HYPERCON JUGATION By V. A. CRAWFORD M.Sc. PH.D. ( WHEATSTONE PHYSICS LABORATORY UNIVERSITY OF LONDON KING'S COLLEGE *) SINCE the review by C. L. Deasy the concept of hyperconjugation has become well established and furthermore has been extended and widely applied ; consequently the present more extensive review was undertaken. Historical Introduction and Definition of the Term.-If electron displace- ment in alkyl groups resulted solely from the operation of the inductive effect then the order But > Pri > E t > Me should always be obtained for those reactions which require accession of electrons to the reaction region. Experimental evidence that this order is not universally applicable was first provided by J. W. Baker and W. S. Nathan,2 who studied the rate of reaction of various alkyl-substituted benzyl bromides with pyridine.It was found that all alkyl substituents increased the reaction rate while with a single p-alkyl substituent the rate decreased in the order Since the reaction studied is one facilitated by electron accession towards the side chain it follows that the accelerating effects of alkyl groups must be related to their capacity for electron release and furthermore that the relative magnitude of this electron release diminishes in the order indicated in (a) which order is exactly the reverse of that expected on the basis of the inductive effect of alkyl groups. The methyl group it would seem is there- fore capable of permitting additional electron release by some mechanism which is either greatly reduced or inoperative in the more highly alkylated groups.Since an anomaly of this kind is not found in every type of system containing alkyl groups Baker and Nathan pointed out that the additional mechanism of electron release by the methyl group in p-methylbenzyl bromide is to be associated with the presence of the attached conjugated system of the aromatic nucleus. It was therefore suggested that when a methyl group is attached to a conjugated system the pair of electrons forming the C-H bond in the methyl group are appreciably less localised than are those in a similarly situated carbon-carbon bond. Hence a methyl group attached to the requisite system permits electron release by what is essentially a type of electromeric effect Me > Et > Pri -N But . - ( a ) This new effect acts in addition to and in the same direction as the inductive effect but since it depends on the number of C-H bonds its magnitude clearly will diminish in the order Me > Et > Pri > But.Hence 1 Chena. Reviews 1945 36 145. * Now Lecturer in Theoretical Chemistry The Durham College University of 226 J . 1935 1844. Durham. CRAWFORD HYPERCONJUGATION 227 bromide the methyl group by this new mechanism electron release at the G-Br bond in p-methylbenzyl permits additional and therefore further facilitates the anionisation of the bromine atom. On the other hand in p-tert.-butylbenzyl bromide CH C"&+H.-Br - CH3 although the inductive effect of' But is greater than that of methyl the postulated electron-release mechanism is no longer possible for the inter- calation of the tertiary carbon atom prevents the additional methyl groups from forming part of the necessary conjugated system.Although the presence of a conjugated system was emphasised it was assumed that the effect could also function in those systems where electron- pair displacements arc rendered possible by fission of a group with its bond electrons as for example If however it requires separation as an anion of a group or atom which normally jonises as a cation the effect is assumed not to occur. The mobility (kl + k,) of the prototropic change in tho azomethine system ki P-RC,H,.CH :N*CH2Ph + P-RC,H,.CH~*N :CHPh Jca which is known t o be facilitated by electron withdrawal from the triad ~ y s t e m ~ was found 4 to decrease in the order But > Pri > Me indicating that the retarding effect of a p-methyl group is. much greater than that of more highly alkylated groups.This result provides good evidence that the Baker-Nathan effect is of a mesomeric character for it is well established that a mesomeric effect is capable of retarding a reaction which is facilitated by electron recession from the region of reaction. This result was confirmed ti by an investigation of the equilibrium p-Rc,$,*CHO + HCN f p-RC,H,*CH(OH)*CN for the stabilities of the aldehydes relative to the cyanohydrins diminished in the order Me > Et > Pri > But > H. This is an observation to be expected if the Baker-Nathan effect predominates over the inductive effect for in the freo aldehyde conjugation extends to the side-chain carbonyl group so that the new effect stabilises the free aldehyde to a greater extent than it would the cyanohydrin.a C . W. Shoppee J . 1933 1117. 4 J. W. Baker W. S. Nathan and C. W. Shoppee J . 1935 1847. J. W. Baker and M. L. Hemming J . 1942 191. 228 QUARTERLY REVIEWS This power of alkyl groups to conjugate with multiple bonds (either double or triple) is known as hyperconjugation i.e. an additional conjugation beyond that ordinarily recognised. Since groups which conjugate with unsaturated systems act as electron donors single and multiple bonds are known as acceptor and donor bonds respectively. Some Theoretical Considerations. -T he decrease of el e c tr on-re pulsive character in ascending the homologous series of alkyl groups had been deduced by N. G. Burkhardt and 31. G. Evans,s on the basis of Mulliken's theory of united atoms. As a united atom group the methyl substituent has the electronic con- figuration ( 1 ~ 2 2 ~ 2 2 ~ 3 5 ) ) which is the same as that of fluorine (except that all the orbitals are more or less deformed) and falls into line with other o,p- directing groups all the electrons (except the carbon 1s) making up the electronic shell of the group.Replacement of the hydrogen atoms of the CH by other alkyl substituents results in (1) destruction of the united atom system with consequent reduction of the nuclear charge and (2) removal to some extent of eloctrons which were available and taking part in the bond when the electronic configuration of ths group resembled that of fluorine. This decrease in the availability of electrons it was