example the optical activity which might be produced by local statistical variation amongst that number of molecules present in a small cell is very much smaller than velocitie~,~~ and this has been confirmed by e~perimentl~J*’~~~ 70 >. T. D. Lee personal communication. 70 J. Henitze 2. Physik 1958 150 134. 71 F. Vester T. L. V. Ulbricht and H. Krauch Naturwiss. in the press. 72 H. Krauch and F. Vester Naturwiss. 1957 44 491. 73 E. Schrodinger “What is Life?” Cambridge University Press 1944. 74 L. von Bertalanffy “Das Biologische Weltbild,” A. Francke Bern 1949. 75 W. Kuhn Experientia 1955 11 429. 76 H. A. Krebs in “The Enzymes” Part 11 i page 499 edited by J. B. Sumner and 77 E. Havinga Chem. Weekblad 1941 38 no.46; Biochim. Biophys. Acta 1954 13 K. Myrback Academic Press New York 1952. 171. 60 QUARTERLY REVIEWS that which might result from asymmetric radiation. Essentially our conclusions depend on what mechanism for the origin of life we propose and at the present time this is the subject more of philosophy than of science. The author thanks Sir Robert Robinson and Professors T. D. Lee H. C. Longuet-Higgins and Sir Alexander Todd for their interest and encouragement and Dr. F. Vester for numerous discussions. This paper was written during the tenure of an Imperial Chemical Industries Limited Fellowship at Cambridge.

ISSN:0009-2681    

DOI:10.1039/QR9591300048

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

OXIDATIONS BY MANGANESE DIOXIDE IN NEUTRAL MEDIA By R. M. EVANS B.Sc. PH.D. D.I.C. F.R.I.C. (GLAXO LABORATORIES LTD. GREENFORD MIDDLESEX) MANGANESE DIOXIDE occurs in many minerals the purest forms being pyrolusite MnO, and psilomelane Mn02,H20. Both these substances have been known from ancient times and manganese dioxide has been used as a decolorising agent in glass manufacture for the past two millennia. Accounts of its properties are met in early chemical literature and since the beginning of the last century its use as an oxidising agent in chemical reac- tions has been established. In organic chemistry it has found many applica- tions being used most frequently in the presence of mineral acids as in the well-known oxidation of methyl-substituted aromatic compounds to the corresponding aldehydes though acetic acid and aqueous solutions of alkalis have also been used as reaction media.These well-known reac- tions in general applicable only to compounds with stable nuclei will not be described here; attention will rather be centred on applications of the recent discovery that a suspension of manganese dioxide (prepared by oxidation or pyrolysis of manganese salts) can act as an oxidising agent in neutral non-aqueous media. This heterogeneous system is often effective at room temperature; under these conditions certain groups may be oxidised selectively in compounds that would be destroyed by more vigorous reagents. This new type of reaction was discovered ten years ago by Ball Goodwin and Morton,l who found that concentrates of vitamin A (I) when dis- solved in light petroleum and shaken with acidified aqueous potassium permanganate gave products whose ultraviolet absorption was similar to that of retinene (11) the highest yields of aldehyde being associated with R = Bk R*CH= CH*CMe= CH*CH= CH=CMe=CH.CH,-OH (I) R*CH=CH.CMe=CH*CH=CH*CMe=CH*CHO (11) 3.the formation of hydrated manganese dioxide during the reaction. Further experiments proved that hydrated manganese dioxide was the agent responsible for the dehydrogenation and that vitamin A in light petroleum solution was converted efficiently into retinene on being shaken at room temperature with a suspension of precipitated manganese dioxide (pre- pared by the oxidation of manganese sulphate with potassium perman- ganate in aqueous solution). Under similar conditions benzyl cetyl and isopropyl alcohol were unaffected and other common oxidising agents including chromic acid barium dioxide lead dioxide and silver oxide were found useless as substitutes.Ball Goodwin and Morton Biochem. J. 1948 42 516. 61 62 QUARTERLY REVIEWS Oxidations at room temperature a/?-EthyZenic and -AcetyZenic AZcohoZs.-After publication of these dis- coveries the possibilities inherent in this type of reaction were rapidly exploited; it proved of particular value for the synthesis of vitamin A and in exploring the chemistry of polyenes and polyenynes. During the next 4-5 years precipitated manganese dioxide became established as a convenient and efficient reagent for the oxidation of ethylenic and acety- lenic c@unsaturated primary and secondary alcohols to the corresponding carbonyl compounds a procedure frequently superior to the alternative Oppenauer reaction.2 Usually the unsaturated alcohol was shaken at room temperature in a neutral organic solvent such as light petroleum chloro- form acetone or ether with finely divided precipitated manganese dioxide or the solution was allowed to percolate through a column packed with the ~ x i d a n t .~ The necessary time of reaction varied from 1 hour to 10 days or more undoubtedly owing to the different methods used to prepare the dioxide to the effect of different solvents and to the use of different ratios of oxidant to substrate. The lack of standardisation of these important factors in the early experiments gave rise to divergent views on the utility of the reagent some of them later found to be erroneous. In this Review the applications will be considered before the influence of these factors and their control.The reactions illustrated (111-XIV) are representative of the synthesis of polyene and polyenyne carbonyl compounds by the use of hydrated man- ganese dioxide. In addition to the ~xidationl>~ of vitamin A (I) to retinene (11) similar efficient dehydrogenations of vitamin A2,4 8,9-dihydrovitamin A,5 and neovitamin A6 were obtained and the mild nature of the reaction was illustrated particularly well by Robeson et al. in their dehydrogenation (111) RCH= CHCMe= CH.CH,.OH (V) CH-CCH=CMe.CH,.OH CH,= CH*CMe=CHCH= CH.CHMe*OH HOC H MeCH = CH-CH = CHCH MeOOH (VW CH2= CH*CH,*OH (IX) (XI) &OH I/f“ C.CHMe*oH (XIII) RCH= CHCMe= CHCHO (IV) CH-C.CH=CMeCHO (VI) CH2=CH*CH0 (VIII) (XI CH,= CHCMe= CH-CM= CH-COMe Me-CO-CH = CH-CH = C H-COMe Oppenauer Rec.Truv. chin?. 1937 56 137 ; Djerassi “Organic Reactions” John Wald J. Gem Physiol. 1948 31 489. Farrar Hamlet Henbest and Jones J. 1952 2657; Cama Dalvi Morton Salah Attenburrow Cameron Chapman Evans Hems Jansen and Walker J. 1952 Dalvi and Morton Biochem J. 1952 50 43. Robeson Blum Dieterle Cawley and Baxter J. Amer. Chem. SOC. 1955,77 4120. Wiley and Son Inc. New York 1951 Vol. VI p. 207. Steinberg and Stubbs Biochem. J. 1952 52 535. 1094. EVANS OXIDATIONS BY MANGANESE DIOXIDE 63 of the four geometrical isomers of vitamin A to the corresponding retinenes without any isomerisation. Wendler Slates Trenner and Tishler* likewise succeeded in oxidising the cis- and the trans-form of the C1 alcohol [ 111; R as for (I)] to the C, aldehyde (IV) without interconversion.The oxidationg of the methylpentenynol (V) to the aldehyde (VI) shows that a free acetylenic group is unaffected and contrary to the observation in the Glaxo Laboratorie~,~ does not inhibit the reaction. The dehydro- genation5 of the simplest ethylenic compound ally1 alcohol (VII) to acraldehyde (VIII) indicates that a single ethylenic bond provides sufficient activation to bring about the reaction; oxidation of the C,alcohol (IX) to the ketone5 (X) and of the dienediol (XI) to the dienedionelO (XII) illustrate the value of the reaction for the oxidation of a/3-unsaturated secondary alcohols. The final examplell (XIII+XIV) proves the adequacy of a single cc/?-acetylenic bond to activate the hydroxyl group. Benzylic Alcohols.-Although Morton and his colleagues1 could not oxidise benzyl alcohol (XV) to benzaldehyde (XVI) more active forms of the dioxide (see below) achieved this in high yield.12J3 Ph.CHz*OH -j.PhCHO HO*CHPh*[CHJz*Ph -+ Ph.CO.[CHz],.Ph (XV) (XV9 (XVII) (XVI I I) Good conversions of a-substituted benzyl alcohols such as (XVII) and (XIX) into the ketones (XVIII) and (XX) respectively,14 together with many other examples in the literature further illustrate the general applicability of the reaction to this type of c o m p o ~ n d . l ~ - ~ ~ Alicyclic ap- Unsaturated Alcohols.-In 1953 workers at the Syntex Laboratories extended the use of this dehydrogenating system to syntheses in the steroid field showing that a/3-unsaturated hydroxyl groups in ring A B or c were converted efficiently into ketone groups16117 and that in contrast to what happens with the Oppenauer reagents ketol side chains were in general unaffected.They used the selective action with particular Wendler Slates Trenner and Tishler ibid. 1951 73 719. Ahrnad and Weedon J. 1953 3286. lo Ahmad Sondheimer Weedon and Woods J. 1952 4089. l1 Attenburrow et al. unpublished results. l2 Harnfeist Baveley and Lazier J Org. Chem. 1954 19 1608. l3 Highnet and \Vildman J. Amer. Chem. SOC. 1955 77 4399. l4 Turner ibid. 1954 76 5175. l5 Rappaport and Masamune ibid. 1955 77 4330. l6 Mancera Rosenkranz and Sondheimer J. 1953 2189. l7 Sondheimer Amendolla and Rosenkranz J Amer. Chern. Soc. 1953 75 5930. 64 QUARTERLY REVIEWS advantage to prepare 1 1 /3-hydroxytestosterone (XXII) in good yield from androst-4-ene-3fY1 1 /3,17/3-triol (XXI).lS Saturated AZcoho2s.The earliest experiments indicated that active manganese dioxide at room temperature had no effect on primary and secondary saturated alcohols,l but Harnfeist and his co-workersl2 have shown that this is not strictly true although the rate of oxidation is small compared with that of ap-unsaturated alcohols.From the foregoing examples it appears that only the ap-unsaturated hydroxyl groups in polyhydroxy-compounds are oxidised to an appreciable extent but an exception is the oxidation of both hydroxyl groups in (XXIII) to give the keto-aldehyde (XXIV) as the major product,18 and Dr. L. Crombiels has found that a cyclopropane ring will also activate an a-hydroxyl group and cause it to undergo oxidation by manganese dioxide. HQ (XXlJl) O f m (XXIV) CHO It may therefore be concluded that in non-aqueous inert organic media at room temperature hydrated (or “active”) manganese dioxide efficiently oxidises a/3-unsaturated primary and secondary alcohols to the cor- responding carbonyl compounds whether the unsaturation is part of an olefinic acetylenic aromatic or alicyclic system whereas tertiary and saturated hydroxyl groups are generally little affected.The exceptions have not so far proved numerous and the reagent has been used with success on several occasions to detect the presence of @-unsaturated Bruderer Arigoni and Jeger Helv. Chern. Acta 1956,39 858. lD Dr. L. Crombie personal communication. 65 EVANS OXIDATIONS BY MANGANESE DIOXIDE primary and secondary hydroxyl groupings the resulting conjugated carbonyl compounds being readily detected by their ultraviolet spectra or as deriva tives.12Jss O N-AZkyZ- and NN-DiaZkyl-aniZines.-A further interesting type of dehydrogenation by hydrated manganese dioxide was provided recently by Henbest and Thomas21 in the ready oxidation of N-alkyl and NN- dialkyl-anilines.Three types of reaction were discerned (4 >NMe -t >NCHO (b) >N.CH,R -+ >NH + RCHO (c) > N*CH,-CH,R -t > N*CH= CH R] +- >N*CHO + R-CHO With several amines the reaction took essentially a single course NN- dimethylaniline gave N-methylformanilide in 80 % yield and N-methyl- aniline was oxidised even more efficiently to formanilide according to route (a). The oxidation of NN-dimethylaniline was facilitated by the introduction of a p-methyl group but a p-nitro-substituent completely inhibited the reaction.The oxidation of diethylaniline proved more complex. The predominant initial reaction was by route (6) to give acetaldehyde (54 %) the monoethyl- aniline thus formed then probably undergoing further oxidation by route (c) to formanilide which was isolated in 65% yield. Small quantities of N-ethylformanilide and azobenzene were also detected. As would be expected N-ethyl-N-methylaniline gave N-ethylformanilide and formanil- ide as follows b Ph-NEt-CHO -?- Ph*NMeEt -f [PhoNHMe] Ph*NH*CHO The expectation that reaction (c) would be promoted by a second amino- group in the p-position was confirmed by the efficient conversion of (XXV) and (XXVII) into (XXVI) and (XXVIII) respectively. (XXV) Ph-N Me*[CHhj,-N MePh >-+ 2Ph-N MeCHO (XXVI) Ph*N/rCH$32\N*Ph -f+ OHC*NPh*[CH,],*NPh*CHO (XXVIII) b % l z / (XXVII) MisceZZaneous Reactions.