2048 DAWSON AND WHEATLEY: REACTIVITY OF KETONES TOWARDSCCXV.--The Reactivity of Ketones towards Iodine andthe Relative Rates of Tautorneric Change.By HARRY MEDFORTH DAWSON and ROBERT WHEATLEY, B.Sc.IN a previous paper (Dawson and Leslie, Trans., 1909, 95, 1860) ithas been shown that iodine reacts readily with acetone at theordinary temperature, and that when aqueous solutions containingacetone and iodine are acidified by addition of a mineral acid, thereaction proceeds at a rate which is suitable for accurate quantitativemeasurements. These observations showed that when the acetoneis present in considerable excess, the rate of disappearance of theiodine is practically constant from the commencement until nearthe end of the reaction, the velocity being proportional to theconcentration of the acetone and of the added mineral acid.To account for these facts, the view was adopted that the reactionbetween acetone and iodine involves two stages.In the first ofthese, the acetone is converted from the ketone into the enolic form,and this is then acted on by the iodine at a relatively very rapidrate with the formation of iodoacetone. In consequence of the verygreat difference in the velocities of the consecutive reactions, therate of disappearance of the iodine is determined solely by the rateat which the acetone is transformed into the enolic form (compareLapworth, Trans., 1904, 85, 30).I f this is the correct interpretation of the facts, it is evident thatthe investigation of the velocity of reaction of iodine with otherketones should lead t o information respecting the rates at whichthe various ketones undergo tautomeric change.With this objectin view, experiments were made to compare the rates of dis-appearance of iodine in dilute acidified solutions of dimethyl, methylethyl, methyl propyl, methyl butyl, methyl hexyl, phenyl methyl,diethyl, and phenyl ethyl ketones. On account of the small solu-bility of certain of these ketones in water, the comparative measure-ments could not be made in aqueous solution, and instead of this, anaqueous alcoholic solution containing forty volumes per cent. of ethylalcohol was employed. In this connexion experiments were madeto ascertain the influence of the medium on t.he reaction betweenacetone and iodine in alcohol-water mixtures ranging from purewater to pure alcohol.The results of this investigation will becommunicated in a further paper. For the present it is sufficientt o state that the reaction between the two substances is of thesame character in alcoholic as in aqueous solution, and that, fora given concentration of acetone and acid, the rate of disappearanceof iodine is practically the same in alcohol-water mixtures as it iIODINE AND THE RELATIVE RATES OF TAUTOMERIC CHANGE. 2049in pure water, if the amount of alcohol present does not exceedsixty to seventy volumes per cent. The only difference to be notedis the displacement of the final equilibrium as the ratio of thetwo components in the solvent is gradually altered.This influenceof the medium is shown by the data in table I, which were obtainedin parallel experiments with solutions containing 0, 20, 40, 60, 80,and 100 volumes per cent. of alcohol. The temperature at whichthese measurements were made wits 25'1O.TABLE I.Initial iodine concentration = 0.0095 ; acetone = 0.272 * ;H,W, = 0.1 mol. per litre.alcohol . . . .. . . . . . . . 0 20 40 60 80 100concentration ... 0*000005 0'000015 0*00040 0.00100 0-00225 OvO0442changed iodine.. 0.05 0.15 4.2 10.5 23.7 46-5* This concentration corresponds with 20 C.C. of acetone per litre.These results show that the limit of the reaction is reached at aprogressively earlier stage as the proportion of alcohol in the solventincreases, and the range of the velocity measurements is corre-spondingly diminished.In presence of potassium iodide, thereaction takes place stJll less completely. I n the case of the firstthree solutions in the above table, the percentage of unchangediodine was found to be 0.8, 2.3, and 16.9 respectively whenpotassium iodide was present to the extent of 0.02 mol. per litre.This effect of the iodide is no doubt due to the removal of iodinein consequence of tshe formation of polyiodides. In choosing a40 per cent. alcoholic solution as the medium for the reactingsubstances, these circumstances were taken into consideration, andthe alcohol-water mixture in question represents the smallest pro-portion of alcohol which is necessary for the solution of the majorityof the ketones in the requisite concentration.The limit of the reaction in the aqueous-alcoholic solution varieswith the nature of the ketone.This is evident from a considerationof the data in table 11, which gives the results obtained for acetone,diethyl ketone, and acetophenone. In these and all other experi-ments the relative concent,rations of ketone and iodine were suchthat the active mass of the ketone could be regarded as constantduring the course of the reaction. Similarly, the concentration ofthe sulphuric acid was so large that no appreciable alteration inits value resulted from the