suggested might contribute to the apparent decrease in o,p-directing power of substituted methyl groups.More recently R. S. Mulliken C. A. Rieke and W. G. Brown 7 studied the phenomenon quantitatively using the method of molecular orbitals. In this method of treatment it is natural to write the methyl group as -CrH, and to compare it with such groups as -C=N and -C=CH. In general a quasi-triple bond is defined as one consisting of three ordinary single bonds from a carbon atom to any three other atoms whenever there is opportunity for conjugation across an intervening carbon-carbon single bond with a second quasi- or ordinary multiple linkage. Since the quasi- triple bond CEH is much more saturated than the C=C triple bond its conjugating power is smaller but that it is still considerable is shown by certain physical data. Second-order conjugation or first-order hyperconjugation involves one quasi- and one true multiple bond whereas third-order conjugation or second-order hyperconjugation involves two quasi-multiple bonds ; ordinary conjugation fits into the scheme as first-order conjugation.The following molecules provide examples of conjugated systems in this gsneralised sense } Ordinary or first-order Conjugation. HCL,C-C=CH NEEC-CEN H,,C-C=CH Second-order conjugation or H,_C-C=N } first -order hyperconjugat'ion. H,~C-C%H Third-order conjugation or second-order hypercon j ugation. 6 Mem. Proc. Manchester Lit. Phil. SOC. 1933 77 37. 7 J . Amer. Chem. SOC. 1941 63 41. CRAWFORD HYPERCONJUGATION 229 On this basis nearly all saturated organic molecules are stabilised by Thus propane third-order conjugation of rather a complicated character.may be written as either H,=C-CFH,Me or MeH,EC-C_H the two ways of writing the formula indicating two different possibilities for hyperconjugation both of which contribute to the stability of the molecule. The three hydrogen atoms of CH have three valency electrons just like the nitrogen atom in the CEZN group. If therefore the three hydrogen atoms of the methyl group can be treated like a pseudo-atom and suitabIe H group molecular orbitals be formulated the procedure for studying N_C-C=N theoretically may likewise be applied to H,=C-C-H,. In this connection it is recalled that a single bond is always formed by a pair of electrons each in a o orbital and in the molecular orbital (M.O.) approximation both electrons occupy a single bonding o M.O. which is symmetrical with respect to rotation about the axis of the molecule.A multiple linkage consists of a o bond and one or more n bonds. Thus the triple bond in acetylene is composed of a o bond together with two pairs of electrons in n A.O. or M.O. If axes x y and x are taken with the x axis dong the symmetry axis of the molecule then each n M.O. occurs in two forms which may be called nz and q, and these will hereafter be denoted simply by x and y. If the three hydrogen atom 1s atomic orbitals of the H group are denoted by a b and c and located a t the vertices of an equilateral triangle group orbitals of two types may be constructed as follows. (i) The linear combination (a + b + c ) may be formed and this has approximate symmetry around an axis passing perpendicularly through the centre of the triangle.This group orbital may therefore be described as a quasi-rr M.O. and can be expected to interact with other a orbitals. Delocalisation of this type is known as o-hyperconjugation and since CJ electrons are relatively tightly bound the resulting energy of delocalisation is likely to be very small. Consequently a-hyperconjugation is generally ignored. (ii) There may also be constructed the linear combination a - +(b + c ) and b - i ( c + a) and these have the same type of symmetry as the dumb- bell orbital of an isolated carbon atom. Pictorial representations of these two types of group orbital are given by C. A. Coulson.8 The three hydrogen atoms of the methyl group can therefore function as a pseudo-atom with a n-type orbital which can interact with other orbitals of the same symmetry such interaction being known as n-hyper- conjugation.Sinco the C-H bonding orbital i s not of the right symmetry the electrons of a single C-H bond clearly cannot be involved in n-hyper- conjugation. If CH is attached to a benzene ring there will be overlapping between the TC orbitals of the ring and that of the H group (via the C atom of CH,) Quart. Reviews 1947 1 144. 230 QUARTERLY REVIEWS resulting in a M.O. in which electrons from the methyl group can migrate into and out of the ring. The resulting delocalisation energy in toluene is small and stabilises the molecule by approximately 1.5 kcals. It might be noted that the hyperconjugation here is analogous to the ordinary con- jugation which occurs in styrenes where the group -CH=CH instead of CH is substituted in the ring.Hypercon jugation in Molecules containing Double Bonds.-Butadiene is the simplest example of ordinary or first-order x conjugation the x axis being taken perpendicular to the plane of the nuclear framework. In propylene H3~C-CH=CH, the C-C single bond can act as an acceptor bond for second-order x conjugation between the x electrons of the C-C double bond and the x electrons of the C-H quasi-triple bond. The y electrons of the CrH bond are inactive in second-order conjugation ; hence for this purpose the CGH bond acts like a double bond. In cyclopentadiene (I) there is a combination of first-order x conjugation as in butadieno and second-order x conjugation as in propylene the molecule having two second- Taking axes as shown with the x axis per- pendicular to the plane of the ring tho two ;% - kTJ-y I I order and one first-order acceptor bonds.c g 6 C II H2 I ;2 hydrogen atoms of the CH group are located one above and the other below the plane of the I 8 I ring. The quasi-double bond between the car- (1.) (11.) bon atom and the pseudo-atom H, involves one o bond and one x bond with M.O. formed by combination of a cr and x carbon A.O. respectively with the following quasi-A.O. of the H group [a] = (a + b ) / 2 / 2 ; [XI = (a - b ) / 2 / 2 The quasi [XI orbital it will be noticed is antisymmetrical with respect to the plane of the ring and consequently