-In addition to the types of reaction mentioned above all exemplified by a group of related compounds the following illustrate more limited applications of the reagent.Like the related alcohols primary and secondary benzylamines are oxidised by hydrated manganese dioxide but the Schiff's bases were only obtained in small yield and the reaction was s10w.l~ A tetrahydroquinazoline (XXIX) has been converted 2o Braude and Forbes J. 1951 1756; Cross Grove MacMillan and Mulholland a1 Henbest and Thomas J. 1957 3032. Proc. Chem. SOC 1958 221. 66 QUARTERLY REVIEWS into the dihydro-derivative (XXX) ;22 and oxidation13 of the hemiacetal (XXXI) to the lactone (XXXII) is also of interest in indicating a further type of application.(XXX I) QOH (XXXll I) (CH,= C H C H J 2 S - c$ (XXXII) -+ (CH,= CH*CHJZSO (X xx IV) Most of the reactions described so far may be classed as dehydrogena- tions but this is not so in the oxidation of diallyl sulphide (XXXIII) to the sulphoxide (XXXIV).23 No oxidising agent other than hydrated manganese dioxide yields any of the desired product and the mild nature of the system is also well illustrated in the successful preparation of a number of 18- amino-aldehydes from ,8-amino-alcohols.24 Oxidations at higher temperatures At temperatures between 70" and 120° the range of reactions brought about by manganese dioxide increases but the selectivity and the yields of polyene-carbonyl compounds are less. In refluxing benzene there is much more dehydrogenation of @-unsaturated 0x0-steroids and /3y-unsaturated hydroxy-steroids such as (XXXV and XXXVI) to the dienone (XXXVII).25 (xxxv) (xxxvr I) (XXXVI) Oxidations involving fission of a C-C bond may also be brought about a-ketols and vicinal glycols yield the corresponding ketones and aldehydes as exemplified by the cleavage in boiling chloroform of the steroid dihydroxyacetonyl side-chain to yield a 17-oxo-steroid and of 9,lO- dihydroxystearic acid to pelargonic and azelaic aldehydes.26 Under the same conditions mandelic and benzilic acid give benzaldehyde and benzo- phenone respectively.26 22 Fales J.Amer. Chem. Soc. 1955 77 5118. 23 Edwards and Stenlake J. 1954 3272. 24 Birkofer and Erlenbach Chem. Ber. 1958 91 2383. 2s Sondheimer Amendolla and Rosenkranz J. Arner. Chem. Sac. 1953,75 5932 26 Padilla and Herran Bol.Inst. Quirn. Univ. nac. auton. Mexico 1956 8 1096, 1:VANS OXIDATIONS BY MANGANESE DIOXIDE 67 An extensive study of other reactions at higher temperatures has been made by Barakat Abdel-Wahab and E1-Sadr,27 who described some fifty different oxidations by manganese dioxide many of them in aqueous media. Several saturated primary and secondary alcohols were oxidised to carbonyl compounds aromatic aldehydes gave the carboxylic acids and or-hydroxy-acids and a-amino-acids readily formed the aldehydes or ketones with one carbon atom less. Aromatic primary amines gave azo- compounds and hydrazones yielded the symmetrical diarylketazines. The efficient oxidation of triphenylphosphine to the oxide was a further application of the reagent. Reagent and conditions Preparation of Active Manganese Dioxide.-Despite its selective action and the efficiency of many of the reactions described the use of manganese dioxide has as yet been somewhat restricted possibly because difficulty has been encountered in obtaining suitable active material and in predict- ing the optimum reaction conditions.Unfortunately hydrated manganese dioxide is difficult to define,28 and material of consistent activity is ob- tained only when the method of preparation has been carefully controlled. The manganese dioxide used in the reactions described above was sometimes of commercial origin in other cases specially prepared. Probably it is for this reason that differences of opinion have arisen over the suit- ability of the reagent for various oxidations ; thus Morton and his colleagues1 and Turner14 found benzyl alcohol unaffected whereas Harnfeist and his co-workers12 and Highnet and Williams13 obtained an efficient conversion into benzaldehyde.Similarly manganese dioxide prepared by the pyrolysis of the carbonate or oxalate failed to oxidise ally1 alcohols whereas good yields of aldehydes were obtained if the dioxide had been first washed with dilute aqueous nitric acid and then dried. Pyrolusite shows low and erratic activity in the types of reaction con- sidered and batches of commercial hydrated manganese dioxide vary widely in their efficiency in neutral media; however suitable material may be selected by checking against a “standard” substrate such as cinnamyl But the most reliable and highly active material is obtained by 27 Barakat Abdel-Wahab and El-Sadr J.1956 4685. 28 Sidgwick “Chemical Elements and their Compounds” Clarendon Press Oxford 1950 p. 1272; Wells “Structural Inorganic Chemistry,” Clarendon Press Oxford 1950 p. 371. za Weedon and Woods J. 1951 2687. 68 QUARTERLY REVIEWS one of the following methods (a) Manganese sulphate and potassium permanganate are treated in hot aqueous solution in the presence of sufficient alkali to ensure that the reaction mixture remains alkaline ; after being washed with water the slurry is dried at 100-120" (b) As above omitting the addition of alkali the aqueous solution becoming strongly acidic.le (c) By heating manganese oxalate or carbonate at 2B0 the product may be used directly or washed with dilute aqueous nitric acid and dried at 230" to produce a more active oxidant.l2 The products obtained by these three methods all show high activity but differ in their relative merits for some types of oxidation; however insufficient comparisons have been made to permit other than broad generalisations.In our experience the product prepared by method (a) has been con- sistently effective in dehydrogenating a/?-unsaturated alcohols and in cleaving a-ketols. The hydrated dioxide prepared by method (b) proved on the other hand the most efficient for preparation of the steroid dienones26 and also had the virtue of not attacking the side chain of a 17a-hydroxy-20- 0x0-steroid at room temperature.17 The dioxide prepared by method (c) seems to have been the least widely used but the more active form is most effective in oxidising ally1 and benzyl alcohols.12 Relative efficiency of reaction is however not the only change brought about by varying the method of preparing the dioxide.Thus Henbest Jones and Owen30 showed that whereas the hydrated dioxide from preparation (a) converted vitamin A quantitatively into retinene (XXXVIII; R = H) one commercial product gave 16% of 3-hydroxyretinene (XXXVIII; R = OH) and another which had been precipitated under acidic conditions gave a considerable proportion of 3-oxoretinene (XXXVIII; R = 0). The last reaction is also of interest in being the first example of the oxidation of ah allylic methylene group by an insoluble reagent at room temper- ature to give an unsaturated ketone and may perhaps be considered analogous to the oxidation of N-methylaniline to formanilide. S'"3 5% ,,+CWCHC=CHCH=CH-C=CH.CHO v (XXXVIII) R Reaction Media.-The most widely used media for the oxidations at room temperature have been saturated hydrocarbons chlorinated hydro- carbons benzene lower alkyl ethers ethyl acetate and acetone. Solvents that compete with the substrate for adsorption on the dioxide surface are obviously unsatisfactory and in our laboratory it has been found that primary and secondary saturated alcohols fall into this category causing rapid and permanent deactivation of the dioxide. Acetone and Henbest Jones and Owen J. 1957 4909. EVANS OXIDATIONS BY MANGANESE DIOXIDE 69 ethyl acetate also bring about deactivation but much more slowly and in contrast to what happens with alcohols the activity is restored by drying the dioxide in a high vacuum. A similar range of solvents has been used for reactions srt higher temperatures where water and pyridine have also proved satisfactory.As in the selection of the dioxide an empirical approach is necessary in choosing the medium. Although many reactions proceed well in a variety of solvents dependence of the yield on the solvent has been noted,12J3 and Morton and his co-workers1 showed that whereas vitamin A in light petroleum solution was converted into retinene the use of diethyl ether resulted in formation of anhydrovitamin A. Reaction Time.-In many of the early experiments in which dioxide of low activity was used several days were often required for complete reaction but with the more active products prepared by methods (a) (b) or (c) reaction may be much more rapid. Dehydrogenation of ccp- unsaturated alcohols is usually complete in a few hours at room tem- perat~re,~ but formation of steroid dienones and oxidation of alkylanilines may require 16-24 hours.21 The time required to complete reactions at higher temperatures varies considerably; whereas at 80" a 50 % yield of acetaldehyde is obtained from ethanol in 20 reactions involving the fission of a C-C bond often require 12-20 hours at 80-100°.26~27 Ratio of Oxidant to Substrate.-As the reaction is heterogeneous and takes place on the surface of the dioxide,l the amount required for efficient reaction depends to some extent on the particle size.A considerable excess is always necessary and ratios of dioxide to substrate ranging from 5 1 to 20 1 by weight are common in the oxidation of hydrbxylic com- pounds ; a ratio of 50 1 was used to oxidise the dialkylanilines efficiently.Reaction mechanism The mechanism of these reactions has not yet been elucidated but a number of relevant observations have been made. Ball et aZ.l postulated that the reaction was triphasic adsorption of the substrate being followed by oxidation and subsequent desorption of the product. On this basis the more saturated compounds would be expected to be the less readily adsorbed. This has been borne out by Harnfeist and his co-workers,12 who observed that the order of ease of oxidation of alcohols was con- jugated polyene> benzyl or furyl>vinyl or ethynyl. However the same authors also pointed out that the addition of 5 % of t-butyl alcohol or 1% of water which would have been expected to alter appreciably the adsorption characteristics of the alcohol did not affect the yields ap- preciably nor could any relation be discerned between the ease of oxida- tion and the oxidation potentials of the alcohols.l3 The more readily oxidised compounds are however those that would be expected to form the more stable carbonium ion which may perhaps play an important part in the reaction mechanism.An equilibrium appears to exist on the dioxide surface between the 3 70 QUARTERLY REVIEWS substrate and the product ;I1 consequently the reactions never appear to go to completion and large increases in the amount of dioxide decrease the yie1d.l Henbest and Thomas21 have observed that only a small proportion of the oxygen in hydrated manganese dioxide is available for the oxidation of organic compounds and have suggested that its unusual properties may be due to the “water” being present as hydroxyl groups linked to the manganese.The hydroxyl groups in the dioxide then lead to the formation of hydroxylated amine intermediates in the dehydrogen- ation of the alkylanilines. The critical effect of moisture content has been noted in unpublished work from our laboratories where it has been found that the degree of hydration had an important effect on the activity of the dioxide prepared by method (a). This is a difficult property to measure since some of the water is so firmly bound that loss of oxygen occurs before dehydration is complete. Heating at 120” under a vacuum however gave a product of constant weight and by using this as a base line it was found that an excess moisture content of ca.4-8% (0.25 equiv.) led to the highest activity. In the same studies it was found that available oxygen of the hydrated manganese dioxide (determined by the Bunsen method”) declined after an oxidation by the theoretical amount required for the reaction; the possibil- ity of “active” oxygen adsorbed on to the dioxide surface being responsible for the oxidations was excluded and it appeared that a portion of the manganese was reduced to a lower valency state during the reaction. X-Ray powder photography and electron-microscopy with a 10,000- fold magnification failed to distinguish between “active” and “inactive” dioxide but clearly indicated its amorphous structure.” It is clear that more work is necessary to elucidate the mechanism of these reactions. They do afford however an elegant method for carrying out a number of oxidations under extremely mild conditions in anhydrous media ; although the most appropriate conditions must be determined empirically the selectivity and efficiency of the reactions often justify the necessary exploration. * These determinations were kindly carried out by Mr. H. P. Rooksby and Mr. B. S. Treadwell and Hall “Analytical Chemistry” Wiley and Sons New York 1942 Cooper at the Research Laboratories of the General Electric Co. Ltd. Wembley. Vol. 11 p. 598.