formation of the hydriodic acid duringthe reaction.The ketone was weighed out into a graduated stoppered flaskVolumes per cent.Equilibrium iodinePercentage of un2050 DAWSON AND WHEATLEY: REACTIVITY OF KETONES TOWARDScontaining water and measured quantities of alcohol and standardsulphuric acid solution.The flask was placed in a thermostat at25'1°, and after'some time a known quantity of iodine was addedin the form of an aqueous potassium iodide solution, and the contentswere made up to the mark with water. After definite time intervals,portions of the solutions were removed, added to excess of a sodiumhydrogen carbonate solution, and titrated with a freshly preparedO'OlN-solution of sodium thiosulphate.In the following tables z1 is the observed iodine concentration inmols. per litre, and x2 the concentration calculated from the equationx z = x o - k t , in which xo is the first observed concentration (t=O),and k the velocity constant=dz/dt. The concentrations of theketone and the acid and the original concentration of the iodine aregiven in every case in mols.per litre.TABLE 11.Acetone.Acetone = 0-1886 ; H,SO, = 0.1 ; iodine = 0*0101.Time(minutes) 0 25 45 65 85 105 140&:,.lo4 91.8 77.8 67'4 57.2 46'9 37.9 27'5x2104 (91'8) 78.3 67'5 56.7 45.9 35'1 16'2k =0*000054.Diethtyl Ketone.Ketone = 0-2532 ; H,SO, = 0.1 ; iodine = 0.0102.Time (minutes) 0 30 60 80 105 120 135x,.lO4 90'4 72.7 54.7 43'0 28.5 20.4 12.9x,.lO4 (90.4) 72.7 55'0 43'2 28'4 19.6 10.8k=0*000059.A cetophenorce.Ketone= 0.1673; H,SO, = O * l ; iodine=0-0101.Time (minutes) 0 15 51 75 105 135 165 255x,.104 98.3 95.6 89,2 84.8 79.3 74.5 70.2 57.7%.lo4 (98'3) 95.6 89.1 84.8 79'4 74.0 68.6 52'4k=OmOOO018.24 hours20.324 hours2.2 -24 hour42.4-From a comparison of the equilibrium iodine concentrationsrecorded under t =24 hours, it is evident that the extent t o whichthe reaction proceeds is dependent on the nature of the ketone insolution.Although the above three experiments are not strictlycomparable because of the differences in the ketone concentrations,it is seen that in the acetone, diethyl ketone, and acetophenonesolutions the unchanged iodine amounts to about 20, 2, and 40 percent. respectively. Of the ketones examined, the reaction proceedIODINE AND THE RELATIVE RATES OF TAUTOMERIC CHANGE. 2051furthest in the case of diethyl ketone, and to the smallest extentin the case of acetophenone, and on this account data are recordedfor these two substances, as well as for acetone, which show theprogress of the reaction throughout the greater part of its course.On comparing the values of x1 and x2, it is seen that in the caseof acetone, the rate of disappearance of the iodine is constant untilabout 50 per cent.of the iodine originally present has reacted, andthat the velocity then diminishes as the equilibrium condition isapproached. For diethyl ketone the velocity remains constant untilabout 75 per cent. of the iodine has disappeared, whereas, in thecase of acetophenone, the velocity shows distinct evidence ofdiminution when about 30 per cent. of the iodine has reacted.The point at which the velocity begins to fall is obviously determinedby the proportion of the original iodine which remains when thecondition of equilibrium has been reached.The more complete thereaction, the greater is the range over which the reaction velocityremains constant.In the communication of further results this circumstance is takeninto consideration, and the measured velocities relate solely to thatpart of the total reaction in which the iodine disappears at aconstant rate. Table I11 contains the data obtained in experimentswith other ketones, the calculated iodine concentrations being placedalongside the observed values, as in table I1 :TLLBLE 111.Methyl Ethyl Ketone.Ketone = 0.174 ; H,S04 = 0'095 mol. per litre.854 12966.1 34-431 *257!i 52.9xp 104 (94.8) 79.8 66.3 52.5Time (minutes) 0 30i9.104 94.8 79 '4k=0*0000495.Methyl Prowl Ketone.Ketone = 0.1727 ; H2S04 = 0.1 mol.per litre.XI. 104 92.1 78.0 69.8 64.0 47'6q. 104 (92'1) 78.0 70.0 63'9 47-5k=O *0000453.Time (minutes) 0 30 47 60 95Methyl Bzltyl Ketone.Ketone = 0-1678 ; H,SO, = 0.1 mol. per litre.x,.104 89.5 76.3 66.2 58.1 45.5Time (minutes) 0 25 45 60 85x2. 104 (89 -5) 7 6 4 66 .O 58'1 45-1k= 0.00005232052 DAWSON AND WHEATLEY: REACTIVITY OF KETONES TOWARDSMethyl Hexyl Ketone.Ketone = 0.0814 ; H,SO,= 0.1 mol. per litre.Time (minutes) 0 30 70 100 130XI. 1 0 4 73 3 65'8 55.7 48 -3 41 '2x,. 104 ('13.3) 65-8 55.8 48'3 40.8k = 0.000025.Phenyl Et h8yl Ketone.Ketone = 0-0855 ; H,SO, =0*251 mol. per litre.Time (minutes) 0 120 235 350 4 $0 565XI.104 96.6 90.0 83 '6 78 *3 71.8 67 '4%. 