two of the four C-H bonding electronsin the CH group must be allocated inM.0. approximation to a quasi- unsaturation M.O. i.e. to a M.O. whose symmetry permits it to interact with the unsaturation M.O.of the ring carbon atoms. The nature of the hyperconjugation here is made clearer by comparing cyclopentadiene with fulvene (11). The quasi-double bond like the C=CH ordinary double bond in fulvene contains one pair of electrons which can enter into conjugation with the two pairs of unsaturation electrons associated with the two double bonds in tho ring. However since the conjugation in cydopentadiene involves ordinary double bonds and one quasi-double bond the resonance here will be less intense than in fulvene where ordinary double bonds only are involved. Third-order Con jugation. -In ethylene only third-order conjugation is present. For each H group there are quasi-A.O. as follows [o] = (a + b ) / d 2 ; [y] = (a - b ) / 2 / 2 a and b being located on opposite sides of the xx plane.CRAWFORD HYPERCONJUGATION 231 The y electrons of the two CH groups of ethylene can give third-order conjugation across the C= C bond which here acts as acceptor in the same way that the [z] and the [y] electrons of the two CH groups conjugate across the single bond of ethane. In both cases the acceptor bond takes on triple-bond character. Bond Orders.-With regard to the analysis of bond lengths and the character of bonds W. G. Penney and C. A. CoulsonlO introduced the term bond order taking values of 1 2 and 3 respectively for the orders of the central bonds in ethane ethylene and acetylene and drew curves relating bond length to bond order. Since there is no conjugation in acetylene the value for the C r C bond in this molecule is correct but in ethylene and ethane however there is third-order conjugation and when allowance is made for this the orders of the central bonds in these molecules become 2-12 and 1.12 respectively.The excess bond order above the usually assumed values of 1 and 2 for normal C-C and C=C may be expressed by saying that normal C-C and C k C bonds contain a certain yo of triple-bond character the values being 6% and 12% respectively. As a result of the inclusion of hyperconjugation the curve relating bond order to length does not reach bond order 1 at 1.54 A. but a t about 1-58 A. Although this value was interpreted as being the normal bond length for an isolated or ideal C-C single bond the analysis does not in general definitely indicate that ethane should have a greater C-C bond length than the normal value of 1-54 A.found in saturated molecules. Effects to be expected if Hyperconjugation occurs.-Hyperconjugakion might be expected to reveal its occurrence in any molecule by its effects on the following properties first-order hyperconjugation being anticipated to produce larger effects than second-order hyperconjugation. The electron-diffraction studies by L. Pauling L. 0. Brockway and J. Y. Beach 11 l2 have shown that first-order or ordinary conjugation causes considerable shortening of single bonds. Hyper- conjugation it might be expected would produce similar changes but since it is weaker in its effects than ordinary conjugation appreciable changes are not likely to be produced. (ii) Dipole moments. Conjugation alters the electron distribution in a molecule and so affects its dipole moment.Whether the moment because of conjugation is increased or diminished depends on whether the con- jugation or resonance moment is in the same or the opposite direction to the normal moment. Dclocalisation whenever it occurs stabilises the system and therefore less heat will be evolved in the hydrogenation process than in the corresponding system not stabilised by hyperconjugation. Furthermore progressive alkylation of a double bond may be expected to result in its progressive stabilisation and this should be reflected in a progressive diminution in the heat of hydrogenation. (i) Bond length. (iii) Heats of hydrogenation. Proc. Roy. SOC. 1937 A 158 318. l1 J . Amer. Chern. Soc. 1935 57 2705. lo Ibid. 1939 A 169 419. l2 Ibid. 1937 59 1223. 232 QUARTERLY REVIEWS (iv) Spectroscopy.It is found empirically that a compound with conjugated double bonds absorbs light of longer wave-lengths than an analogous compound with isolated double bonds. Moreover as the number of double bonds in the conjugated system increases the absorption is progressively shifted to longer wave-lengths. Conjugation causes (a) the unoccupied antibonding M.O. to become less strongly antibonding and (b) a raising of the occupied bonding M.O. and hence absorption occurs at longer wavc-lengths an effect which increases with progressive conjugation. Since hyperconjugation is generically similar to ordinary conjugation it too might be expected in general to result in displacement of absorption bands towards longer wave-lengths when alkyl substituents are attached to unsaturated atoms. Highly unsaturated substituents may be expected to exert pronounced effects but with alkyl groups where the unsaturation is small the system itself may be expected to play a controlling part.This it will accomplish by enhancing the unsaturation in the substituent by means of its own unsaturation either as this exists permanently or as it is developed during the transition state of the reaction. l3 The clectromeric effect therefore becomes of importance in those transition states which require an enhanced alkyl conjugation and in such reactions the pheno- menon will manifest itself. The remainder of this review will be devoted to tho presentation of experimental evidence confirming these expectations and also to such other experimental work in which the concept of hyperconjugation has becn applied.Bond Length.-It has been pointed out that hyperconjugation should produce detectable changes in the lengths of acceptor bonds. The results of electron-diffraction studies by L. Pauljng H. D. Springall and K. J. Palmer l4 are shown below (in A.). (v) Kinetic studies. Single bond Single bond bond. bonds. adjacent to triple between two triple H,=C-CrCH . . 1.46 H 3 ~ C - - - C ~ C - C ~ C - C H 3 . . 1-47 1.38 HC=C-C=uH . 