ISSN:0009-2681    

DOI:10.1039/QR9591300061

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

CURRENT CONCEPTS IN THE THEORY OF FOAMING By J. A. KITCHENER D.Sc. and C . F. COOPER PH.D.* (DEPARTMENT OF CHEMISTRY TMPERIAL COLLEGE LONDON s.w.7). Introduction the problem of foams IT IS perhaps surprising to find in 1958 that no thoroughly satisfactory explanation has yet been given as to why certain liquids foam strongly others feebly and many not at all. This situation does not arise from any lack of scientific interest in foams nor are they without practical import- ance. On the contrary the uses or disadvantages of foaming in technical chemistry are too well known to need comment in this Review. It will suffice simply to mention such useful applications as froth-flotation fire-fighting foams and foamed rubber on the one hand and the in- convenience of foams on boilers rivers fermentation tanks and engine oils on the other.The literature contains innumerable papers about foams and several monographs1” and reviewslb on the subject have appeared in recent years. A broad survey of this literature suggests several reasons for the con- fusion of ideas which has prevailed for so long. One is the implicit assump- tion which many workers have made that there exists one simple explana- tion for all types of foam under all conditions. But it is surely obvious that a dynamic foam is a complicated physicochemical system. As there is no well-defined physical property of “coefficient of foamability” different experimenters have studied a great variety of systems including foams in bulk either static or dynamic single soap films single bubbles pairs of bubbles bubbles under monolayers etc.and it is hardly to be expected that one simple factor will explain all the observations. Another source of confusion has been the multiplicity of isolated ad hoc experiments which have been carried out and the diversity of materials often impure which have been used. (Only quite recently has the great importance of impurities been realised.) Too often the experimenters have ignored the other evidence and unjustifiably generalised their con- clusions with the result that the literature appears to contain several mutually exclusive theories of foaming each of which explains one group of observations but not the others. A more logical attitude to the data is to suppose that the quantitative properties of foams depend on a number of distinct factors the relative importance of which varies according to the system chosen for study.(a) J. J. Bikerman Foarffs Theory and Industrial Applications ” Reinhold New York 1953; E. Ma::gold Schaum ” Strassenbau Heidelberg 1953; (b) A. J. de Vries “ Foam Stability Rubber-Stichting Delft 1957; T. G. Jones K. Durham W. P. Evans and M. Camp “ Proceedings of the Second International Congress on Surface Activity ” Butterworths London 1957 Vol. I p. 225. See also ref. 32. * Carnegie Institute of phnology Pittsburgh U.S.A. 71 72 QUARTERLY REVIEWS It is significant that a very wide range of foam “persistence” is observed with different materials-in fact from fractions of a second to years- and there appear to be no sharp transitions from a feebly-foaming to a non-foaming or to a strongly-foaming solution but only a continuous transition of properties.However despite chemical differences there must be certain basic mechanical characteristics common to all strongly- foaming liquids and differentiating them from non-foaming liquids. In this Review an attempt will be made to clarify the problem by first con- sidering this mechanical aspect and then examining the several physico- chemical mechanisms that have been proposed to account for it. It will be seen that only one important new theory of foam stability has emerged in recent years namely the electrical double-layer theory of the eminent Russian colloid scientist B. V. Derjaguin.t Most of the other concepts still current (and largely valid) date back to the last century but development of a precise theory of foaming has been excluded by lack of quantitative data to support the qualitative ideas.Consequently it will be pertinent to include a brief account of certain recent experimental developments in fundamental surface chemistry. The outstanding problem in the theory of foams now seems to be to assess the relative importance of these various physicochemical factors in the different systems. The occurrence and classification of foams First it is well established that pure liquids do not foam. Secondly the foaming of solutions shows an obvious correlation with surface activity of the solutes. For example foaming is not pronounced in mixtures of liquids of similar chemical type and surface tension (e.g. benzene + carbon tetrachloride) or in aqueous solutions of highly hydrophilic solutes (e.g. pure glycerol or sucrose) transient foams are obtained with solutes which lower the surface tension moderately (e.g.the short-chain aliphatic alcohols and acids); and really persistent foams arise only with solutes which lower the surface tension strongly in dilute solution (e.g. the soaps synthetic detergents? proteins etc.) i.e. the highly surface-active sub- stances. The onset of foaming on addition of a solute is hard to define experi- mentally and in fact little reliable evidence can be found in the literature about such borderline systems. The older literature contains many examples of allegedly feebly-foaming solutions but these reports are highly suspect because the experiments were not carried out with sufficiently pure materials. The quantity of a suitable impurity capable of producing a transient foam is often infinitesimal particularly with water as the solvent e.g.0-0002~-sodium cetyl sulphate or 0.0005 % of saponin. In addition surface films of greasy materials were certainly not avoided by earlier t We adopt this English spelling as used by Professor Derjaguin himself; trans- literated according to the Royal Society’s system he would be B. V. Deryagin. (See also Quart. RPv 1956 In- 395 1 KITCHENER AND COOPER THE THEORY OF FOAMING 73 workers and it is now known that monolayers containing as little as 10-lo mole per cm.2 are able to retard the escape of bubbles from water and retain them in the surface for several seconds. Reports of transient foams with solutions of purely inorganic salts such as sodium chloride are almost certainly erroneous ; they have never been thoroughly checked$ with scrupulously purified materials and are contrary to modern theo- retical expectations.Nevertheless genuine foams are formed with such a great variety of chemical substances that the chemistry of the materials does not provide a useful basis for a classification. The physical chemistry of the surface layers of the solutions is what determines the widely different orders of “stability” observed with different solutions and a rational scheme of classifying foams can be developed on the basis of the order of “stability”. The loose term “stability” does not of course imply complete thermodynamic stability since foam is a disperse system and contains a higher surface area and hence higher surface free energy than would the segregated gas and liquid; consequently all foams tend to collapse spontaneously.Foams must therefore be either unstable structures (ix. transient monotonously changing in the direction of decreasing surface area) or at best metastable (i.e. remaining in a state of suspended transformation). Actually both types exist and the distinction between them provides a fundamental criterion of classification in the light of which many existing observations begin to fall into place. Unstable foams are well exemplified by dilute solutions of short-chain aliphatic alcohols or acids (e.g. pentyl alcohol butyric acid rn-cresol aniline etc.). These foams are constantly breaking down as the liquid drains from between the bubbles. Their life-time depends on the con- centration of the solution but at best it is only around 20 seconds for aqueous solutions of these non-ionic and weakly surface-active solutes.The life-time of the foams can be considerably extended by adding a second non-foaming solute such as glycerol to increase the viscosity of the solution and so slow down the drainage of liquid from between the bubbles. In unstable foams there clearly exists a surface force opposing the coales- cence of the bubbles; it is too weak to stop the process entirely but it does retard it considerably. Metastable foams are characterised by the fact that the process of drainage of liquid from between the bubbles eventually stops and the foam would persist indefinitely if it could be absolutely protected from dis- turbing influences. “Disturbing influences” include vibration draughts evaporation radiant heat temperature differences dust and other 5 The Reviewers have however carried out enough tests to be sure that the results of Talmud and Suchowolskaja (1931) on the life-time of bubbles on solutions of inorganic salts quoted by Bikerman (ref.1 p. 51) are worthless. For example these authors claimed that bubbles on N-potassium sulphate persisted for 40 sec. ; but the truth is that if the salt is first heated to redness in a platinum crucible to destroy dust etc. bubbles do not persist on the solution for more than 1 sec. 74 QUARTERLY REVIEWS impurities. Dewar’s classic experiments (19 16) in which he succeeded in preserving horizontal soap films in closed tubes for up to three years are well known. After a certain drainage time such films remain unchanged in thickness. Their eventual breakdown must be ascribed to some un- controlled disturbance.Such films must undoubtedly have a strong stabilising factor; in other words in these systems there exists a potential energy barrier dependent on the structure of the surfkce layers which is powerful enough to counterbalance the forces making for collapse and because of this barrier the system rests at metastable equilibrium. The class of metastable foams includes besides the classic soaps the numerous modern synthetic detergents of the anionic and cationic types and presumably the many proteins and other naturally-occurring macro- molecular substances such as the glucoside saponin all of which give very persistent foams. The conclusion that these substances (and probably the non-ionic detergents also) give true metastable foams is presumptive and based only on comparison of foam persistence with that of the soaps but no one seems to have tried to preserve single films of these solutions for very long.A “black” gelatin films can certainly be kept for days. Ordinary three-dimensional foams of these substances generally persist only for a matter of hours in a closed vessel if no special precautions are taken to eliminate all the sources of instability mentioned above. One reason why they do not last for say months is that gas slowly diffuses from the small bubbles into the large ones since the pressure and hence the thermodynamic activity of the gas within the bubbles is inversely pro- portional to the bubble radius. Diffusion of gas eventually leads to a mechanically unstable packing which suddenly rearranges.The shock is sufficient to rupture the extremely thin lamellae in a well-drained meta- stable foam. It is of course a characteristic property of any system in metastable equilibrium that a certain minimum energy is needed to dis- place it. Drained foams are stable against random molecular fluctuations but direct experiment shows that the “resilience” of soap films to mech- anical shocks decreases as they become thinner. A clear difference can be seen in the structure of extreme examples of the two types of foam. In unstable foams the bubbles jostle together as an assembly of spheres only slightly distorted by their neighbours whereas metastable foams persist long enough for drainage to proceed extensively so that the films of liquid between the bubbles become planar or slightly curved lamellae of practically uniform thickness (Plate 1).Manegoldl appropriately calls the two structures “kugelschaum” and “polyeder- schaum” i.e. round-bubble foam and polyhedral foam. However this observation does not provide a practical criterion for distinguishing fundamentally unstable from fundamentally metastable types since there is a continuous transition from one structure to the other. Some funda- 9 “ Black ” films are those which are too thin to show nterference colours i.e. less than about 2 x cm. 74 QUARTERLY REVIEWS impurities. Dewar’s classic experiments (19 16) in which he succeeded in preserving horizontal soap films in closed tubes for up to three years are well known. After a certain drainage time such films remain unchanged in thickness.Their eventual breakdown must be ascribed to some un- controlled disturbance. Such films must undoubtedly have a strong stabilising factor; in other words in these systems there exists a potential energy barrier dependent on the structure of the surfkce layers which is powerful enough to counterbalance the forces making for collapse and because of this barrier the system rests at metastable equilibrium. The class of metastable foams includes besides the classic soaps the numerous modern synthetic detergents of the anionic and cationic types and presumably the many proteins and other naturally-occurring macro- molecular substances such as the glucoside saponin all of which give very persistent foams. The conclusion that these substances (and probably the non-ionic detergents also) give true metastable foams is presumptive and based only on comparison of foam persistence with that of the soaps but no one seems to have tried to preserve single films of these solutions for very long.A “black” gelatin films can certainly be kept for days. Ordinary three-dimensional foams of these substances generally persist only for a matter of hours in a closed vessel if no special precautions are taken to eliminate all the sources of instability mentioned above. One reason why they do not last for say months is that gas slowly diffuses from the small bubbles into the large ones since the pressure and hence the thermodynamic activity of the gas within the bubbles is inversely pro- portional to the bubble radius. Diffusion of gas eventually leads to a mechanically unstable packing which suddenly rearranges.The shock is sufficient to rupture the extremely thin lamellae in a well-drained meta- stable foam. It is of course a characteristic property of any system in metastable equilibrium that a certain minimum energy is needed to dis- place it. Drained foams are stable against random molecular fluctuations but direct experiment shows that the “resilience” of soap films to mech- anical shocks decreases as they become thinner. A clear difference can be seen in the structure of extreme examples of the two types of foam. In unstable foams the bubbles jostle together as an assembly of spheres only slightly distorted by their neighbours whereas metastable foams persist long enough for drainage to proceed extensively so that the films of liquid between the bubbles become planar or slightly curved lamellae of practically uniform thickness (Plate 1).Manegoldl appropriately calls the two structures “kugelschaum” and “polyeder- schaum” i.e. round-bubble foam and polyhedral foam. However this observation does not provide a practical criterion for distinguishing fundamentally unstable from fundamentally metastable types since there is a continuous transition from one structure to the other. Some funda- 9 “ Black ” films are those which are too thin to show nterference colours i.e. less than about 2 x cm. 74 QUARTERLY REVIEWS impurities. Dewar’s classic experiments (19 16) in which he succeeded in preserving horizontal soap films in closed tubes for up to three years are well known. After a certain drainage time such films remain unchanged in thickness.Their eventual breakdown must be ascribed to some un- controlled disturbance. Such films must undoubtedly have a strong stabilising factor; in other words in these systems there exists a potential energy barrier dependent on the structure of the surfkce layers which is powerful enough to counterbalance the forces making for collapse and because of this barrier the system rests at metastable equilibrium. The class of metastable foams includes besides the classic soaps the numerous modern synthetic detergents of the anionic and cationic types and presumably the many proteins and other naturally-occurring macro- molecular substances such as the glucoside saponin all of which give very persistent foams. The conclusion that these substances (and probably the non-ionic detergents also) give true metastable foams is presumptive and based only on comparison of foam persistence with that of the soaps but no one seems to have tried to preserve single films of these solutions for very long.A “black” gelatin films can certainly be kept for days. Ordinary three-dimensional foams of these substances generally persist only for a matter of hours in a closed vessel if no special precautions are taken to eliminate all the sources of instability mentioned above. One reason why they do not last for say months is that gas slowly diffuses from the small bubbles into the large ones since the pressure and hence the thermodynamic activity of the gas within the bubbles is inversely pro- portional to the bubble radius. Diffusion of gas eventually leads to a mechanically unstable packing which suddenly rearranges.The shock is sufficient to rupture the extremely thin lamellae in a well-drained meta- stable foam. It is of course a characteristic property of any system in metastable equilibrium that a certain minimum energy is needed to dis- place it. Drained foams are stable against random molecular fluctuations but direct experiment shows that the “resilience” of soap films to mech- anical shocks decreases as they become thinner. A clear difference can be seen in the structure of extreme examples of the two types of foam. In unstable foams the bubbles jostle together as an assembly of spheres only slightly distorted by their neighbours whereas metastable foams persist long enough for drainage to proceed extensively so that the films of liquid between the bubbles become planar or slightly curved lamellae of practically uniform thickness (Plate 1).Manegoldl appropriately calls the two structures “kugelschaum” and “polyeder- schaum” i.e. round-bubble foam and polyhedral foam. However this observation does not provide a practical criterion for distinguishing fundamentally unstable from fundamentally metastable types since there is a continuous transition from one structure to the other. Some funda- 9 “ Black ” films are those which are too thin to show nterference colours i.e. less than about 2 x cm. KITCHENER AND COOPER THE THEORY OF FOAMING 75 mentally unstable foams persist long enough for considerable thinning to take place so that polyhedral faces (though still relatively thick) are formed between bubbles but nevertheless the lamellae shortly break down.Conversely a soap foam in its early stages is still practically of the round-bubble type and it remains so for a long time at the bottom because of drainage of liquid from the column above. The question of distinguish- ing between a spherical-bubble transient foam and a mere mechanical dispersion of bubbles in a liquid will be considered later. In addition to the well-known foaming substances already mentioned foaming occurs with certain mixed organic liquids (for example lubricat- ing oils particularly those with “additives”) and with certain molten glasses and metallurgical slags. As few of these non-aqueous systems have yet been thoroughly studied it can only be assumed that they do not differ fundamentally from aqueous ones. Many solid “foams” (e.g.rubber latex foam) are simply spherical bubbles of gas trapped in a flexible solid but some of the modern highly expanded “cellular polymers” (e.g. expanded polystyrene) contain as little as 1 ”/o of solid matter and are composed of polyhedral cells with very thin walls like a true drained foam2 (Plate 1E). The key questions to be answered therefore reduce to two (a) What is the physical factor common to the great variety of chemically different foaming solutions which distinguishes them from non-foaming solutions ? (b) What mechanism (or mechanisms) can account for the metastability of certain liquid lamellae such as soap films? When these fundamental questions have been answered it should be possible to explain at least descriptively the reactions of different foams to physical and chemical treatments.The mechanism of thinning and rupture of liquid lamellae A fruitful approach to the first of the two fundamental problems is to consider the thinning of a liquid film with and without the addition of a foaming agent the simplest example being the rise of a single bubble to the surface of a liquid. The sequence of events as revealed by high-speed cinematography3 is shown in Fig. 1. On a clean water surface a bubble bursts within a short time (roughly sec. from the time its upperside touches the water surface) whereas as little as an incomplete monolayer of insoluble surface film is sufficient to prolong its life to seconds and on a dilute detergent solution single bubbles last for many minutes or even hours. The surface tension which is a consequence of the intermolecular cohesion is fully capable of counteracting the buoyancy of the bubble as is proved by the fact that static equilibrium is achieved by a bubble A.Cooper Trans Plastics Inst. 1958 26 299. D. M. Newitt N. Dombrowski and F. Knelman Trans. Inst. Chem. Engrs. 