Acharya, R. C.

Mohanty, S. B.

Nanda, C. N.

Rout, M. K.

English

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Indian Journal of ChemistryVol. 14A; January 1976, pp. 42-44Kinetics & Mechanism of Oxidation of Benzoin & Its Derivativesby TICIII) in Aqueous Acetic AcidS. B. MOHANTY, R. C. ACHARYA, C. N. NANDi\- & M. K. ROUTDepartment of Chemistry, Raven shaw College, Cuttack 3Received ~ November 1974; accepted 19 February 1975The kinetics of oxidation of benzoin and its derivatives by Tl(III) in aqueous acetic acid havebeen investigated. The reactions obey second order kinetics. The observed rate decreases withincreasing [TI(III) acetate]. Tl(OAc)~ is found to be the most electrophiIic species in aq. aceticacid. Activation parameters have been calculated and the structure reactivity relationships dis-cussed. A mechanism involving oxythallation-dethallation is suggested.THE kinetics of Tl(III) oxidation of a numberof organic substrates, viz, olefins-->, acety-lenes", arylcyclopropanes'', alcohols", ketones"and arenes? have been studied by several workers.It appears that Tl(III) oxidation of aromaticketo-res has not been i-ivestigated. Hence, for thepresent study we have chosen benzoin and itsderivatives,Materials and MethodsThallic oxide (BDH) dissolved in hot acetic acid(GR) and the solution filtered to give a stocksolution of thallic acetate which was standardizediodometrically. Benzoin and its derivatives wereprepared in the laboratory. Demineralized distilledwater was used for preparing the solvent mixtures.All other chemicals used were of either analar orreagent grade.The reaction was initiated by mixing equalvolumes of the equilibrated reactant solutions in.definite acetic acid-water mixtures. The experi-ments were conducted at constant ionic strengthand acidity. The reaction was followed by estimat-ing the rate of disappearance of Tl(III) by iodometry-at different time intervals. The ketone was alwaystaken in sufficient excess over Tl(III) so that theobserved kinetics were pseudo-first order and a parti-cular run gave a linear plot for about 70% of thereaction. The duplicate rate measurements werereproducible to ± 3%.Results and DiscussionStoichiometry - The results for Tl(III) oxidationof benzoin conform to 1: 1 stoichiometry. The pro-duct of oxidation, benzil, has been identified bymixed m.p. and IR spectra. The reaction can berepresented asOH 0 0 0, \I \I \IPh-CH-C-Ph+TI(OAc)3-Ph-C-C-Ph+TIOAc+2HOAc42Tl(III) oxidation of. benzoin has been studiedin greater detail and the results of the kineticexperiments at fixed compositio.is of acetic acid-water mixtufes (v (v) at coistant acidity and io+icstrength are recorded in Table LThe order with respect to Tl(III) 8.~.dberzoin hasbeen fouid to be one each. The first order ratecO.1s.tant for the dis~ppearance of Tl(III) at a givenacidity a~d [aromatic ketone] has been found to de-c:ease WIth 1ll1cre2S111g[Tl(III)J. Simi18.r observa-tlO?S ~ave also ?~e;1 ~nade in the case of Tl(III)oxidatio.i of fenzllic acid and a-phenylethyl alcohol".However, this type of behaviour has pot beenobserved in the case of aliphatic ke to-es (unpublishedwork). Second order rate constant has been fOll,dt.o be 9·1 X 1~-4 mole+ secl from the slope of theIme~r plot of ko.bS Versus [benzoin]. The empiricalrate law co.iforrnir-g to these orders may be written as:- d[Tl (III) ] - d[keto.'1e ]dt = at = k[TI(III)J[ketone]Acid dependence - The data given in Table 2show. that. the rate.is .first order with respect to [per-chloric acid]. A similar rate dependence on [acid]JTABLE 1 - DEPENDENCEOF REACTIONRATE ON [TI(III))AND [BENZOIN]{[HCIO.]=l·071M; solvent: 90% HOAc-H20 (v/v);!J.=H24M; temp.