1.36 H 3 r C - C ~ C - C ~ H 3 . . I 1-47 It is seen. that in those compounds in which the C-C single bond is adjacent to the C-C triple bond its value is appreciably lcss than the normal value of 1.54 A. and this has been regarded as being due partly to con- tributions to the normal state from hyperconjugated structures. H+ H H* H I - I C=C=C-H C=C=C-H I H I H The carbon-carbon distance in acetaldehyde is approximately 0.04 A.1 3 M. L. Dhar E. D. Hughes C. K. Ingold A. M. M. Mandour G. A. Maw and L. I. Woolf J . 1948 2095. l4 J . Amer. Chem. SOC. 1939 61 927. CRAWFORD HYPERCONJUGATION 233 less than the normal value,15 and here again the shortening has been attributed to hyperconjugated structures. The shortened bond length in methylacetylene has been confirmed by the spectroscopic studies of R. M. Badger and S. H. Bauer and also of G. Herzberg F. Patat and H. Verleger,l7 and according to R. S. Mulliken,ls when the formula is written as H,=C-C=CH it would indicate hyper- conjugation almost as strong as the conjugation in N%2--C=N where the C-C length is 1.43 A. It has also led to the conclusion that the methyl group conjugates more with C=C than with C-C.Indeed two conjugated triple bonds can interact through either their x or their y orbitals so that for this system conjugation twice as great as for two conjugated double bonds might be expected. However in the molecules C2H6 C2H, and C2H2 in which there is no conjugation (ignoring third-order conjugation in C,H6 and C2H,) there is an abrupt shortening on passing from C-C to C=C to C%C. Thus part of the observed shortening is no doubt due to changes of hybridisation.18a Recently X-ray analysis l9 of the di-isoprene derivative geranylamine hydrochloride (111) has revealed a shortening of the central bond accom- CH3 CH3 I 1-51 1-44 1-51 I CH~-C=CH~CH,~CH~-C=CH-CH~-NH,Cl (111.) panied by the planar arrangement of the adjacent groups and L. Bateman and G.A. Jeffrey 2O have suggested that this unique bond character which simulates ordinary conjugation is the result of hyperconjugation. Hyperconjugation has also been invoked to account for the partial double- bond character of the C-C bond in dibenzyl 21 which is a similar system. In this molecule (IV) the three formally single carbon-carbon bonds have lengths of 1.523 1.510 and 1.523 A . ~ ~ ~ and make angles of 115" with each other.22 Aniongst other causes the observed shortening in these two molecules may be due to third-order conjugation between the two quasi-double bonds but this is hardly likely to produce decreases in bond length of the observed magnitude. Further- more just such third-order conjugation occurs in paraffin hydrocarbons and in these molecules no such decrease in bond length has been obtained.The angle abc is 115" and this is intermediate between 109" 28' and 120° the values required by 8p3 and sp2 hybridisation respectively. The l5 P. Stevenson H. D. Burnham and V. Schomaker J . Amer. Chem. Xoc. 1939 61 2922. l7 J . Physical Chem. 1937 41 123. lSa C. A. Coulson Victor Henri Memorial Vol. Desoer LiBge 1948 16. 2o Nature 1943 152 446. 21 M. Szwarc Faraday SOC. Discussions 1947 No. 2 39. 21a E. G. Cox and D. W. J. Cruickshank Acta Cryst. 1948 1 921. 2 2 G. A. Jeffrey Proc. Roy. XOC. 1947 A 188 222. @ \ (IV. ) l6 J . Chem. Physics 1937 5 599. Is J . Chem. Physics 1939 '4 339. G. A. Jeffrey Proc. Roy. SOC. 1945 A 183 388. 234 QUARTERLY REVIEWS bond bc therefore has more s character and is consequently stronger than the normal C-C single bond.23 This is a sufficient and probably the sole explanation of the observed shortening in these two molecules.Application of the concept of hyperconjugation with caution is therefore indicated. Dipole Moment.-Dipole-moment measurements have yielded significant evidence as to hyperconjugation in unsaturated molecules. Some pertinent values for the vapour state 2 4 7 25 are shown in Table I. Dipole moment D. j/ Substance. I-____- ~______I_ 2.27 Crotonaldehyde . . . 2.72 1 -Methylbutadhe . 2.73 2-Methylbutadiene . . 2.72 2 3-Dimethylbutadiene TABLE I Dipole moment D. 3.67 0.68 0.38 0-52 ________ Substance. Formaldehyde . . . Acetaldehyde . . . Propaldehyde . . . n-Butaldehyde . . . If the large increase in moment from formaldehyde to acetaldehyde is due solely to the inductive effect it might be expected to have produced a further increase from acetaldehyde to propaldehyde.That this does not occur was regarded as indicating that resonance is partly responsible for the increase from formaldehyde to acetalde- I - hyde. Thus structures of the type (V) among others would be expected to contribute to the observed increase in moment. The observed shortening of the C-C bond by 0.04 A. indicates that this bond contains about 8% of double-bond character. The solution value of acraldehyde is 0.4 D. greater than that of acetalde- hyde an increase due to the transfer of charge to the oxygen atom facilitated by the conjugation of the molecule I3 H H H+H H H+- H H-C=C-O I (V.1 H I I I - I I - H-C+-C=C-O H-C-C-C-0 The effect of hyperconjugation is more strikingly shown in the large rise in moment of nearly 0.6 D.in tmns-crotonaldehyde. In addition to polar structures analogous to those which have been written for acraldehyde three further highly polar ones (as in VI) can be written for crotonaldehyde. H+H H H I I I - H-C=C-C=C-O I H H+H H I I I I H-C=C-C- H H (VI4 (VII.) In like manner the moments of propylene and but-l-ene are accounted 23 A. D. Walsh Faraday SOC. Discussions 1947 No. 2 18. 