1954 32,244. 76 QUARTERLY REVIEWS floating on the surface of a detergent solution (the surface tension of which is lower than that of pure water). The point at issue is therefore why the shell tears in some liquids and not in others. FIG. 1 Stages in the bursting of a single bubble on the surface of water. (Drawings by Arthur Smith from the high-speed photographs of Newitt Dombrowski and Knelman3.) Nothing is known for certain about the actual mechanism of rupture of thin liquid films except that it happens within microseconds.The hypothesis has sometimes been put forward that rupture under the surface tension occurs when a certain critical thickness is reached. Now pure degassed water in bulk can withstand a static negative pressure of at least 270 atm. without ~avitating.~ The negative pressure tending to tear a static water lamina of thickness d (cm.) and surface tension y (dyne cm.-l) is 2yld and hence the critical thickness for rupture under its own surface tension could be estimated as of the order cm. Lamellae of this order Q€ thickness are indeed observed with carefully stretched “black” films from dilute soap solutions. On the other hand nothing approaching this degree of attenuation can be observed with pure liquids such as water for if attempts are made to form very thin lamellae of water the sheets disintegrate within micro- seconds at a stage when their average thickness is still in the range lo4 cm.Experiments of this kind were carried out by Dombrowski and Fraser5 in which expanding sheets of liquid issuing from a slit orifice were photographed by a flash method. However in these experiments the cause of rupture of the laminae was not simply the gradual thinning of the sheet as it expanded radially from the orifice but the growth of transverse waves which subjected the sheet to rapid local thinning6. It is perhaps significant that cavitation occurs in water containing dissolved air at a negative pressure of only 1 atm. in an acoustical field.4 Thus a liquid under tension breaks by nucleation of gas bubbles if the stress is applied L. J. Briggs J.Appl. Phys. 1950 21 721. N. Dombrowski and R. P. Fraser Phil. Trans. 1954,247 A 101. N. Dombrowski personal communication. KITCHENER AND COOPER THE THEORY OF FOAMING 77 so rapidly that the supersaturated gas cannot diffuse out. It seems reason- able to conclude that an expanding bubble of water probably tears by nucleation of dissolved gas or vapour at a thickness of the order of lo4- cm. and that waves of microscopic amplitude probably play a part in causing rapid local thinning to this extent. The slow thinning of horizontal liquid films between two gas bubbles was investigated by Derjaguin and his co-worker~.~ These experiments are so important at the present stage of development of the theory of foams that they deserve closer examination. The apparatus used is shown in Fig.2. The two small bubbles are formed on the cups a and b and as they are in direct communication via the tube c the gas pressure within the bubbles is equal and the small lamina separating them is planar over an area of about 1 mm.2. There is therefore no difference of hydrostatic pressure across the gas-liquid boundary (as there is at a curved surface) and a little consideration shows that the easily measured pressure difference Ah (see Fig. 2) is equal to the pressure forcing the liquid from between bubbles (which Derjaguin calls the “disjoining pressure”). I I ii FIG. 2 Principle :[ the experiments ofl,Derjaguin and Titijev~kaya~ (diagrammatic) for determining the disjoining pressure between two bubbles. The more elaborate part of the apparatus not shown is an optical arrangement for determining .the thickness d of the horizontal lamella of liquid.This is obtained from a precise measurement of the intensity of monochromatic light reflected from it the apparatus consisting of a microscope with microphotometer attachment. By adjusting the head of liquid in the connecting tubes one can gradually force the bubbles together and simultaneously measure the thickness of the lamella and the hydrostatic pressure acting on it. The qualitative results obtained with this apparatus were described by Derjaguin as follows “As might be expected the observation of films of B. V. Derjaguin and A. S. Titijevskaya “ Proceedings of the Second International Congress on Surface Activity ” Butterworths London 1957 Vol. I p. 210. 78 QUARTERLY REVIEWS pure water is impossible.Even with the most careful approach of the lower bubble to the upper (with the aid of a micromanipulator) the break occurred imperceptibly rapidly at the first instant of contact between the bubbles without any detectable flattening. However a high degree of purity is needed.to obtain this result. The bubble holders must be of silica as with glass holders . . . after a waiting period of about 1 minute the life- time of the film is of the order of 1-2 seconds. “In presence of surface monolayers of organic compounds the picture changes. The effect of alcohols of the aliphatic series dissolved in water was first studied. At the first instant of compression of the bubbles a film of apparently uniform thickness is formed but this gradually and steadily becomes thinner generally remains uniform and bursts after becoming almost black.The thinning process is well reproducible as regards duration and evidently occurs according to a definite law as a function of time. The initial thickness of the film is about 0-lp. The life-time of the film falls with increase of pressure P. “We then studied the effect of fatty acids. The addition of 0-2 mole/litre of acetic acid to water gave films which became thinner and broke after 16 seconds. . . . A solution of 040025~-octanoic acid gave a film of constant thickness (0*’11p at P = 560 dyne/cm2) . . . Thus soluble fatty acids differ sharply from soluble alcohols in their capacity to form films of a thickness which is quite stable with time; apparently this property is due to the formation of a soap.” (The quantitative results obtained by Derjaguin for the film thickness in different solutions will be considered later.) These experiments point clearly to the property which distinguishes foaming from non-foaming liquids.It is simply the ability to resist excessive Zocalised thinning of a membrane of liquid while a controlled general thinning proceeds. Under these conditions a thin lamella of fluid can be formed though when formed the lamella may subsequently prove to be either transient or persistent. To show such special rheological properties the fluid must possess a special form of elasticity such that any applied stresses which tend towards local thinning are rapidly opposed and counterbalanced by restoring forces generated during the initial displacements of the material. The restoring force of what may be called “film-elasticity” must increase with displacement like that in a stretched rubber membrane though it may not necessarily follow Hooke’s law; in other words the thinner the film becomes the greater becomes its tension.Pure liquids are completely lacking in the property of elasticity because their surface tension is independent of extension. By analogy with other types of elasticity Gibbs defined surface elasticity E as the stress divided by the strain per unit area. The tension in a lamella is 27 dynes/cm. and therefore if an extension of the area (A) by an amount dA causes a rise of surface tension of dy E = 2A (dy/dA) = 2 (dy/d In A). It is easy to demonstrate experimentally the increase of tension with KITCHENER AND COOPER THE THEORY OF FOAMING 79 extension in a soap film by means of the DuprC frame apparatus; if the film is rapidly extended at one end coloured bands further along the film will move outwards in the same direction showing that a gradient of tension has been established.Similarly Gibbs pointed out that the tension near the top of a vertical soap film must be greater than that at the bottom because of the weight of the film. But it is not easy to measure the changes of tension with extension and thus determine the modulus of surface elasticity. Numerous tests of the effect of thickness on the surface tension of soap films have been made and are reviewed by Bikerman,la but these are static experiments in which the film is allowed to thin by draining. No detectable change of surface tension (< 1 %) then occurs even down to about 100 A.Derjaguin’s experiments are a much more sensitive method of detecting any static effect of thickness on properties. The results (Fig. 3) show that a 800 700 o& 600 - - c L . 500 \ al F .!? 400 0- ~ r e 5 s u r e ( ~ ) ( d y n es cm:” ) 0 ZOO0 4000 FIG. 3 Typical results of the experimetits of Derjuguiti and Titijevskirya showing the thickness of Iumella (11) as a function of ‘‘ disjoining pressure ” (P) for various solutions. 1 4 . 10-3~-Sodium oleate containing sodium chloride at 1 lCF4~; 2 1 0 - 3 ~ ; 3 10-2~; 4 10-l~; 5,0-3 yi “ Aerosol OT ” + lO-%-sodium chloride. 6,0.1% Saponin + 1 O-lN-sodiurn chloride. detectable force opposing thinning can be measured with certain very thin films but the effect on surface tension would be negligibly small. Further- more since such forces were absent from foamable solutions of alcohols they cannot account for the dynamic elasticity of thick films of such solu- tions.It is not established at what lamina thickness the restoring forces of film elasticity first come into action. Soap films when first formed are too 80 QUARTERLY REVIEWS thick to show interference colours (say 5 x cm.) so presumably elasticity develops already by this stage. From a thermodynamic point of view the important quantity for protection of a lamina against shocks is the work of displacement measured by the restoring force multiplied by the distance of displacement since it is this work which must be capable of cancelling the kinetic energy of possible disturbances before a danger- ously thin spot (100-500 A) is produced.A good shock-absorbing film must therefore be capable of providing a large work of displacement before rupture. Of major importance for securing resilience is the capacity of a film to suffer large displacements without rupturing. This is the distinction between an “elastic” material and a brittle one not the value of the modulus of elasticity. Hence for example a thick soap film is more robust than a thin one of the same surface tension because it con- tains more reserve material permitting considerable extension before the critical thinness is reached. FIG. 4 Local thinning of a soap film under the impact of a gas jet (see text). The mechanism of film-elasticity can be illustrated by an imaginary experiment in which a narrow jet of air suddenly impinges on a soap film; Fig.4 shows schematically the state of affairs a moment after the jet is switched on and while the bubble is still expanding. It is thinnest at the apex a and thickest at c. If the theory is correct that the local instanta- neous surface tension y of a foamable liquid increases with extension then ya> Y b >yc. Such a gradient of surface tension along the line abc means that each section of liquid will be subject to a net force towards Q and the liquid will therefore stream outwards and feed the expanding bubble from its edges. The same mechanism on a microscopic scale would explain the resistance of thin soap films to rupture under the action of disturbances ; a thinning spot automatically draws liquid from its perimeter. It follows therefore that any liquid that shows a positive value of the coefficient dy/dA under dynamic conditions should be capable .of foaming to some extent.The great variations in foaming power of different solu- tions are then probably related to differences in the effective value of d y/dA under practical conditions although in addition the relative rates KITCHENER AND COOPER THE THEORY OF FOAMING 81 of the local displacement the flow of the liquid and the relaxation of surface tension changes are certainly important (see later). There are at least four distinct theories proposed by Gibbs (1878) Marangoni (1 865) Plateau (1 873) and Derjaguin (1953) to account for film elasticity. They are complementary rather than mutually exclusive mechanisms and are probably all operative though generally in different circumstances.These theories will be considered in turn in the subsequent sections. Before leaving the general aspects of film elasticity however it is of interest to note the formal analogy between this property and the rheological property of “spinnability” the capacity shown by certain melts and solutions of long-chain substances to be drawn out into fibres. In that case the “flow elasticity” (or “elastico-viscosity”) of the liquid is due to orientation and extension of the molecular chains along the direction of flow. In film formation the liquid is flowing in two dimensions. It is therefore conceivable that a spinnable fluid would also show film elasticity and thus form a polyhedral foam. Expanded polystyrene seems to provide an example of this class in which elastico-viscosity gives the necessary protection against local over-thinning of the expanding lamellae.If this suggestion is correct such systems form an entirely distinct class of foams and have only the film elasticity in common with the usual foam- ing liquids. Gibbs’s surface-elasticity theory Gibbs showed that if a thin film of a solution of a surface-active solute (e.g. a soap) is subjected to local stretching the surface tension of that part increases because the solute is positively absorbed in the surface and any increase of surface area therefore leads to a decrease of average solute concentration within the film and hence a rise of equilibrium surface tension. This effect is clearly significant only with rather thin films where the amount of solute absorbed on the surface is comparable with that contained within the film.Further it is more important with strongly absorbed solutes than weakly absorbed ones and for solutions of low rather than high concentration. The operation of Gibbs elasticity in thin stretched films can scarcely be doubted but there seems to have been no direct test of the theory. Its magnitude can be calculated from thermodynamical formulae as shown from the following example. Fig. 5(a) shows the equilibrium surface tension of solutions of pure sodium dodecyl sulphate as a function of concentration.8 The quantity of the solute which is adsorbed per cm.2 of surface (T) can be calculated from these results by use of the well-known Gibbs adsorption equation which for a 1 :I electrolyte takes the farm r= (where c is the concentration of sodium dodecyl sulphate in moles per E.J. Clafield and J. B. Matthews ref. 7 p. 172. 82 QUARTERLY REVIEWS ~ 1 1 1 . ~ and f* is a mean ion. activity coefficient for the detergent at this concentration). Results so calculatedS show that the adsorbed monolayer behaves substantially as a two-dimensional vapour. Suppose now that a quantity of the solution is expanded without change of overall composition into the form of a sheet of thickness d cm. A quantity ( 2 r ’ l d ) moles of solute would be removed from each ~ m . ~ of the liquid to bring it to adsorption equilibrium with the remaining solution which would have the reduced bulk concentration of (c -2r’ld) mole~/cm.~. By simultaneous solution of the conditions of conservation of solute with the Gibbs equa- tion one obtains the required values for the surface tension of solutions 2 4 0 0-O/ 0.02 0.03 0.04 30 toncentrotion (mo/es per /itre) Thickness of stretched film (cm.) FIG.5 Surface acrivity and the Gibbs efect in solutions of sodium dodecyl sulphate. (a) Surfacc tension as a function of concentration.* The arrow represents the critical micelle concentration (0.008~). (b) Calculated surface tension of A 0 ~ 0 0 1 ~ ; B O.OlM-(static); and C 0-lmsolutions when extended as thin sheets of thickness d cm. [calc. from results in (a)]. (The surface elasticities are proportional to the slopes of these graphs.) The broken line is the hypothetical curve for the conditions of B with t = 0.01 sec. KITCHENER AND COOPER THE THEORY OF FOAMING 83 expanded to various thicknesses. Some approximate results so calculated for sodium dodecyl sulphate at three initial concentrations are shown in Fig.5(b). Consider for example the 0-OlM-solution. If the film were stretched to lo3 cm. thickness the surface tension would rise from ca. 38 to ca. 45 dynes cm.-l. This solution would have a well-developed elasticity over the range 10-2-10-5 mm. thickness. The restoring forces developed are less than 0.01% of the strength of those in a sheet of rubber of the same thickness but they are nevertheless significant in relation to the mass of liquid-for example lo3-lo4 dynes per g. of film which would impart accelerations of lo3-lo4 cm. sec. -2 to the portions of liquid. Although these calculations are crude it is evident that the Gibbs effect provides a plausible source of surface elasticity. Fig. 5(b) shows also that the range of film thickness over which the Gibbs effect operates depends on the concentration of the solution.With 0.001~- sodium dodecyl sulphate the total reduction of y is too small to provide a strong elasticity at any thickness. With 0.1 M-sodium dodecyl sulphate negligible elasticity develops above lo4 cm. partly because of the small dependence of y on c above the critical micelle concentration and partly because of the large reserves of solute present in the solution. Around 10-5 cm. elasticity is strong and it persists down to lo4 em. so that this solution should provide very thin lamellae. Nothing is to be gained by attempting to press the Gibbs elasticity theory further because it is based on a static model. Marangoni’s surface-elasticity theory Gibbs was well aware that the surface elasticity of solutions under dynamic conditions would be greater than that calculated by the above static theory since a rapidly expanding surface would have a tension greater and a compressed surface less than the equilibrium value.This had been pointed out a decade earlier by Marangoni. But at that time no quantitative data were available about the difference between dynamic and equilibrium tensions. Since then a good deal of evidence has been forthcoming. The oldest observation is apparently that of Duprk (1869) who deduced from the height of rise of fine vertical jets of solution that the apparent tension of a soap solution was similar to that of water in these experiments. Then came the introduction by Rayleigh (1879) of the oscillating-jet method of determining dynamic surface tension.Rayleigh showed in 1890 that a freshly formed surface of a soap solution has a surface tension close to that of pure water for periods less than 0.01 second provided that the concentration is low (< 0.04~). SutherlandO has given a useful review of work on dynamic tensions. K. L. Sutherland (a) Rev. Pure Appl. Chem. (Australia) 1951 1 3 5 ; (b) Austral. J. Sci. Res. 1952 A 5 683. See also R. Defay and J. Hommelen Ind. chim. belge 1958 23 597. 