104 (96.6) 90'4 84'4 78.4 71.6 67 '2I n addition to the above experiments others were made in whichthe concentrations of the various ketones and of the acid wereapproximately doubled or halved. These indicate that the variousreactions take place at a rate which is proportional to the ketoneand acid concentrations, as was found to be the case for the actionbetween iodine and acetone in aqueous solution. Apart from theconstancy of the reaction velocity, the fact that the influence ofketone and acid concentration is the same for all the ketonesexamined affords strong evidence in support of the view that thecause of the uniform speed of the reaction is the same in all cases,that is to say, a change in the ketone from the ketonic to the enolicform.On the basis of the observed proportionality between the velocityand the ketone and acid concentrations, the several values obtainedfor the speed of the reactions may be reduced to a uniform ketoneand acid concentration (ketone = 1 / 6 mol. per litre ; sulphuricacid=0.1 mol.per litre). I n this way the velocities of reactionrecorded in the second column of table IV are obtained. Thesenumbers may be taken as representing the relative rates at whichthe ketonic forms of the various members of the series are convertedinto the corresponding enolic forms. The velocit.ies with referenceto acetone as standard are given in the third volume.k=0'0000052.TABLE IV.k (niols. per litreAcetone .................................48 x I 0-6Methyl ethyl ketone ............... 50 xMethyl propyl ketone ............... 45 xMethyl butyl ketone ............... 53 xAcetophenone ........................ 18 x loF6Diethyl ketone ........................ 39.5 xPhenyl ethyl ketone ............... 4 *O xKetone. per minute).Methyl hexyl ketone ............... 51 xRe1 ativek values.11'040.941-101 '060 *370 '820.08IODINE AND THE RELATIVE HATES OF TAITTOMERIC CHAKUE. 2053From the above values of k, it is seen that the replacement ofone of the methyl groups in acetone by ethyl, propyl, butyl, orhexyl does not cause very much alteration in the rate at whichthe substance reacti9 with iodine. On the other hand, the reactivityis reduced to nearly one-third when the methyl group is replacedby phenyl.For diethyl ketone the velocity is only about 20 percent. smaller than in the case of acetone, and substitution of phenylfor one of the ethyl groups reduces the reactivity to about one-tenth.On the assumption that the measured velocities are determinedby the respective rates of tautomeric change, it is not surprisingthat the first five ketones should be found to react with iodine atapproximately the same rate, for in each case the transformationinvolved may be supposed to be that represented byOn the other hand, the fact that the reactivity of diethyl ketoneis nearly as great as that of the methyl ketones would seem toshow that the change represented byCH,*CH,*CO*R -3 CH,*CH:C(OH)*Emay take place almost as readily as the previous one.That thisapproximate equality of the rates of change of the groups CH,*CIO*and CH,*CH,*CO* is not general, however, is evident from a com-parison of the values for acetophenone and phenyl ethyl ketone.As shown by experiments with benzophenone, the phenyl groupdoes not react a t all with iodine under the conditions of the dynamicexperiments, and it might be expected that the ratio of thereactivities of these two substances would be the same as the ratiofound in the case of acetone and diethyl ketone. This is not thecase, the observed velocity of reaction in the case of acetophenonebeing about four and a-half times as great as that found for phenylethyl ketone.I n other words, the relative rates at which thegroups CH,*CO* and CH3*CH,*CO* undergo tautomeric change isdependent on the nature of the radicle with which the two groupsare combined. On the other hand, the approximate equality ofthe rates of reaction of dimethyl and diethyl ketone with iodineleads us to anticipate that methyl ethyl ketone will react with iodinein two ways, which are determined respectively by the tautomericchanges :CH,*CO*CH,*CH, -+ CH,:C(OH)*CH,*CH, andCH,*CO*R -+ CH,:C(OH)*R.CIJ ,*CO*CH,-CH, -+ CH,*C(OH):CH*CH,.The amounts of the corresponding iodo-substitution products willbe conditioned by the relative rates at which these two changestake place. On the basis of the results which have been obtainedwith the aliphatic ketones, it appears probable that aliphatic ketone2054 TUTIN : THE CONSTlTUTION OF ERIODICTP OL,will in general give rise to two substitution products, for thevelocities of the two possible tautomeric changes are apparently ofthe same order of magnitude.Preliminary experiments have been made which show that certainaldehydes react with iodine in a similar manner to that observedfor the various ketones investigated in this paper. This is the casefor propaldehyde, whereas the kinetic investigation of the reactionbetween acetaldehyde and iodine indicates that the mechanism ofthis change is of a different kind. This question is being furtherinvestigated.The chief results obtained in this inquiry are :1. Evidence is adduced to show that the mechanism involved inthe reaction of iodine on various ketones is of the same kind, theprogress of the change being determined by the rate at which theenolic form of the ketone is formed.2. From the measurement of the velocities with which the ketonesreact with iodine, the relatdve rates a t which the ketones undergotautomeric change have been obtained.PHYSICAL CHEMISTRY LABORATORY,THE UNIVERSITY,LEEDS