2 4 E. C. Hurdis and C. P. Smyth J . Amer. Chern. SOC. 1943 65 89. a 5 N. B. Hannay and'C. P. Smyth &id. p. 1931. CRAWFORD HYPERCONJUGATION 2 35 for by polar structures of the kind written for aldehydes. Thus for propylene may be written three structures of the type (VII). The moments of the methylbutadienes provide further evidence of the polarity resulting from hyperconjugation.Thus the moment of 2-methyl- butadiene is experimentally indistinguishable from that of propylene suggesting participation in the resonance hybrid of structures of the type (VIII). However the extent to which structures of this type contribute H H H I I I I II I c-c=c -C- H C H H / \ H H+ (VIII. ) should be rather less than in the case of propylene for the amount of double- bond character of the central butadiene bond should reduce that of the bond to the methyl carbon. In l-methylbutadiene (penta-1 3-diene) polarity should arise from polar structures analogous to those proposed for propylene and 2-methylbutadiene but here the negative charge instead of being displaced three carbon atoms away from the methyl hydrogens is displaced five carbons away to give (IX).H+H H H H H+ I3 C1 I I I I (X. 1 c=c c1- H C1 Since three such structures are the principal source of the moment of the molecule as in 2-methylbutadiene the moment of the molecule should be to that of 2-methylbutadiene approximately as the charge separation in the l-mothyl is to that in the %methyl. Measurement of tho molecular models shows that the ratio of these distances is approximately 1-5 if l-methylbutadiene is cis with respect to the central single bond and 14 if it has the more probable trans-structure. Tho fact that the ratio of the two moment values is 1.8 is considered by Smyth to give striking evidence in support of the validity of hyperconjugation as applied to unsaturated compounds. The argument however is of rather doubtful validity because in 2-methylbutadiene the conjugation is crossed whereas in l-methylbuta- diene it is not ; hence the 2-methyl isomer should have a lower moment.Hyperconjugation has also been invoked to account for the large increase in moment obtained on passing from chloroform to methylchloroform 26 for which there are possible nine resonance structures of the type (X). Similar considerations have been applied to account for the moments of acrylonitrile 26 E. C. Hurdis and C. P. Smyth J . Amer. Chem. SOC. 1942 64 2829. 236 QUARTERLY REVIEWS and isocrotyl chloride,25 some unsaturated aldehydes ethers and halogen compounds.27 The observed zero moments of all cyclohexanes combined with the known absence of a moment in benzene itself show that the moments of the corre- sponding alkylbenzenes must arise from electron H+ H+ displacement in the aromatic electronic system stimulated by the polar effect of the alkyl sub- stituent.Thus hyperconjugated structures (XI) and (XII) contribute to the small moment of -0 0 toluene.24 However the observed sequence of moment values relating to the vapour state28 in- creases from toluene to tert.-butylbenzene which is the order of the inductive effect-a result ex- plained by J. W. Baker 29 on the basis of the simultaneous operation of both electron mechanisms. Heats of Hydrogenation.-In Table I1 are recorded the heats of hydro- genation of certain relevant unsaturated That the heat of hydrogenation of cyclopentadiene is 50.9 kcals. per mole as compared with 57.1 for butadiene is regarded by Mulliken l 8 as evidence suggesting an H*-C-H H-C-H - (XII.) Substance.Substance. Ethylene . . . . Propylene . . . . isoPropylethyleno . . tert.-Butylethylene . . But-2-ene (trans) . . , (cis) . . . Trimethylethylene . . Heat at 356' K. in kcals. TABLE I1 Tetramethylethylene . cydopenta-1 3-dieno . Benzene . . . . . Ethylbenzene . . . Mesitylene . . . cycZoHcxa-1 3-dime . o-Xylene . . . Heat at 355" I(. in licals. 26.6 55.4 50.9 49.s 48-9 47.6 47.3 32-8 30.1 30.3 30-3 27.6 2S.G 26.9 added stabilisation of cyclopentadiene by hyperconjugation. However it was also pointed out that the low heat of hydrogenation may be due equally well to instability of the saturated alicyclic five-membered ring as to the stability of the unsaturated compound. For 1 3-cycZohexadiene where the aliphatic six-membered ring would be expected to have normal stability the value 55.4 kcals.would indicate that hyperconjugation has a smaller though appreciable stabilising effect. The progressive substitution by methyl groups of the hydrogen atoms of ethylene diminishes the heat of hydrogenation of the resultant com- pounds the diminution being greatest for tetramethylethylene for here 6.2 kcals. less heat is evolved than in the case of ethylene. However the diminution from ethylene to propylene is 2-7 kcals. whereas that from trimethyl to tetramethylethylene is only 0.3 kcal. thus indicating that the a7 M. T. Rogers J. Arner. Chem. SOC. 1947 09 1243. 28 J. W. Baker and L. G. Groves J . 1939 1144. a. J. B. Conant and G. B. Kistiakowsky Chem. Reviews 1937 20 181. 29 Ibid. p. 1150. CRAWFORD HYPERCONJUGATION 237 effect of progressive substitution of hydrogen atoms of ethylene by methyl groups is not additive.I n the alkylbonzenes substitution likewise results in diminished heats of hydrogenation although hero the stabilisation produced is lower than in ethylene. This is probably duo to the large resonance stabilisation already present in the aromatic ring.31 Thermal data show therefore that double bonds are considerably affected by the nature of the groups attached to them. Since the n electrons are tho ones involved in the hydrogenation process it follows that such substitutions affect their energy states and hence it might be expected that such changes will be reflected in the spectra and ionisation potentials of these electrons. Absorption Spectra and Ionisation Potentials.-(A) BZkyZethyZenes. The work of E.P. Carr and her collaborators 32p 33 clearly shows that progressive substitution in ethylene by methyl groups results in progressive shift to longer wsve-lengths of the bands in the Schumann region. Some of their results are shown in Fig. 1 whore the wave-number of the first band is plotted against the number of alkyl groups. The diminution in heat of hydrogenation and the long wave- length shift show parallel effects. Thus the largest fall in heat of hydrogenation (2.7 kcals.) occurs in passing from ethylene to pro- pylene and this also corresponds to the greatest long wave-length shift (3500 cm.-l) while the smallest diminution in heat of hydrogena- I I I 42' 0 ; 2 3 4 Number of Me groups around the doubje bond FIG. 1. tion (0.3 kcal.) occurs in passing from trimethyl- to tetramethyl-ethylene corresponding to the smallest shift (2000 cm-l).Table I11 shows the experimentally observed term values of methyl- substituted ethylenes for both the ground and the excited state.34 It will be noticed that the term values of both states decrease with progressive methylation. TABLE I11 8-75 8.3 No. of Me groups . . . . . . . 