84 QUARTERLY REVIEWS With extremely dilute solutions of highly surface-active agents such as soaps the surface tension falls steadily during many seconds correspond- ing to the time required for the molecules to travel chiefly by diffusion to the surface from the interior of the solution. These changes can be followed easily by the Wilhelmy plate method or the drop-shape method.In addition many authors have recorded much slower ageing effects extending to minutes hours or even days; but these are now known to be due to traces of impurities (for example multivalent metallic ions) from the glass or water or trace amounts of agents which form condensed mixed films with the principal detergent. Slow ageing with detergents largely disappears on purification.1° Similarly the slow ageing of surfaces of protein solutions saponin etc. is partly due to the slowness of diffusion from dilute solutions and partly to irreversible “denaturation” processes which some of these substances suffer when adsorbed at an interface. In any case such very slow surface changes have little relevance to the problem of foams as surface elasticity must come into operation in very short periods of time if it is to protect a thinning lamella against breakage.The precise time-scale concerned in the development of foam lamellae seems never to have received attention; but judging by the times concerned in various experiments involving expanding films and jets of water one can guess that the order of magnitude of time during which surface elasticity must come into effect is around to second. Unfortunately it is not possible at present to carry out calculationsof the magnitude of the Marangoni effect in say an expanding lamella of solution because of the complicated hydrodynamics. Even the interpretation of experimental studies of dynamic surface tension is obscured by convection effects in the liquid as it seems to be impossible to devise an experiment in which a fresh surface is formed without some disturbance.S~therland,~~ for example has shown that the oscillating-jet method gives different results for the ageing of solutions according to the size of jet used. The rate at which solute arrives at a surface depends on (a) the rate of diffusion to the surface (b) convective transport (c) possible electrical repulsion in the case of a surface-active electrolyte and (d) possible steric hindrance to the entrance of large molecules into an already fairly crowded monolayer. Factors (c) and (d) are probably unimportant for the foaming of fairly dilute solutions being relevant chiefly to the slow processes. Most of the work with the oscillating-jet technique has been concerned with the soluble aliphatic alcohols and for these the “half-time” for the adsorption is of the order of 0.01 sec.(e.g. for 0*005~-heptyl alcohol). Sutherlandga and others have shown that adsorption occurs somewhat more rapidly than can be accounted for by diffusion alone; the discrepancy is.greater the narrower the jet. There seems no doubt that transport to the surface is partly convective and this is true even for the plate method.Qb Con- sequently a precise treatment of the Marangoni effect is impossible without lo See e.g. A. P. Brady J. Phys. Chem. 1949 53 56. KITCHENER AND COOPER THE THEORY OF FOAMING 85 more knowledge of the flow which occurs when bubbles are pressed to- get her. Data on dynamic tensions in solutions of ionized detergents in the time interval 10-3-10-2 sec. are sparse.Much discussion has centred round the slower processes commonly observed with slightly impure materials and hydrolyzable soaps and at one time the opinion was widely held that electrical or steric energy barriers must exist to account for these slow rates. But in fact with well purified detergents,lO.ll using the oscillating- jet method Burcik12 has obtained results for sodium laurate sodium dodecyl sulphate and sodium oleate which show rates of change of surface tension closely similar to those found for non-ionic agents i.e. the order of magnitude of 0.01 sec. for about half the final change of tension to occur with a solution about 0.001~. Fig. 6 shows a rough comparison of ionic and non-ionic materials. Burcik’s results with detergents (e.g. curve C in Fig. 6) seem to show a fast process followed by a much slower FIG.6 Typical examples of the surface tension of solutions under dynamic conditions surface tension as a function of surface age. A 0.005~-Heptyl alcohol with jet diam. 0.035 cm.:g B as A ditto with jet diam. 0.1 cm. (Addison) C 0.005~-sodium dodecyl sulphate:12 D 0.1 % “Tween” (poly- oxyethylene sorbitan mono1aurate.12 ye marks equilibrium tension; for curve D it is 36.7 dynes cm.-l). one. The rate is naturally faster the more concentrated the solution and vice versa.12 Thus with 1o4M-“Aerosol OT” (sodium dioctyl sulpho- succinate) Padday13 found ageing in the range 0.1-1 sec. Burcik showed that addition of sodium chloride increases the rate of adsorption of ionic detergents but not of a non-ionic detergent “Tween 20” (polyoxyethylene l1 G.D. Miles and L. Shedlovsky J. Phys. Chem. 1944 48 57. l2 E. J. Burcik J. Colloid Sci. 1950 5 421 ; 1953 8 520. l3 J. F. Padday ref. 7 p. 1. 86 QUARTERLY REVIEWS sorbitan monolaurate). These observations might be supposed to support the theory that diffusion in the former is opposed by an electrical potential and this might also account for the slower process mentioned. However there is also the common-ion effect of sodium chloride on the thermo- dynamic activity of these surface-active electrolytes. The added salt enhances the lowering of surface tension as well as its rate of change and a knowledge of the effect of sodium chloride on the diffusion coefficient of the detergent ion would also be needed before the effect on the rate of adsorption could be fully analysed.The above evidence being reviewed it is clear despite the incomplete experimental data that solutions of all types of surface-active solute show marked time effects in the range 10-3-10-2 sec. which provides strong support for Marangoni’s theory. The Marangoni effect operates on any expanding surface carrying an adsorbed layer irrespective of the thickness of underlying liquid and provides a restoring force tending to protect a film against local thinning. It operates even on a deep pool of solution opposing local displacements and therefore damping capillary waves. This effect of soluble detergents (as distinct from insoluble oil films) was recorded by l3r0wn.l~ It is easily demonstrated by adding a drop of detergent solution to a trough of water on which ripples are being produced by a vibrating plate a film “flashes” across the trough at the rate of about 30 cm./sec.and greatly reduces the amplitude and extent of the ripples until in a few seconds the surface film dissolves in the substrate. Such damping of capillary waves may well be an auxiliary factor in foam stabilisation by the Marangoni effect operating even on thick films and therefore promoting smooth thinning of the liquid between bubbles. The surface transport theory The rapid movement of liquid monolayers in a surface-tension gradient has been suggested by Ewers and Sutherland15 as the basis of a theory of foam stability and foam-breaking. Their paper cites many interesting points in favour of the idea. A spreading monolayer dragging with it sigdicant quantities of the underlying solution would rapidly repair thinning spots in a lamella.This phenomenon of “surface transport” is well established. It was first studied by Schulman and TeorelP who showed that a monolayer of oleic acid spreading at the rate of 5 cm./sec. carried with it a water layer effectively 0.03 mm. thick. The work was continued later by Crisp1’ and recently Blank and La M e P found by using a dye solution that liquid can even be raised against gravity by the surface transport of a film of e.g. cyclohexyl myristate from one trough to another l4 R. C. Brown Proc. Phys. SOC. 1936,48 312. l5 W. E. Ewers and K. L. Sutherland Austral. J. Sci. Res. 1952 A 5 697; cf. L. T. Shearer and W. W. Akers J. Phys. Chem. 1958 62 1264. J. H. Schulman and T. Teorell Trans. Faraday Soc. 1938 34 1337. D. J. Crisp Trans. Faraday SOC.1946 42 619. l8 M. Blank and V. K. La Mer ref. 7 p. 102. KITCHENER A.ND COOPER THE THEORY OF FOAMING 87 at a slightly higher level. The effect does not imply any special structure or viscosity of the film or adjoining water layer and it arises simply from the viscous drag of the expanding monolayer on the water and can readily be ca1culated.l The surface transport theory can be regarded as an extension of the Marangoni effect from which it differs only in specifying a mechanism for the movement of solution once a gradient of tension has been estab- lished. The classical explanation of the “healing” effect of a local increase of tension focused attention on the restoring forces opposing displacement or thinning i.e. the modulus of elasticity. The surface transport theory advances the idea a stage further by pointing out that a gradient of surface tension (such as that envisaged in Fig.4) will first lead to a surfaceflow and this in turn will move adjoining liquid layers. The speed of response will therefore be much greater than if the force were thought of as operating over the whole of the liquid lamella. It would be interesting to determine the velocity at which a sudden relaxation of tension is transmitted by a monolayer. Here again absence of quantitative data prevents further development and testing of a qualitatively attractive theory. Surface viscosity and bulk viscosity Plateau was responsible for the concept of surface viscosity which indeed he believed to be the chief factor in the development of foam lamellae. His own experimental measurements were inadequate but his main contentions have since been fully proved ; thus weakly-foaming alcohol solutions have little surface viscosity soaps a moderate amount and some solutions of proteins saponin etc.a high surface viscosity even amounting to surface rigidity. In recent years several different instruments have been developed for measuring surface viscosity. This property is the exact 2-dimensional analogue of ordinary viscosity and its coefficient is analogously defined and determined. The coefficient of surface viscosity qs is the force (in dynes) which must be applied per cm. of length along a surface to maintain a gradient of surface flow (between two parallel lines 1 cm. apart) of 1 cm. sec.-l. The unit is the surface poise having dimensions MT-l. The simplest instrument for detecting surface viscosity is the oscillating-disc viscometer the logarithmic decrement of its torsional oscillations being determined with and without the surface film.However as the damping due to the water alone is relatively large with a disc it is better to use a ring or the edge of a cylinder. For insoluble monolayers the canal method is well founded on theorylg and is accepted as a standard. For monolayers showing non-Newtonian flow a sensitive rotating-disc surface viscometer has recently been described by de Bernard20 (who also gives references to other instruments). l9 W. D. Harkins and J. G. Kirkwood J. Chem. Phys. 1938,6.53,298. 2o L. de Bernard ref. 7 p. 1. 88 QUARTERLY REVIEWS The limitations of these viscometers for use with soluble films were pointed out by Ewers and Sack21 who devised a very sensitive instrument based on the canal model but in which the underlying liquid was caused to flow and the rate of movement of the surface was determined by floating graphite particles.The theory was worked out so that the instrument gives absolute results for the surface monolayer. A similar idea was used in a new viscometer described recently by Davies,22 but here a circular canal was used and the liquid was moved by rotation on a turntable. This instrument was calibrated by means of insoluble monolayers of well- established surface viscosity. There is some confusion in the literature as to whether “surface viscosity” includes the drag due to the water below the monolayer. Ewers and Sack21 and Davies22 point out that in the canal methods this drag is allowed for theoretically so the true viscosity due to the monolayer alone is obtained whereas with the oscillating visco- meter23 a much higher apparent viscosity (y8’) is recorded because no correction is (or can be) made.Some typical results for surface viscosity are collected in Table 1 which also illustrates the two kinds of “surface viscosity”. It is seen that very dilute solutions of alcohols or soaps have a scarcely detectable surface viscosity even with solutions which foam appreciably. On addition of a small amount of lauryl alcohol to sodium dodecyl sulphate the foaming increases greatly23 and the surface viscosity increases ten-fold. Proteins give yet higher viscosities and on concentrated solutions the films behave as non-Newtonian pastes with a definite yield point and structural vis~osity.~* According to Trape~nikov~~ the surface layers of saponin solutions show visco-elasticity.The great problem to be settled is what contribution surface viscosity makes to foam stabilization. So,me authors have claimed for it a major importance while others have considered it of quite secondary value. It is generally conceded that (a) there is a strong correlation between foam stability and surface viscosity but (6) actual rigidity (as in solid “brittle” monolayers such as that of stearate monolayers on hard water) is detri- mental to foam life. This is probably because of the very short range of areas over which such films show elasticity. A viscous fluid but elastic layer is required. The‘most important function of enhanced viscosity is to retard drainage of liquid from between the bubbles.This factor has been studied particu- larly by Miles Ross and ShedlovskyZ6 who determined the rate of thinning of vertical soap lamellae by observing their interference colours and proved that this corresponded closely with the rate of drainage of liquid from 21 W. E. Ewers and R. A. Sack Austral. J. Chem. 1954,7 40. ” J. T. Davies ref. 7 p. 220. 23 A. G. Brown W. C . Thuman and J. W. McBain J. CuZZuidSci. 1953,8,491. 24 Eg. S. C. Ellis A. F. Lanham and K. G. A. Pankhurst J. Sci. Instv. 1955,32 70. 25 A. A. Trapeznikov ref. 7 p. 242. 26 G. D. Miles J. Ross and L. Shedlovsky J. Phys. Chem. 1945 49 93. KITCHENER AND COOPER THE THEORY OF FOAMING 89 columns of the foam a process which can be distinguished from actual breakdown of foam at least with the more stable systems.It is clear that partial or complete immobilisation of the outer surfaces of lamellae will have an increasingly strong effect in retarding flow the closer the sides come together. Consequently high surface viscosity favours the develop- ment of lamellae since the liquid can drain much more rapidly from thick regions than from thin. Miles et al. also discovered interesting phase TABLE 1. Typical values for true (qS) and apparent (rp') surface viscosities (g. sec.-l) of monolayers. Pure water. . . . . . . Stearic acid monolayer at 20 A2/ molecule . . . . . . Octadecyl alcohol monolayer at 20 A2/molecule . . . . n-Octanol (3.8 x 10-4~) . . , (7.7X 104M) .. Sodium dodecyl sulphate (3.5 x 10-3~) .. . . .. * . Sodium dodecyl sulphate (3-5 x 10-3~)+lauryl alcohol .. Sodium laurate (0.1 % pH 10) . . Serum albumin monolayer (3 dynes/cm.)* . . .. Serum albumin monolayer (6 dyneslcm.)? . . .. Serum albumin monolayer (10 dynes/cm.)? . . .. rls (absolute) 0 6 ~ 1 0 ~ 4~ 10-3 1 x 10-4 3 x 10-4 30 x 10-4 1 x 10-4 0 - - - 9 s (apparent) 0 - - - - 2~ 10-3 40 x 10-3 I x 10-3 2 x (1 x 10-1) (3 x 10-1) 22 22 21 21 21,23 21,23 21,23 24 24 24 * Newtonian fluid. t Plastic with structural viscosity decreasing with increasing rate of shear. changes occurring at definite temperatures with certain mixed films above the transition temperature the films were fast-draining and below it slow-draining corresponding closely to low and high viscosities and conversely high and low foam per~istence~~. These experiments seem clear enough but Burcik and Newman2* also showed that those mixtures such as sodium dodecyl sulphate + lauryl alcohol which give viscous mixed films also show a slower rate of adsorption (despite a lower final tension) 27 M.B. Epstein J. Ross and C. W. Jakob J. Culluid Sci. 1954 9 50. 28 E. J. Burcik and R. C. Newman J. Colloid Sci. 1954 9 498. 90 QUARTERLY REVIEWS than pure sodium dodecyl sulphats (probably because of formation of complexes in solution). Hence the Marangoni effect would be stronger with them. This is another example of the great difficulty of “separating the variables” in research on foaming. However there is no real doubt in this case since viscosity of any sort retards coalescence of bubbles. Even in the absence of added surface- active solutes a spherical-bubble “foam” can be built up by injecting a sufficiently dense stream of small enough bubbles into any viscous liquid.The rate of coalescence of the bubbles decreases with decreasing bubble size and with increasing viscosity. The reviewers have confirmed that the rate of collapse of a column of this kind of “foam” depends predominantly on the rate of drainage. This was proved by comparing the rates with “pure” liquids such as glycerol medicinal paraffin and silicone fluids. By using multiple jets a foam of bubbles of uniform and controlled size was built up and its rate of collapse was then determined at several temperatures. The rate of collapse was the same with different liquids if they were compared at temperatures such that their viscosities were equal. Their surface tensions were different showing that surface properties are largely irrelevant in these “foams”.the rate of collapse of such foams was found to be proportional to the viscosity; in paraffin foams the activation energy for collapse was the same as that for viscous flow. The addition of a detergent to glycerol of course enhances the persistence of the foam. Thus the presence of a foam-stabilising factor can be detected even in viscous liquids such as certain molten silicates by com- parison with a pure liquid of similar viscosity. It can be concluded therefore that true foaming requires film elasticity but the lifetime of a practical foam can be greatly extended by viscosity either of the liquid as a whole or of the surface layer only. The question whether a liquid with surface viscosity only would foam is hypothetical since surface viscosity does not occur without surface activity.Further in agreement with Brady and Electrical double-layer repulsion (Derjaguin) The preceding paragraphs have been chiefly concerned with the forma- tion of foam lamellae under dynamic conditions. Turning now to the forces responsible for metastability of lamellae under static conditions we think it worth quoting the words of Gibbs himself “That which is most difficult to account for in the formation of black spots is the arrest of the process by which the film grows thinner. It seems most natural to account for this if possible by passive resistance to motion due to a very viscous or gelatinous condition of the film.” The ‘‘viscous or gelatinous condition” is essentially Plateau’s theory and it undoubtedly operates in certain systems (as already mentioned).29 A. P. Brady and S. Ross J . A i m - . Chetn. Soc. 1944 66 1348. KITCHENER AND COOPER THE THEORY OF FOAMING 91 However the recent work has shown that many metastable foams do not possess great surface viscosity; the adsorbed layer on pure detergent solutions is gaseous or a mobile liquid. Derjaguin has put forward the theory that the very thin metastable “black” films owe their stability chiefly to electrical repulsions between the ionic double-layers which are formed by adsorption on two sides of the lamellae. The existence of such layers not only follows from the well-established orientation of surface- active electrolytes but is also indicated by high electrical conductivity of thin soap lamellae and the migration of liquid through them under the influence of an electric current both of which are doubtless due to electro- osmosis.Derjaguin and Ti tievskaya’s experiments on solutions of undecanoic acid in water dilute hydrochloric acid and potassium chloride showed that the equilibrium thickness of these thin films is very much dependent on the electrolyte. In 10-3~-hydrochloric acid the lamellae thin slowly but eventually collapse (as do those of solutions of short-chain alcohols). But on water and 10-3~-potassium chloride an equilibrium thickness is reached depending on the concentration of the carboxylic acid and inversely on the applied hydrostatic pressure. With increase of concentra- tion of potassium chloride the equilibrium thickness is reduced and collapse sets in at 1 0 - 2 ~ .Sodium oleate films could be studied in up to 0 . 