=30oq[TI(III)] [BENZOIN] k X 105xl03M xl01M seer!6'23 2'0 23·06'23 2·4 26·86'23 3'0 32'66'23 3·4 36·36'23 4·0 41·76'85 2·0 17'67'08 2·0 14·28'31 2'0 11·68'54 2·0 8·38'76 2·0 5'2MOHANTY et ai.: TI(III) OXIDATION OF BENZOIN & ITS DERIVATIVEShas been observed in the case of Tl(III) oxidationof a-hydroxy acid.. Effect of solvent composition - The reaction rateIncreases with the increase in acetic acid contentin the solvent mixture. The values of rate constant(k X 105 see-I) in the benzoin oxidation are foundto be 10'7, 13·5, 16·8, 20·7 and 23·0 for 70, 75, 80,85 and 90% acetic acid.The plot of log k versus reciprocal of dielectricconstant has been found to be linear with a positiveslope indicating a positive ion-dipole reactions. Con-sidering a monopositive thallium species as the activeoxidant, the value of r, the minimum distance ofapproach between the ion and the dipole, hasbeencalculated from the slope of the above plot and foundto be 0·96 A. This value is of expected magnitude,Effect of added salt - Addition of either sodiumsulphate or sodium perchlorate seems to havenegligible effect on the reaction rate. However,the rate has been found to decrease significantly onadding sodium chloride or sodium acetate (Table 3).This may be due to the formation of less electrophilicTABLE 2 - ACID DEPENDENCE ON REACTION RATE([TI(III)] =6,23 x 10-3M; [benzoin] =2 X 10-1M; solvent: 90%HOAc-H20 (v/v); temp.=30°C1[HClO.] k X 10· k X loaM see'? [HeIO.]mole:" sec10'65 14'9 0·231-07 23·0 0'221'61 36·3 0·231'86 42·7 0·232·14 49·2 0'232·45 55·7 0'23TABLE 3 - DEPENDENCE OF RATE ON ADDED SALT([Tl(III)] =6'23 X 10-3M; [benzoin] =2 X 10-1M; [HClO.l=l'071M; solvent: 90% HOAc-H20 (v/v); temp.=30oqAdded [Salt] k X 10·salt x10'M sect23·2NaClO. 1·6 23·1Na2SO. 2·0 22·8NaCI 0·2 9·4NaOAc 2·0 11·3and more covalent species of TI(III), viz. Tl(OAc)2'Tl(OAc)2Cl or Tl+C12 (refs 9-11).Nature of Tl(III) species - Since the oxidationhas been carried out in aq. acetic acid, the followingspecies of Tl(III) are expected to be involved in theoxidation: Tl(OAch, Ti(OAcH, Tl(OAc)4' Tl(OAc)tOA·~- and TJ2(OAcJ4, The io.i pair Tl(OAc)~OAc-and the double salt Tl2(OAc)4 have been shown toplay insignificant role In redox reactions in aceticacid mediums. The possibility of Tl(OAc);i partici-pating in the reaction can be ruled out since it isle.is reactive and a negative ion is not warranted inthis reaction. Although Tl(OAc)a has been sug-gested- to be the active species in redox reactions,the rate depe.idence 0_1 dielectric constant of themedium suggests a positive thallic ion species as anactive oxidant in the present study. Tl(OAc)~ seemsto be the most active electrophilic species in theoxidatio.i of benzoins by D(III) in aq. acetic acidmedium. This has also been show.i to be theimportant reactive species in the oxidation ofolefins-.Struct-ure-reactivity relation - The reaction ratesfor all the benzoins have been measured at fourdifferent temperatures ranging from 30 to 45°.Arrhenius equation has been found to be valid withinthe temperature range studied. The rate data forthe substituted benzoins are recorded in Table 4 andthe reactivity conforms to the following order:P-OCHa > p-CHa > P-Cl > H > p-p' -dinitro.Nitro substituent retards the reaction in con-sonance with electronic effect-s. p-Chloro-substitutedcompound, however, exhibits an anomalous be-haviour as its reactivity is higher than that ofunsubstituted compound. This may be due to thepredominance of electrorner ic effect at the instant ofthe reaction. The entropy of activation values havebeen calculated by the use of Eyring's equation andthese indicate rigid tranaition states. The logarithmof the rate constants at 30° correlates well withBrown's 0'+ values. The value of fl+ was calculatedto be -1·00 which indicates an electron deficientcarbon centre in the transition state of the reaction.Mechanism - In oxidation reaction, a ketonemay react directly or through its enol form. Directoxidations1a-15 by cobalt ic, manganic and eerie saltshave been shown to involve one electron transferand seem to have a free radical mechanism. Oxida-tions proceeding through enol intermediates-v " havebeen reported for Tl(III) and its isoelectror.ic saltslike Pb(IV) and Hg(II). Since the enol form ofTABLE 4 - EFFECT OF VARIOUS SUBSTITUENTS ON REACTION RATE{[Tl(III)]=6'23xl0-aM [Benzoin]=2x10-1M; [HClO.]