1 2 ' 0 Term values Ground state . . . . 10.45 ev. {Excited state . . . . 3.04 1 :& 1 29:328 ~ 2.04 1 1-72 31 W. C. Price Chem. Reviews 1947 41 258. 32 E. P. Carr and M. K. Walker J . Chem. Physics 1936 4 751. 33 E. P. Carr and H. Stiicklen ibid. p. 760. 34 R. S. Mulliken Rev. Mod. Physics 1942 14 265. Q 238 QUARTERLY REVIEWS These results are explained as being due to (i) charge transfer or inductive effect of the methyl group and (ii) hyperconjuga,tion of the CH group and the double bond.(i) Since the inductive effect is short-ranged it is likely to be of greater importance in small molecules and in terms of hybridisation may be described in the following way.,5 When the less electronegative CH group replaces a hydrogen atom in ethylene the bond linking the carbon atom of CH contains more s character and therefore the cr bond of the C=C double bond contains more p character. The cr electrons of the C-C double bond therefore become less tightly bound and consequently the repulsion between them and the n electrons is increased whence removal of the latter is more easily effected i.e. the inductive effect results in a raising of the ground-state orbital. However the excited state must also be considered F I G .2 Result on excited state of (a) inductive effect and (b) hyperconjugation efSect. Result on ground state of ( c ) inductive effect and (cl) hyperconjzigation effect. for the wave-length a t which an ab- sorption band appears depends upon the energy difference between the ground and the excited state. Com- putation shows that the inductive effect results in a decrease in the excited term value the change in the excited however being smaller than that brought about in the ground state. (ii) Hyperconjugation of the CH group with the ethylene double bond not only raises the ground state but also causes an increase in the excitcd- state torm value. The hyperconjugation and induc- tive effects are therefore opposed in the excited state and when allowance is made for the greater effect of charge t,railsfer the observed net decreases in excited-state term values are accounted for.Since hyperconjugation raises the ground state 0.14 e ~ . ~ ~ of the total drop in ionisation potential (0.80 ev.) on passing from ethylene to propylene is regarded as being due to this cause. Fig. 2 shows the influence of the two effects (acting independently) on the ground and the excited state. (B) AZkyEbenzenes. F. A. Matsen W. W. Robertson and R. L. Chuoke 36 have compared the near ultra-violet spectra of toluene ethylbenzene iso- propylbenzenc and tert.-butylbenzene and found that the bands representing transitions allowed by the lowered symmetry relatively to benzene due to migration of charge into the ring become stronger and shift to longer wave-lengths on passing from tert.-butylbenzene to toluene This result was ascribed to increase of hyperconjugation between the ring and the side chain as the latter changes from tert.-butyl to methyl.a 5 A. D. Walsh Ann. Reports 1947 44 32. 36 Chem. Reviews 1947 41 273. CRAWFORD HYPERCONJUGATION 239 In Table IV are recorded the ionisation potentials of some alkylbenzenes. That hyperconjugation is more important here than in the ethylene system would seem to be indicated by the smaller lowering in ionisation potential on passing from benzene to toluene (0.32 ev.) compared with (0-80 ev.) on TABLE IV I Substance. Benzene . . . Toluene . . . Ethylbenzene . . isoPropylbenzene . Ionisation potential ev. 11 Substance. 1 Ionisation potential ev. 1 9.24 8.92 8.75 8.6 fed-Butylbenzone . 1 o-Xylene . . . I . . . . I 8.5 8.3 8.3 s.3 passing from ethylene t o propylene.Indeed if the inductive effect were equally important in larger molecules then as R. N. Jones 37 points out it is difficult to understand why the absorption spectra of the ions of aromatic amines should resemble those of the parent hydrocarbon so closely. The polar effect of the positive charge on the nitrogen atom must produce a greater electron displacement in the -C-NH linkago than the relatively weak dipole displacement in -C-CH, yet in several cases the NH sub- stituent has been observed to produce shifts in aromatic hydrocarbons no larger than those produced by the at introduction the same position; of a methyl e.g. substituent the wave- 9 3 + c L - @ length and intensities of the maxima of 3-aminopyrene hydrochloride (XIII) and cyclohesadiene show marlredly and 8.4 ev.respectively) and absorb at relatively long wave-lengths compared with open-chain dienes and it was to explain this characteristic that Mullikcn independently invoked the concept of hyperconjugation. However T. M. Sugden and A. D. Walsh 38 obtained values of 8.71 and 9.02 ev. respectively for the ionisation potentials of s-cis- and s-trans-forms of butadiene and pointed out that the problem arose mainly because of the comparison of cyclic dienes with the spectra of predominantly s-trans-butadione. Walsh maintains that when the more satisfactory comparison of cyclic dienes with s-cis-open-chain dienes is made the changes in the ground states of the cyclic dienes may be explained as being due entirely to strain and charge-transfer effects.Further applications of the concept of hyperconjugation to spectra are discussed by E. A. Fehnel and M. Carmack 39 and by I. M. K l o t ~ . ~ * + ‘7 and 3-methylpyrene (XIV). / lower first ioilisation potentials (8.62 (XI11 .) (XIV.) \ (C) Cyclic dienes. cycZoPentadiene 37 Chem. Reviews 1943 32 1. 38 Trans. Faraday SOC. 1945 41 76. 40 Ibid. 1944 66 88. J . Amer. Chem. Xoc. 1949 71 84. 