1 ~ - sodium chloride with results shown in Fig. 3. Such results point clearly to electrical double-layer effects. (A similar pointer is given by older work on the effect of salts in reducing the stability of foams of soaps but these observations were not certain proof because of the effect of salts on the adsorption of the soaps.) The results were shown by Derjaguin and Titievskaya to be interpretable in terms of the Gouy- Chapman theory of flat double-layers. The theory of the interaction (repulsion) of double-la yers was worked out previously by Derjaguin and his co-workers in their important contributions to the theory of stability of colloid^.^ Essentially equivalent treatments have been developed independently by Verwey and Overbeek whose well-known monograph30 provides the only full account of this theory in English.The relationships between h the distance between the double layers and P the repulsive force per cm.2 is simple only if the double-layer potential is small (e.g. (25 mv) when it takes the form (Derjaguin 1937) where (T is the surface charge density of ions $o the surface potential D the dielectric constant of the medium in the double-layers and K the reciprocal thickness of the double-layer (as employed also in the Debye- Huckel theory). For high potentials more elaborate formulae are available. Colloids ” Elsevier New York 1948. 30 E. J. W. Verwey and J. T. G Overbeek “ Theory of the Stability of Lyophobic 92 QUARTERLY REVIEWS With the more dilute salt solutions the results with sodium oleate were well represented by assuming a nominal charge density in the double layers of 160 e.s.u.per cm.2 which corresponds to only a fraction of the actual soap molecules in the monolayer. Full agreement with the results for sodium oleate could be obtained only by introducing certain additional assumptions amounting to a rigid “hydration” layer 60 A thick because the film thickness at the higher salt concentrations did not decrease as much as the double-layer theory would predict. If correct these results are obviously of great importance. They have not yet been confirmed by any other experimenters for films between gas bubbles but van den TempePl has recently carried out very similar experiments with oil droplets in O-OlM-sodium dodecyl sulphate + sodium chloride and has come to very similar conclusions namely that the lamella thickness does not decrease with increasing salt concentration in the region of 0 .0 1 ~ as much as the double-layer theory predicts. He concludes that “the experimental results suggest the presence of still other repulsive forces which become operative at distances between the interfaces smaller than about 125 A”. It seems therefore that the old ideas of “steric hindrance” or “solvation sheaths” may yet have to be revived in colloid chemistry for very close approach to surfaces though more evidence is needed. The evidence for electrical double-layer repulsions in the range of h between 100 and 800 is strong. The influence of other factors on foam persistence (a) Concentration of Solute.-Many studies have been reported of the effect of the concentration ofthe active solute on foaming and it is gener- ally found that foaming passes through a maximum at intermediate con- centrations.This conclusion applies to soluble alcohols fatty acids soaps synthetic detergents and even insoluble monolayers. It is observed with very different methods of test such as those depending on the life-time of single bubbles the dynamic foam height or the persistence of a column of foam although these different tests do not always give the “optimum” at exactly the same concentration. Numerous examples are quoted in Bikerman’s m0nograph.l With sparingly soluble agents such as octyl alcohol and pine oil the foaming drops to zero when the solution becomes saturated. It is generally considered that these observations support the surface elasticity theory of foaming.Too dilute a solution gives a small range of surface tension for the effects to range over while too concentrated a solution carries too large a reserve of solute which can diffuse to the surface and restore a low tension during the thinning of a lamella. (This applies theoretically to both the Gibbs effect and the Marangoni effect and careful experiments with a favourable system might help to decide which of these is the more important.) 31 M. van den Tempel J. Colloid Sci. 1958 13 125. KITCHENER AND COOPER THE THEORY OF FOAMING 93 Der~ichian~~ emphasises another aspect of this phenomenon the compressibility of the adsorbed monolayer. By reference to experiments on the persistence of bubbles under insoluble monolayers he concludes that the adsorption layer should be neither a “gaseous” monolayer nor a rigid condensed film; for optimum stabilisation it should be a compressible two-dimensional fluid such as the “liquid expanded” type of monolayer.Such a film can respond appropriately with moderate changes of tension to both expansion and compression of area whereas a gaseous monolayer needs large changes of area to give comparable changes of tension and a “solid” monolayer crumples on compression and has only a short range of expansion over which a changing tension is operative. (6) Temperature.-Change of temperature has a relatively greater effect on viscosity than it has on surface tension Nevertheless local differ- ences of temperature cause gradients of surface tension and Plateau observed that a warmed spot in a soap film becomes thinner.(The fact that it does not continue to thin to breaking point under the greater tension of the surrounding cooler foam shows again that the tension increases with decrease of film thickness.) In a column of bubbles in a viscous pure liquid quite small differences of temperature cause rapid collapse. Particularly with solvents like glycerol oils and silicate slags rise of temperature has a marked effect in increasing the rate of drainage of foams owing to reduc- tion of the bulk viscosity. An interesting special effect has been studied by Epstein and his c o - ~ o r k e r s ~ ~ y ~ ~ who found that certain mixed adsorbed films such as sodium dodecyl sulphate+lauryl alcohol have a sharp transition temperature below which they are two-dimensional solids (and hence give slow-draining foams) and above which they are fluid (and hence give fast-draining foams).(c) Mixed Surface-active Agents.-Numerous examples have been recorded of the co-sorption of two or more agents often producing stronger foaming than either separately. Other examples besides sodium dodecyl sulphate+lauryl alcohol (or lauric acid) are alkyl arylsulphonate detergent +NN-bis-2-hydroxyethyl-lauramide and sodium stearate+ stearic acid (resulting from hydrolysis or the action of carbon dioxide). Some of these mixed films are “plastic” i.e. solid films.34 Such surface “complexes” seem to depend on the convenient packing of the non-ionic agents between the chains of the ionic ones thus causing condensation of a monolayer which would otherwise be “expanded” on account of electrostatic repulsion between the head groups.Crystalline intermolecular adducts containing two sulphate molecules to one alcohol molecule can be prepared in some of these systems. Certain pairs of surface-active agents are mutually antagonistic as 32 D. G. Dervichian Bull. Soc. chim. France 1956 15. 33 M. B. Epstein A. Wilson C. W. Jakob,.L. E. Couroy and J. Ross J. Phys. Chem. 1954,58 860; cf. G. D. Miles J. Ross and L. Shedlovsky J. Amer. Oil Chemists’ Soc. 1950 27 268. 34 E. J. Burcik and R. C. Newman J. Colloid Sci. 1957 12 10. 94 QUARTERLY REVIEWS regards foaming.35 For example stearates spoil the performance of oleates for soap-bubbles and small amounts of sodium palmitate or stearate in the presence of calcium salt render the foam of sodium p-dodecylbenzene- sulphonate unstable.36 Tnese all appear to be cases where the two agents are structurally unable to pack well together.Perhaps the adsorbed layers separate into two phases; heterogeneity of structure would account for the ready disintegration of lamellae since local tensions would be unbalanced under dynamic conditions. These effects are fairly specific sodium laurate or oleate renders the foam of sodium p-dodecylbenzenesulphonate more stable as does calcium palmitate with straight-chain detergents. (d) Miscellaneous Factors.-Amongst other factors not mentioned that may affect foaming are (i) evaporation (which can increase or decrease film life in different circumstances) (ii) solid particles (which can also act in either manner according to their size and wetting characteristics) and (iii) the layer structure in “black” soap films.It is well known that “black” films become thinner in discrete steps which are supposed to correspond to double layers of soap molecules oriented end-to-end but there seems to have been no progress on this problem in recent years. It is not clear whether the films showing these effects are concentrated enough to consist throughout of oriented liquid crystalline phases or whether multi- molecular surface micelles are formed by adsorption. Little reliable infor- mation is available about adsorption at the surface of detergent solutions above the critical micelle concentration. (According to Clayfield and Matthews,a phase transitions occur with sodium dodecyl sulphate.) The surface chemistry of protein solutions is even more nebulous at present and foaming of highly purified proteins seems not to have been investi- gated.S ~ h u t z ~ ~ and Others have demonstrated how bile acids and various other substances of biological origin can be fractionated by foaming. Antifoam agents The technical literature contains many examples of substances which are claimed to prevent foaming or destroy foams; but different agents are recommended for different foams and they seem to have been developed largely empirically. It is evident from the above discussion of the mechan- ism of foaming that the most important action of a completely effective antifoam agent must be to eliminate surface elasticity i.e. to produce a surface which has substantially constant tension when subjected to expansion. To do this it must displace any foam stabiliser that may be present.Hence it must have a low intrinsic tension in the pure state and be able to spread when applied to the foam l-amella. Further it must be present in sufficient quantity to maintain a high surface concentration even under dynamic conditions low solubility is therefore an advantage. Many antifoams are insoluble “spreading” oils. 35 F. Schiitz Trans. Faraday SOC. 1946 42 437. 36 H. Peper J. Colloid Sci. 1958 13 199. KITCHENER AND COOPER THE THEORY OF FOAMING 95 There are two types of antifoam agent namely foam-breakers and foam-inhibitors. As is well known many foams can be made to collapse rapidly by applying a few drops of ether octyl alcohol etc. Even the vapour is sufficient. Ether has an exceptionally low surface tension and therefore if local regions of a foam lamella receive ether these regions are rapidly pulled out by the higher tension of the surrounding parts of the film.Now ether is not itself a foaming agent being only weakly surface-active. Hence the ether-bearing region tears on being stretched by tension from the sides. Ether has no foam-breaking action if the foaming solution is previously saturated with ether. Evidently further reduction of tension cannot then occur on adding more ether but an original powerful foaming agent present can still be responsible for a positive dy/dA factor during stretching. Certain moderately surface-active substances which alone in dilute solution are actually foamers though feeble ones (e.g. pentyl alcohol) act as foam-breakers when applied neat to soap foams and also to foams formed from their own dilute solutions.But pentyl alcohol in excess will neither foam itself nor permit a dilute soap solution to do so; it is then a foam inhibitor. This is a case of swamping the surface with a high con- centration of rapidly-diffusing molecules so that any transient rise of tension on stretching is rapidly annulled. (Pentyl alcohol will also inhibit foams on concentrated detergent solutions but large quantities are needed probably because of solubilisation of the pentyl alcohol in the detergent micelles.) Generally more effective and more versatile than any soluble antifoam agents are materials such as the silicone fluids which are insoluble in water and have tensions as low as 20 dynes cm. -I. As foam-inhibitors they are applied as emulsions or with a diluent so that every bubble rising to the surface of the liquid catches enough silicone to spread over the air- liquid interface as a “duplex” film or a monolayer with excess of liquid as lenses.37 Quantities of the order of 1-60 p.p.m.are said to prevent foaming of adhesives dyebaths fermentation vats sewage tanks tars and even engine oils. (Pure hydrocarbons do not form true foams but compounded engine lubricants do.) Polyfluorinated hydrocarbons also seem likely to be useful antifoams for oils as they have even lower surface tensions around 10 dynes cm.-l. All the above examples of antifoam action seem to depend on eliminat- ing the surface elasticity but S. Ross and his co-workers3* have shown that while some commercial antifoam agents (e.g. 1,3-dirnethylbutyl alcohol) do indeed cause the rupture of thick lamellae before drainage can occur others (e.g.tributyl phosphate) act by promoting drainage in the early 37 L. T. Shearer and W. W. Akers J. Phys. Chem. 195S 62 1264. 38 (a) S. ROSS J. Phys. Colloid Chem. 1950 54 429; (b) S. Ross and G. J. Young Znd. Eng. Chem. 1951,43,2520; (c) S. Ross and M. J. Cutillas J. Phys. Chem. 1955,59 863; ( d ) S. Ross and J. N. Butler ibid. 1956,60 1255; (d) S. Ross and R. M. Haak ibid.. 1958. 62. 1260. 96 QUARTERLY REVIEWS stages of a foam’s history and so shorten its life-time. Viscous surface layers were found to assume practically the viscosity of water on the addition of traces of tributyl phosphate and formation of a plastic surface film on (impure ?) sodium lauryl sulphate solutions was inhibited. Similarly 0.001 % of an unspecified substance used to prevent foaming of egg white during dehydration completely inhibited formation of the normal gelatinous film on 0.01 % egg albumin and dimethyl- silicone used as a foam inhibitor for hydrocarbons has been reported to remove surface viscosity of Silicones may therefore have a dual action.In a recent paper Ross and Haak38e have shown by the oscillating jet technique that foam inhibitors may also accelerate the rate of change of dynamic surface tension with time thus possibly reducing the Marangoni effect in another way. Finally to complete the series one may note that the other possible contribution to stability the electrical double-layer repulsion may be reduced by increase of the concentration of indifferent electrolytes. Summary of conclusions On the basis of the evidence which has been reviewed here the following main conclusions have been drawn 1.Dispersion of gases in liquids can be divided into two classes (A) Spherical-bubble dispersions the life-time of which depends only on viscous drainage of liquid from between the bubbles; (B) True foams having longer life-times than comparable dispersions of class A . 2. True foams can be sub-divided into (a) transient (b) metastable and (c) solidified foams. Transient foams form polyhedral bubbles with relatively thick walls which subsequently become thin and collapse at some definite stage (typical example pine oil foam). Metastable foams form lamellae which become thin to the “black” state and then remain at a definite thickness until they are destroyed by external disturbances (typical example soap film).Solidified foams may start as either type (a) or (b) in the liquid state but the lamellae are preserved by solidification (e.g. of a plastic) or possibly by drying. The primary requirement for true foaming is a rheological property which may be called “film-elasticity”. If a lamella has this property a restoring force is produced under the action ofany stress that tends to pull out the film to greater areas; conversely a reduction of area induces a restoring force opposing thickening. This elastic property may have a relaxation time as short as a fraction of a second (in transient foams) or it may be much longer and may include also a static component as in metastable foams. Exceptionally the restoring force of film elasticity can arise from 3.4. 39 D. W. Criddle and A. L. Meader J. Appl. Phys. 1955 26 840. KITCHENER AND COOPER THE THEORY OF FOAMING 97 flow elasticity (“elastico-viscosity”) of the homogeneous fluid itself (as in the production of expanded polystyrene); but in the majority of foamable solutions it is due to surface elasticity caused by depletion of an adsorption layer. Both the Gibbs effect and the Marangoni effect should contribute to surface elasticity but their relative importance is not clearly established. A contributory effect of surface elasticity is probably the damping of ripples which would otherwise cause dangerous local thinning. 5. The mechanism by which liquid is drawn towards a thinning spot in a lamella is probably surface flow of the adsorption layer under the action of a gradient of surface pressure and the simultaneous dragging along of some of the underlying liquid.This is the “surface transport” theory of Ewers and Sutherland. 6 . The life-time of foams is increased by increase of fluid viscosity or by the presence of surface viscosity or surface plasticity; these factors prolong the drainage process and perhaps also help to reduce ripples. Metastable “black” soap films depend for their persistence largely on an electrical double-layer repulsion between the opposite sides of the lamellae. In addition there is some evidence to suggest that steric effects possibly due to hydration layers may also be involved with extremely thin films. Foaming may be either enhanced or reduced by addition of a second surface-active agent according to whether or not the two agents can pack together in a mixed adsorption layer. The most effective anti-foam agents are spreading insoluble liquids. Several other subsidiary factors affect the properties of foams in practice e.g. diffusion of gas from small to large bubbles the presence of any suspended particles evaporation temperature gradients or slow ageing processes. 7. 8. 9. It is evident that further progress in the understanding of foaming requires more precise quantitative studies of the flow process and of the magnitude of the restoring forces in thinning lamellae.