240 QUARTERLY REVIEWS 6 C- I H* H M. Szwarc 41 determined the C-H bond energy in toluene and the xylenes from pyrolysis experiments and found the weakest bond to be the C-H bond in the methyl group. The energy of that bond was found to be 77.5 Bcals. for toluene and m-xylene 75 for p-xylene and 74 for o-xylene. The data were taken to indicate that hyperconjugation in p-xylene decreases the C-H bond energy in the CH group by 2.5-3 kcals.the weakening of the C-H bond in the methyl group of p-xylene as compared with that in toluene being expected on the basis of hyperconjugation occurring for example in the p-x~dyl radical as in (XV). Tho pyrolysis of the three fluorotoluenes gave a value for the C-H bond energy very nearly equal to that obtained for toluene.42 Since therefore the field effect has very little in- fluence on the bond energy this result was regarded as sup- porting the suggestion that hyperconjugation is responsible for the weakening of the C-H bond of the CH in p-xylene. Recent kinetic studies have indicated that hyperconjaga- .I tion might be of some significanco in the oxidation of hydro- (xv.) c a r b o n ~ . ~ ~ R. S. Mulliken and C. C. J. Roothaan 4 4 9 45 have given a theoretical discussion of the twisting frequency and barrier height for free rotation in ethylene and find that a 90" rotation of the two parts of the molecule does not entirely destroy tho TC bond.When the two CH groups have been twisted through 90" relatively to each other a kind of hyperconjugation must exist between the x unsaturation electron of each CH group and a pair of quasi-unsaturation electrons y involved in C-H binding in the other CH group. Tho unsaturation electrons which would have no bonding power in " perpendicular " ethylene contribute somewhat to the bonding because of this hyperconjugation and the barrier to free rotation must be lowered thereby. As far as the methylothylenes are concerned it might be thought that hyperconjugation would tend to stabilise certain orientations of the CH group bLit this is probably not so ; for only tho four conjugating electrons in propylcne H,=C-C-C being considered and other electrons and also the other hydrogen atoms being neglocted tho energy of the system is unchanged for successive rotations of the CH group by 60" around its axis and is probably very little if at all changed for rotations through intermediate angles.In ethane also hyperconjugation has been shown to have little or no direct effect in rostricting free rotation.ls Molecular Refractivities.-In the alkylbenzenes there is a progressive increase in exaltation with increase in the number of methyl groups and furthermore the exaltation increases with increasingly symmetrical dis- tribution of the methyl groups. In aliphatic systems e.g.substituted 4 1 J. Ghem. Physics 1948,16 128. 4 2 M. Szwarc and J. S. Roberts ibid. p. 609 4 3 C. F. Cullis C. N. Hinshelwood and M. F. R. Mulcahy Proc. Roy. SOC. 1949 45 R. S. Mulliken Physical Rev. 1932 43 301. A 196 160. 4 4 Chena. Reviews 1947 41 219. CRAWFORD HYPERCONJUGATION 241 butadienes changes in exaltation may be due to changes in the shapo of the molecule but since this is hardly possible here R. S. Mulliken 46 regards hyperconjugation as being the constitutional cause of the observed increasing exaltation. It has been shown that equilibrium and kinetic studies provide chemical evidence for the tautomeric electron displacement in alkyl groups and it was in the field of kinetics that the first unambiguous evidence such as a well-spaced and complete sequence of rate constants substantiated by corresponding activation energies was obtained by E.D. Hughes C. K. Ingold and N. A. Taher.47 Furthermore it was pointed out why the effect had not clearly been observed before in such studies. Thus in reaction rate for example the initial and the transition state must be treated in a differential manner and since each will be subject to both inductive and mesomeric displacement the net result dopends upon a com- bination of four effects. In this way the clectron displacement is classified as a polarisation Le. a mesomeric effect if the unsaturation permitting alkyl conjugation is present in the initial state but as a polarisability i.e. an electromeric effect when the conjugation is either only present or enhanced in tho transition state.Because of these complications it becomes clear why the complete inversion of the inductive-effcct order had not previously been obtained. Indeed the extreme rate range H Me was only 1 1-66 in Baker and Nathan's experiments and moreover since the reaction was a bimolecular nucleophilic substitution witJh only a small electron demand because of mutually accommodating electron transfers no clear result should have been obtained. Hughes Ingold and Taher studied the unimolecular hydrolysis of p-alkylbenzhydryl chlorides which is a strongly eloctron-demanding reaction involving only a single electron transfer. Thus the factor involving alkyl conjugation swan ps the others and the following unequivocal results were obtained x = - ~~ lo% . . E kcals. . I I I I €1. 1 Me. 1 Et. ~ Pri. 1 But.2.82 63.5 62.6 46.95 35-9 21.0 1 18.9 I 19-4 1 19-8 1 20.05 More recent rate data concern the competitive bromination of toluene and tert.-b~tylbenzene,*~ which yielded relative rates of 4 1 a result readily explicable on the basis of hyperconjugation. The relative rates of chlorination of a series of alkylbenzenes also show the operation of hyperconjugation. 49 For tert.-butylbenzene and benzene 60 tt relative rate of 115 1 was obtained a result which cannot be accounted for by hyperconj ugation involving hydrogen atoms and therofore the 46 J . Chem. Physics 1939 7 356. 47 J . 1940 949. 48 E. Berliner and F. J. Bondhus J. Arner. Chem. Suc. 1946 68 2355. 49 P. B. do la Mare and P. W. Robertson J. 1943 279. 5O E. Berliner and F. J. Bondhus J. Amer. Chem. SOC. 1948 70 854. 242 QUARTERLY REVIEWS difference in rates was explained as being due to release of electrons from the tert.-butyl group through structures of the type (XVI) contributing to the resonance hybrid.The experimental data however are not sufficient to warrant such a postulate for partial rate factors were not determined. Furthermore the order of electron release by this mechanism is the same as that due to the inductive effect and thus would be created the difficulty of two opposing orders based on the same effect.50 Anionotropy.