ISSN:0009-2681    

DOI:10.1039/QR9591300071

出版商:RSC    

年代:1959

数据来源: RSC

摘要:

CUMULATIIIVIE INDEXES VOLUMES I-XI11 (1947-1 959) CUMULATIVE INDEX OF AUTHORS Abrahams S. C. 10,407 Abrikosova I. I. 10,295 Addison C. C. 9 115 Ahrland S. 12 265 Albert A. 6 197 Allen G. 7 255 Amphlett C. B. 8 219 Anderson J. S. 1 331 Angyal S. J. 11 212 Amstein H. R. V. 4,172 Atherton F. R. 3 146 Avison A. W. D. 5 171 Bacon R. G. R. 9,287 Baddeley G. 8 355 Baddiley J. 12 152 Badger G. M. 5 147 Bagnall K. W. 11 30 Baker W. 11 15 Baltazzi E. 9 150 Barker S. A. 7 58 Barltrop J. A, 12 34 Barnartt S. 7 84 Barrer R. M. 3 293 Barton D. H. R. 3 36; 10 44; 11 189 Bassett H. 1 247 Bateman L. 8 147 Baughan E. C. 7 103 Baulch D. L. 12 133 Bayliss N. S. 6 319 Bell R. P. 1 113; 2 132; 13 169 Bentley R. 4 172 Bergel F. 2 349 Bethell D. 12 173 Bevington J. C. 6 141 Birch A. J. 4,69 ; 12 17 Bircumshaw L.L. 6,157 Bockris J. O’M 3 173 Bolland J. L. 3 1 Bond G. C. 8 279 Bourne E. J. 7 58 Bowen E. J. 1 1 ; 4,236 Bradley R. S. 5 315 Braude E. A. 4 404 Bremner J. G. M. 2 1 Brink N. G. 12 93 Brown B. R. 5 131 Brown R. D. 6 63 Buchanan J. G. 12 152 Buckingham A. D. 13 183 Bu’Lock J. D. 10 371 Bunnett J. F. 12 1 Burkin A. R. 5 1 Burnett G. M. 4 292 Burton H. 6 302 Cadogan J. I. G. 8 308 Caldin E. F. 7 255 Challenger F. 9 255 Chatt J. 12 265 Coates G. E. 4 217 Collinson E. 9 31 1 Cook A. H. 2 203 Cook J. W. 5 99 Cookson R. C. 10,44 Cooper C. F. 13 71 Cottrell T. L. 2 260 Coulson C. A. 1 144 Cowdrey W. A. 6 358 Cox E. G. 7 335 Crawford V. A. 3 226 Crofts P. C. 12,341 Crombie L. 6 101 Cruickshank D. W. J. Curran S. C. 7 1 Cuthbert J. 13 215 7,335 Dainton F. S. 12 61 Dalgliesh C. E.5 227 Davies A. G. 9 203 Davies D. S. 6 358 Davies M. 8 250 Davies N. R. 12 265 Davies R. O. 11; 134 Dawton R. H. V. M. 9 De Heer J. 4 94 de la Mare P. B. D. 3 de Mayo P. 11 189 Derjaguin B. V. 10,295 Dickens P. G. 11 291 Dubinin M. M. 9 101 Duncan J. F. 2,307; 12 Dunning W. J. 9 23 1 126 133 Eley D. D. 3 181 Emelkus H. J. 2 132 Errede L. A. 12 301 Evans M. G. 4 94; 6 376 186 Evans R. M. 13 61 Fensham P. J. 11 227 Ferrier R. J. 13 265 Foster A. B. 11 61 Freidlina R. Kh. 10 Gascoigne R. M. 9,328 Gaydon A. G. 4 1 Gee G. 1 265 Gent W. L. G. 2 383 Gibson D. T. 3 263 Gillespie R. J. 2,277; 8 Gilman H. 13 116 Glenn A. L. 8 192 Goehring M. 10 437 Gold V. 9 51 ; 12 173 Gowenlock B. G. 12 Gray B. 9 362 Greenwood N. N. 8 1 Griffith J. S. 11 381 Gunstone F. D. 7 175 Gutmann V. 10,451 Halpern J.10 463 Hamer F. M. 4 327 Hardy D. V. N. 2 25 Harman R. E. 12 93 Harris M. M. 1 299 Hartley G. S. 2 154 Hassel O. 7 221 Hawkins E. G. E. 4 Hawkins J. D. 5 171 Haynes L. J. 2 46 Heaney H. 11 109 Hey D. H. 8 308 Hickling A. 3 95 Holt R. J. W. 13 327 Hughes E. D. 2 107; 5 245; 6 34 Hush N. S. 6 186 Ingold C. K. 6 34; 11 Irving H. M. 5 200 Ivin K. J. 62 61 Jacobs P. W. M. 6 238 Jain A. C. 10 169 Janz G. J. 9 229 Jeffrey G. A. 7 335 330 40; 11,339 321 25 1 1 CUMULATIVE INDEX 377 Jenkins E. N. 10 83 Jennings K. R. 12 116 Jones D. G. 4 195 Kapustinskii A. F. 10 283 Katritzky A. R. 10 395; 13,353 Kenyon J. 9 203 Khorana H. G. 6 340 Kipling J. J. 5,60; 10 1 Kitchener J. A. 13 71 Lagowski J. J. 13 233 Lamb J. 11 134 Lamberton A. H. 5 75 Law H. D. 10 230 Lea F. M. 3 82 9 Leech H. R. 3 22 Leisten 3.A. 8 40 Levy N. 1 358 Lewis E. S. 12 230 Lewis J. 9 11 5 Lifshitz E. M. 10 295 Linnett J. W. 1 73;' 11 291; 12 116 Lister B. A. J. 2 307 Lister M. W. 4 20 Long L. H. 7 134 Longuet-Higgins H. C. Loudon J. D. 5 99 Luttke W. 12 321 Lythgoe B. 3 181 11 121 Maccoll A. 1 16 McCoubrey J. C. 5 MacDiarmid A. G. 10 McGrath W. D. 11 87 McKenna J. 7 231 Maddock A. C. 5,270 Maitland P. 4 45 Manners D. J. 9 73 Marsh J. K. 1 126 Martin F. S. 13 327 Martin R. L. 8 1 Megson N. J. L. 2 25 Millar I. T. 11 109 Millen D. J. 2 277 Morgan K. J. 8 123; Morrison A. L. 2 349 Musgrave W. K. R. 8 364; 11 87 208 12 34 33 1 Nelson Smith R. 13 Nesmeyanov A. N. 10 Newth F. H. 13 30 Norrish R. G. W. 10 Nyholm R. S. 3 321; 287 3 30 149 7 377; 11 339 O h W. D. 11 15 Orgel L. E. 8 422; 11 Orville-Thomas W.J. Overend W. G. 11 61 ; Owston P. G. 5 344 38 1 11 162 13 265 Page J. E. 6 262 Paneth F. A, 2 93 Parsonage N. G. 13 Pauson P. L. 9 391 Pepper D. C. 8 88 Percival E. G. V. 3 369 Phillips F. C. 1 91 Pople J. A. 11 273 Praill P. F. G. 6 302 306 Reid C. 12 205 Richards R. E. 10,480 Riddiford A C. 6 157 Riley H. L. 1,59; 3,160 Rose J. D. 1 358 Rowlinson J. S. 8 168 Satchell D. P. N. 9 51 Saxton J. E. 10 108 Schofield K. 4 382 Seshadri T. R. 10 169 Sexton W. A. 4 272 Sharpe A. G. 4 115; Shchukina L. A. 10 Shemyakin M. M. 10 Sheppard N. 6 1 ; 7 19 SillCn L. G. 13 146 Simes J. J. H. 9 328 Simons P. 13 3 Simpson D. M. 6 1; 7 Smales A. A. 10 83 11,49 261 26 1 19 Smith H. 12 17 Smith J. A. S. 7 279 Smith M. L. 9 1 Springall H. D. 10 230 Stacey M. 1 179 213 Staveley L. A. K. 3,65; Stern E.S. 5 405 Stone F. G. A. 9 174 Sutton L. E. 2 260 Swallow A. J. 9 3 11 Symons M. C. R. 12 13 306 230 13. 99 Synge; R.'L. M. 3 245 Szwarc M. 5 22; 12 301 Taylor A. W. C. 4 195 Thomas S. L. 7 407 Thomson R. H. 10 27 Thrush B. A. 10 149 Tipper C. F. H. 11 313 Tomkins F. C. 6 238 Topley B. 3 345 Trapnell B. M. W. 8 Trotman-Dickenson A. Truter E. V. 6 390 Turner E. E. 1 299 Turner H. S. 7 407 404 F. 7 198 Ubbelohde A. R. 4 356; 5,364; 11,246 Ulbricht T. L. V. 13,48 Uri N. 6 186 Walsh A. D. 2 73 Warburton W. K. 8 67 Warhurst E. 5 44 Waters W. A. 12 277 Weedon B. C. L. 6,380 Wells A. F. 2 185; 8. Wells R. A. 7 307 Whiffen D. H. 4 131; Whytiaw-Gray R. 4 Wilson H. N. 2 1 Wittenberg D. 13 116 Woodward L. A. 10 Yoffe A. D. 9 362 Zakharkin L. I. 10,330 330 12 250 153 185 CUMULATIVE INDEX OF TITLES Acetylenes as natural products 10 371 Acetylenes infrared and Raman spectra of 6 1.Acid use of the term 1 113 Acids carboxylic anodic syntheses with 6 380 Acids carboxylic association of 7 255 Acids straight-chain fatty natural and synthetic recent developments in the preparation of 7 175 Acid-base reactions simple rates of 13 169 Addition reactions free-radical of olefinic systems 8 308 Adsorption of non-electrolytes from solution 5 60 Affinities relative of ligand atoms for acceptor molecules and ions 12,265 Age geological determination of by radioactivity 7 1 Aldehydes polymerisation of 6 141 AIkaioids ergot 8 192 Alkaloids indole excluding harmine and strychnine 10 108 Alkaloids steroidal 7 23 1. Alkaloids veratrum 12 34 Alkanes infrared and Raman spectra Alkanes tetra- and tri-chloro- and related compounds 10 330 Analgesics synthetic 2 349 Analysis conformational principles of 10 44 Analysis inorganic applications of solvent extraction to 5 200 Analysis radioactivation 10 83 Anionotropy 4 404 Anodic syntheses with carboxylic Antibiotics newer chemistry of 12,93 Association of carboxylic acids 7,255 Asymmetry the non-conservation of parity and optical activity 13 48 Attraction molecular direct measure- ment of between solids separated by a narrow gap 10 295 Base use of the term 1 113 Biological reactions r61e of phosphoric Bond aromatic 5 147 Bonds dissociation energies of 5 22 of 7 19 acids 6 380 esters in 5 171 Bonds interpretation of properties of Bonding chemical and nuclear quad- Boron hydrides chemistry of 9 174 Boron hydrides and related com- Boron trifluoride co-ordination com- 2 260 rupole coupling 11 162 pounds 2 132 pounds of 8 1 Carbides of iron 3 166 Carbohydrate epoxides 13 30 Carbohydrate phosphates 11 61 Carbohydrate sulphates 3 369 Carbohydrates newer aspects of stereochemistry of 13 265 Carbon amorphous and graphite 1 59 Carbon and oxygen surface com- pounds of 13,287 Carbon-carbon bonds oxidative hydrolysis of in organic molecules 10,261 Carbon-carbon double bonds geo- metrical isomerism about 6 101 Carbon-hydrogen bond polarity of 2 383 Carbon-hydrogen bonds mechanism of breakage of 12,230 Carbon-phosphorus bonds com- pounds containing 12 341 Carbonitrides of iron 3 160 Carbonium ions structure of 12 173 Carbons active study of porous structure of by a variety of methods 9,101 Carbons adsorbent properties and nature of 10 1 Carbonyls of metals chemistry of 1 33 1 Catalysis by metals specificity in 8 404 Catalysis of reactions involving hydro- gen mechanisms of 3 209 Catalysis and semiconductivity 11 227 Catalysts redox initiation of poly- merisations by 9 287 Cations organic reactions of 6 302 Charcoals active study of porous structure of by a variety of methods 9,101 Chromatography inorganic 7,307 CUMULATIVE INDEX 379 Chromium mechanisms of oxidation Collisions in gases energy transfer in Colloidal electrolytes state of solution of 2 154 Colour and constitution 1 16 Combustions slow in the gas phase elementary reactions in 11 313 Complex compounds stabilities of 5,l Conductance ionic in solid salts 6 238 Configuration of flexible organic molecules 5 364 Conformational analysis principles of 10 44 Conjugated compounds free-electron approximation for 6 319 Co-ordination compounds of boron trifiuoride 8 1 Crystal structure and melting 4 356 Crystal structures of salt hydrates and complex halides 8 380 Crystals location of hydrogen atoms in 10,480 Crystals ionic lattice energy of 18 283 Cyanine dyes 4 327 Cyclohexane stereochemistry of 7 221 Decarboxylation thermal mechanism of 5 131 Degradation biological of trypto- phan 5 227 Densities limiting 4 153 Dielectric absorption 8 250 Dihalogen compounds Grignard and organolithium compounds derived from 11 109 Disproportionation in inorganic com- pounds 2 1 Diterpenoids chemistry of 3 36 Dyes effect of light on 4 236 Dyes cyanine 4 327 Dyes organic and their constitution Earth distribution of the elements in Electrode processes in aqueous solu- j3ectrolytes effects of ultrasonic waves Electrolytes colloidal state of solu- by compounds of 12 277 11 87 1 16 the 3 263 tions mechanism of 3 95 on 7 84 tion of 2 154 Electromagnetic separation of stable isotopes 9 1 Electron correlation and chemical consequences 11 291 Electrons structures of molecules deficient in 11 121 Elements terrestrial distribution of 3 263 Elements heavy radioactivity of 5,270 Elements of Group VIII recent stereochemistry of 3 321 Elements of Groups IVB and IV comments on the thermochemistry of 7 103 Elements of the rare-earth series separation of 1 126 Elements transuranic chemistry of 4 20 Energy transfer of in gaseous collisions 11 87 Enzymes degradation of polysacchar- ides by 9 73 Enzymes synthesis of polysaccharides by 7 58 Epoxides of sugars 13 30 Equilibria hydrolytic quantitative studies of 13 146 Equivalent-orbital approach to mole- cular structure 11 273 Ergot alkaloids structure of 8 192 Esters carboxylic and related com- pounds alkyl-oxygen heterolysis in 9 203 Exchange reactions of hydrogen iso- topes in solution principles of 9,51 Extraction liquid-liquid in inorganic chemistry 13 327 Ferrocene and related compounds 9 Flames emission spectra of 4 1 Flash photolysis and kinetic spectro- Flavones nuclear methylation of 10 Fluorescence and fluorescence quench- Fluorine laboratory and technical production of 3 22.Fluorine compounds general aspects of the iaorganic chemistry of 11,49 Fluorine compounds laboratory and technical production of 3 22 Fluorine compounds organic reac- tions of 8 331 39 1 scopy 10 149 169 ing,.1 1 380 QUARTERLY REVIEWS Foaming current concepts in theory Force constants 1 73 Forces intermolecular and the pro- Free-electron approximation for con- Friedel-Crafts reaction modern Furans some aspects of the chemistry of 13 71 perties of matter 8 168 jugated compounds 6 319 aspects of 8 355 of 4 195 Gases elementary reactions in slow Gases energy transfer in collisions in Graphite and amorphous carbon I Grignard reagents derived from di- combustions in 11 313 11,87 59 halogen compounds ll 109 Halides reactions of in solution 5 245 Halides complex crystal structures of 8 380 Halogens kinetics of thermal addition of to olefins 3 126 Heats of formation of simple inorganic compounds 7 134 Heteroaromatic compounds infrared spectra of 13 353 Heterogeneous reactions transport control in 6 157 Heterolysis alkyl-oxygen in carbo- xylic esters and related compounds 9 203 Hydrocarbons infrared and Raman spectra of Part I acetylenes and olefins 6 1.Part 11 paraffins 7 19 Hydrogen molecular homogeneous reactions of in solution 10 463 Hydrogen atoms location of in crystals 10 480 Hydrogen catalysis mechanisms of 3 209 Hydrogen isotope exchange reactions in solution principles of 9 51 Hydrogen peroxide its radicals and its ions energetics of reactions involving 6 186 Hydrogenation catalytic and related reactions mechanism of 8 279 Hyperconjugation 3 226 Ice structure of 5 344 Inimuno chemistry aspects of 1 179 Indole alkaloids excluding harmine Induction asymmetric and asym- Inorganic chemistry and magnetism Inorganic compounds disproportiona- Inorganic compounds Raman spectra Inorganic compounds stereochemistry Inorganic compounds simple heats of Inositols 11 212 Insecticides synthetic structure and activity in 4 272 Interhalogen compounds and poly- halides 4 115 Intermolecular forces and some pro- perties of matter 8 168 Iodine compounds inorganic some reactions of 8 123 Ion exchange 2 307 Ionisation potentials and far ultra- violet spectra their significance in chemistry 2 73 Iron carbides nitrides and carbo- nitrides of 3 160 Isoflavones 8 67 Isomerism geometrical about carbon- carbon double bonds 6 101 Isotopes exchange of between different oxidation states in aqueous solution 8 219 Isotopes synthesis of organic com- pounds labelled with 7 407 Isotopes tracer techniques involving 4 172 Isotopes stable electromagnetic separation of 9 1 21 3 and strychnine PO 108 metric transformation 1 299 7,377 tion in 2 l of 10 185 of 11 339 formation of 7 134 Lactones physiologically active un- Lanthanons separation of 1 126 Lattice energy of ionic crystals 10 283 Ligand atoms relative affinities of for acceptor molecules and ions 12 265 saturated 2 46 CUMULATIVE INDEX 381 Ligand-field theory 11 381 Light absorption of and photo- chemistry 4 236 Liquids transitions in 3 65 Liquids ultrasonic analysis of relaxa- tion processes in 11 134 Magnetic resonance absorption Magnetism and inorganic chemistry Manganese mechanisms of oxidation Manganese dioxide oxidations by in Mass spectrometry application of to Mass spectrometry of free radicals 13 Melting and crystal structure 4 356 Meso-ionic compounds 11 15 Metal-amine solutions reduction by; applications in synthesis and deter- mination of structure 12 17 Metal-ammonia solutions reduction of organic compounds by 4,69 Metals nature of solutions of 13 99 Metals specificity in catalysis by 8 404 Methyl radicals reactions of 7 198 Methylation biological 9 255 Methylation nuclear of flavones and related compounds 10 169 Molecules determination of structure of by X-ray crystal analysis modern methods and their accuracy 7,335 Molecules molecular-orbital and equivalent-orbital approach to structure of 11 273 Molecules electron deficient struc- tures of 11 121 Molecules electronically excited reac- tions of in solution 13 3 Molecules flexible organic configura- tion of 5 364 Molecules organic oxidative-hydro- lysis of carbonxarbon bonds in 10 261 Molecules simple representation of by molecular orbitals 1 144 Molecular-orbital approach to mole- cular structure 11 273 Molecular-sieve action of solids 3 293 nuclear 7 279 7,377 by compounds of 12 277 neutral media 13 61 chemical problems 9 23 21 5 Morphine synthetic approaches to structure of 5,405 Nitramines some aspects of the chemistry of 5 75 Nitration of aromatic compounds 2 277 Nitration of heterocyclic nitrogen compounds 4,382 Nitrides of iron 3 160 Nitro-compounds aliphatic 1 358 Nitrogen active 12 116 Nitrogen compounds heterocyclic nitration of 4 382 Nitrogen dioxide-dinitrogen tetroxide system structure and reactivity of 9 362 C-Nitroso-compounds structure and properties of 12,321 Nitrosyl group chemistry of 9 11 5 Non-electrolytes adsorption of from solution 5 60 Non-electrolytes theories of solutions of 13 306 Nuclear chemistry quantitative 12 133 Nuclear magnetic resonance absorp- tion 7,279 Nuclear quadrupole coupling and chemical bonding 11 162 Nucleation in phase changes 5 315 Nucleotide coenzymes recent develop- ments in biochemistry of 12 152 Oceans salt deposits from 1 91 Olefinic systems free-radical addition Olefins infrared and Raman spectra Olefins kinetics of oxidation of 3 1 Olefins kinetics of thermal addition of Olefins oxidation of 8 147 Optical activity and non-conservation Orbitals molecular approach to mole- Orbitals molecular and organic Orbitals molecular representation Organic compounds action of ionising Organic compounds behaviour of in reactions of 8 308 of 6 l halogens to 3 126 of parity 13 48 cular structure through 11,273 reactions 6 63 of simple molecules by 1 144 radiations on 9 31 1 sulphuric acid 8 40 382 QUARTERLY REVIEWS Organic compounds estimation of thermodynamic properties for 9 229 Organic compounds polarography of 6.262 Organic compounds reduction of by metal-ammonia solutions 4 69 Organic compounds isotopically labelled synthesis of 7 407 Organic reactions and molecular orbitals 6 34 Organolithium reagents derived from dihalogen compounds 11 109 Organometallic compounds of the first three periodic groups 4 2 17 Organosilylmetallic compounds for- mation and reactions of 13 116 5-Oxazolones chemistry of 9 150 Oxidation by compounds of chro- mium and manganese mechanisms of 12 277 Oxidation of olefins 3 1; 8 147 Oxidation-reduction potential of quinones relation of to chemical structure 4 94 Oxides of metals structures of 2 185 N-Oxides aromatic heterocyclic chemistry of 10 395 Oxygen and carbon surface com- pounds of 13 287 Parity non-conservation of 13 48 Penicillins chemistry of 2 283 Peptides methods of synthesis and terminal-residue studies of 10 230 Peptides structural investigation of 6 340 Peptides naturally occurring 3 245 Perfluoroalkyl derivatives of metals and non-metals 13,233.Peroxides organic and their reactions 4 251 Phase changes nucleation in 5 3 15 Phenols tautomerism of 10,27 Phosphates of carbohydrates 11 61 Phosphates condensed 3 345 Phosphoric esters r61e of in biological reactions 5 171 Phosphorus oxyacids some aspects of the organic chemistry of derivatives of 3 146 Photochemistry and light absorption 4,236 Photography cyanine dyes in 4 327 Photonolvmerisation.4. 236 Polarity of the carbon-hydrogen bond 2 383 Polarography of organic compounds 6 262 Polonium chemistry of 11 30 Polyhalides and interhalogen com- pounds 4 115 Polymerisation initiation of by redox catalysts 9 287 Polymerisation of aldehydes 6 141 Polymerisation addition some thermodynamic and kinetic aspects of 12 61 Polymerisation induced by light 4 236 Polymerisation ionic 8 88 Polymerisation radical rate constants in 4,292 Polymers based on silicon chemistry of 2 25 Polymers high thermodynamic pro- perties of and their molecular interpretation 1 265 Polysaccharides enzymic degradation of 9 73 Polysaccharides enzymic synthesis of 7 58 Portland cement constitution of 3 82 Proteins structural investigation of 6 340 Pteridines 6 197 Purines some aspects of the chemistry of 3 181 Pyrans some aspects of the chemistry of 4 195 Pyrimidines some aspects of the chemistry of 3 181 Pyrrole pigments biogenetic origin of 4 45 QuadrupoIe coupling nuclear and chemical bonding 11 162 Quadrupole moments molecular 13 183 Quenching of fluorescence I 1 Quinones relation between the oxida- tion-reduction potential and chem- ical structures of 4 94 Radiations ionisicg action of on organic compounds 9 31 1 Radicals free electron resonance spectrcscopy of 12 250 Radicals free mass spectrometry of 13 215 Radioactivation analvsis.10. 83 CUMULATIVE INDEX 383 Radioactivity determination of geo- logical age by 7 1 Radioactivity of the heavy elements 5 270 Rearrangements aromatic 6 34 Redox potentials of quinone relation of to chemical structure 4 94 Reduction by metal-amine solutions ; applications in synthesis and deter- mination of structure 12 17 Reduction by metal-ammonia solu- tions of organic compounds 4 69 Relaxation processes molecular in liquids ultrasonic analysis of 11 134 Salt hydrates crystal structures of 8 380 Salts deposits of from oceans 1 91 Salts basic structure of 1 247 Salts solid ionic conductance in 6 Sandmeyer reactions 6 358 Seniiconductivity and catalysis 11 227 Sesquiterpenoids recent advances in chemistry of 11 189 Silicon chemistry of polymers con- taining 2 25 Silyl compounds 10 208 Sodium " flame " reactions 5 44 Solids molecular-sieve action of 3 293 Solids thermal transformations in 11 246 Solids transitions in 3 65 Solids separated by a narrow gap direct measurement of molecular attraction between 18 295 Solutions aqueous mechanism of electrode processes in 3 95 Solutions of non-electrolytes theories of 13,306 Solvation ionic 3 173 Solvent extraction and its applications to inorganic analysis 5 200.Solvents ionising non-aqueous reac- tions in 10,451 Specificity in catalysis by metals 8 404 Spectra charge-transfer and related phenomena 8,422 Spectra emission of flames 4 1 Spectra far ultraviolet ionisation potentials and their significance in chemistry 2 73 238 Spectra infrared and Raman of hydrocarbons. Part I acetylenes and olefins 6 1. Part 11 paraffms 7 19 Spectra infrared of heteroaromatic compounds 13 353 Spectra Raman of inorganic com- pounds 10 185 Spectra rotation 4,13 1 Spectroscopy electron resonance of free radicals 12 250 Spectroscopy kinetic and flash photo- lysis 18 149 Stabilities of complex compounds 5 l Stereochemistry of inorganic com- pounds 11 339 Stereochemistry of cyclohexane 7 221 Stereochemistry of elements of Sub- group VIB of the Periodic Table 10 407 Stereochemistry of elements of Group VIII of the Periodic Table 3 321 Steric hindrance 2 107; 11 1 Steroidal alkaloids 7,23 1 Subs ti tu t ions aromatic nucleop hilic mechanism and reactivity in 12 1 Sugar epoxides 13 30 Sulphur nitride and its derivatives Sulphuric acid behaviour of organic Surface compounds the chemistry of Sydnones 11 15 Tautomerism of phenols PO 27 Thermochemistry of the elements of Group IVB and IV comments on 7 103 Thermodynamic properties estima- tion of for organic compounds and chemical reactions 9 229 Thermodynamic properties of high polymers and their molecular inter- pretation 1 265 Tracers radioactive preparation of 2 93 Transformation asymmetric and asymmetric induction 1,299 Transformations thermal in solids 11 246 Transitions in solids and liquids 3 65 Transport control in heterogeneous reactions 6 157 Triplet state 12 205 10 437 compounds in 8 40 carbon-oxygen 13 287 384 QUARTERLY REVIEWS Triterpenes tetracyclic 9 328 Tropolones 5 99 Tryptophan biological degradation of 5 227 Ultrasonic analysis of molecular relaxation processes in liquids 11 134 Ultrasonic waves effects of on electrolytes and electrolytic pro- cesses 7 84 Veratrum alkaloids 12 34 Wool wax constitution of 5 390 A'-Ray crystal analysis modern methods of determination of mole- cular structure by and their ac- curacy 7 335 p-Xylylene chemistry of and of its analogues and polymers 12 301

ISSN:0009-2681    

DOI:10.1039/QR9591300375

出版商:RSC    

年代:1959

数据来源: RSC