-The concept of hyperconjugation has also been used by A. G. Catchpole E. D. Hughes and C. K. Ingold 51 to account for equilibrium in anionotropic systems that isomer being preferentially formed in which the methyl group is hyperconjugated with the double bond.The crude approximation being made that only the energy of hyperconjugation makes a direct contribution to the free energy of one of the isomerides and this being talien to be 3 kcals. for the gem-dimethyl group the system ( CH3),C:CR-CH,I3r + (CH,),CBr*CR:CH + H,C-C-cH C"3 - 6 (XVI.) a t 300" K. should contain 1 % of the aa-dimcthylallylbromide. present is known to bo experimentally undetectable. A similar calculation being made for the system The amount CH,*CH:CH*CH,Br + CH,-CHRr.CH:CH which has been experimentally investigated the following results were obtained 7 :; 1 12 13 :i 1 15 Temp. OK. 273 373 In this table y and a refer to y- and a-methylallyl bromide respactively. the agreement being fortuitously better than could have been expected. Prototropy.-With regard t o t hI ee-carbon prototrop y hyper conjugation has been used to account for the following facts.52 A single activating group in three-carbon systems does not much influence the equilibrium between a@- and By-unsaturated forms whereas alkyl substituents in certain positions do quite markedly influence the ratio in which the isomers are formed.For instance in the case of an unsubstituted y-position the ap-unsaturated form is the one present a t equilibrium. As a result of introducing a methyl group in the y-position the equilibrium is shifted to the side of the By-isomer which is now the important form and 51 J . 1948 8. 5 2 P. B. ds la Mare E. D. Hughes and C. IS;. Ingold ibid. p. 17. CRAWFORD HYPERCONJUGATION 243 y-Substituent. H. 1 GH,. Et. I 1 - ____. Py-Form present at equilibrium yo 1 0 1 9 3 67 .CHNe,. 51 Also pentenoic acids and their methyl derivatives show that an a-methyl substituent does not favour the production of &forms which is the opposite effect to that produced by a y-mothyl substituent. This is well illustrated in the following table Acid. -~ Py-Form at equilibrium yo . _______ 94.4 - 24.6 19.3 The abovo facts may be explained as follows. The ap-form preponderates in the non-y-substituted compounds such as (XVII) for in that isomer there CH,*CR :CHCO,H CH,CH,*CR:CH*CO,H (XVII. ) (XVIII. ) is conjugation between the olefinic bond and the activating group whereas there is no such conjugation in the &form. Consequently hyperconjuga- tion between the @-substituent and the double bond produces only second- order effects. I n the y-methyl compounds (XVIII) the hyperconjugation energy of the y-methyl group with the double bond in the &-form compensates quite appreciably the conjugation energy in the ap-form.That the methyl group should be more effective is characteristic of alkyl conjugation. The data relating to pentenoic acid and its methyl derivatives are readily explained for there is hyperconjugation between the y-methyl and the By-double bond whereas the hyperconjugation between the ap-double bond is offset by conjugation between that bond and the activating group. Ingold has also explained on the basis of hyperconjugation data relating to the proportions in which ap- and ad-dihydro-compounds are formed by reduction of vinylacrylic acid and certain methylated derivatives. Halogen Addition.-Recently halogen additions to conjugated un- saturated systems have been explained on the basis of hyperconjugation as exemplified by the systems n n C=C-C-HaL a n d H-C-C-HaL For instance the equilibrium mixture obtained when bromine is added to 244 QUARTERLY REVIEWS butadiene contains 80% of the ccd-dibromide a result regarded as a mani- festation of the doubled bromine hyperconjugation in this compound whereas in the ap-isomeride there is only a single halogen hyperconjugation CH,Rr*C€I:CHCH,Br + CK,Br*CHBrCH:CH (80%) (20%) In 8- and y-allrylated butadienes alkyl hyperconjugation is also of importance and so in /3-methylbutadioneY since yb-addition is excluded by C initiation only the ab-compound CH,X-CMe :CH*CH,X of remaining possible addition compounds maintains the hyperconjugation of the methyl group.Since halogen hyperconjugation reinforces alkyl hyperconjugation the ab-dihalide may be expected t o preponderate a t equilibrium a conclusion verified by experiment .53 Elimination.-The Saytzeff rule namely that in elimination reactions of sec.- or tert.-alkyl halides with alkali the most alkylated ethylene is formed is readily explained as being govorne'd by electromeric electron displacement. In the transition state the quasi-unsaturation electrons of the alkyl group hyperconjugate with the unsaturation electrons of the developing double bond. Since such conjugation creates a system of lower energy the elimination is facilitated by the development of the latent hyperconjugation. l3 Experimental investigations in which alkyl groups are conjugated with unsaturated olefinic bonds have normally been confined to cases where there is conjugation between C-H bonds and an aromatic nucleus.No such restriction is applicable theoretically and so conjugatioii of a methyl group with a double bond should result in increased reactivity of a-methylenic hydrogen atoms. Recently J. W. Baker 64 carried out the Prins reacbion with propylene and obtained as products (i) the diacetate of n-butane- 1 3-diol (64%) (ii) the cyclic formal of this diol (la%) (iii) 4-acetoxy- tetrahydro-y-pyran (22%). Acid-catalysed addition to the double bond resulted in the first two compounds being formed whereas the third involves the direct reaction of the hydrogen of the methyl group activated by its conjugation with the olefinic linkage. Further evidence for such cc-methylenic activity in olefinic and poly- olefinic systems will be found in references 55 56 and 57. I am indebted to Prof. C. A. Coulson for suggesting to me the investigation of the subject here reviewed and for friendly discussions. Prof. Coulson also read the manu- script and made valuable criticisms and suggestions which have helped to overcome certain infelicities of presentation. Dr. D. H. R. Barton checked the manuscript and this service as well as beneficial comment is warmly acknowledged. 6 3 W. J. Jones and H. G. Williams J . 1934 829. 5 4 J . 1944 296. 5 5 E. H. Farmer Trans. Paraday SOC. 1942 38 340. sE E. H. Farmer G. F. Bloomfield A. Sundralingham and D. A. Sutton ibid. p. 348. 67 E. H. Farmer {bid. p. 356.