=1·071M; Solvent: 90% HOAc-H20 (v/v). tJ.=1-124M}Substituent kx10. see"! E -LlSt LlFt (30°)kcal mole'? cal kcal30° 35° 40° 45° deg.'? mole'< mole'<4-0CHa 64'7 86'9 112'8 160'3 10'0 41·6 22·04-CHa 64·7 67·9 88·6 118'7 10'2 41'5 22'24-CI 38·3 49·6 64·9 81-6 10'3 42·2 22-5H 23-0 30·1 40·2 51-1 11·1 40·7 22-82.2' -Dinitro 4'8 8·8 14'5 22·4 19·1 17·0 23·643I\INDIAN J. CHEM., VOL. 14A, JANUARY 1976OH 0I IIPh-CH-C -PhOH OHI IPh- C=C- Ph(±)Haq.HOAc\\\\\"(\OH OH ~\o-..tl OH OHI I Heterolysis I IPh- CH-C€""Ph . ~,P,h- C::;;C- Ph<, /..../6 H20 I I/ TI HO OH,~o 0n IIPh-C-C-PhScheme Ithe ketone contains a double bond, TI(III) saltis expected to undergo facile reaction with olefinsand acetylenes>", The oxidation of benzoin tobenzil via enediol finds further support from thestudy of the electrolytic oxidation of phenylacetylenest" where it has been concluded thatstilbenediol diacetate is the precursor to benzil.Formation of enol is, however, not a rate determin-ing step as the observed rate is not independent of[Tl(III)).TI(III) acetate essentially behaves as a two-electron oxidant and the experimental results showthat each a-C-H bond ruptures by a two-electrontransfer. Therefore, the oxidation of benzoin tobeizil in aq. acetic acid medium in the presence ofperchloric acid may be interpreted by invoking oxy-thallation of the enol form of the ketone followedby dethallation (Scheme 1).The above mechanism finds support from the factthat the stable oxythallation adducts of the type"- /H-O~C-C-TI(OAc)2 have actually been isolated/ "-in the oxidation of olefins2o,21 in aq. acetic acid.Tl(III) oxidation of alkenols in aq. HCl04 also shows44the formation of oxythallation adduct". These factsreveal that the oxidation of ketomethylene com-pounds through enol form would involve carbonthallium bond formation and this is supported bythe p+= -1·00 which indicates an elect ron deficientcarbon centre in the transition state. Organothal-lium derivatives of the type R-Tl(OAc)2 are knownto be highly unstable to solvolysis= and decomposeeasily to giVE[a diketone. A similar mechanism hasalso been envisaged for the Tl(III) oxidation of cyclicand aliphatic ketones studied by us.References1. HENRY, P. M., J. Am. chem . Soc., 87 (1965), 990,4423;88 (1966), 1597.2. BYRD, J. E. & HALPERN, J., J. Am. chem . Soc., 95 (1973),2586.3. McKILLOP, A., OLDENZIEL,O. H., SWANN,B. P., TAYLOR,E. C. & ROBEY, R. L., J. Am. chem, Soc., 95(1973), 1296.4. SOUTH (Jr), A. & OUELLETTE, R. J., J. Am. chem . Soc.,90 (1968), 7064.5. SRINIVASAN,V. S. & VENKATSUBRAMANIAN,N., IndianJ. Chem., 8 (1970), 57.6. MEENAKSHl, A. & SANTAPPA, M., Indian J. Chem., 11(1973), 3?3.7. (a) BRIODY, J. M. & MOORE,R. A., J. chem, Soc., Perkin I I(1972),179. (b) ELSON, I. H. & KOCHl, J. K., J. Am.chem . Soc., 95 (1973), 5060.8. AMIS, E., J. chem . Educ., 30 (1953),351.9. GUPTA, K. S. & GUPTA, Y. K., Indian]. cu«; 11(1973), 1285.10. SYKES, A. G., Kinetics of inorganic reaction (PergamonPress, Oxford), 1966, 147.11. HALVERSON,H. N. & HALPERN, J., J. Am. chem . Soa.,78 (1956), 5562.12. TAFT (Jr), R. W., Steric effects in organic chemistry,edited by M. Newman (John Wiley, New Yer k),1956, 59~.13. LITTLER, J. S., J. chem . Soc., (1962), 827, 832.14. BANERJEE, K. K., NATH, P. & BAKORE, V., Bull. chem .Soc. Japan, 43 (1970), 2027.15. VENKATAKRISHNAN,V. & SANTAPPA,M., Z. phys. Chem,(Frankfurt), 16 (1958), 73.16. KHANDUAI" N. C, SATAPATHY,K. K. & NAYAK, P. L.,Proc. Indian Acad . Sci. (A), 77 (1973), 163.17. ICHIKAWA,K. & TAKEUCHI,Y., Nippon Kagaku Zasshi,79 (1958), 1060.18 PATAI, S. <:t SHENFELD, L, J. chem: Soc. (B) (1966), 366.19. KOCHEL, \r. J., J. org. Chem., 32 (1967), 614.20. KABBE, H. J., Liebig's Ann., 656 (1962),204.21. KRUSE, W. & BEDNARSKI,J. M., J. argo Chem., 36 (1971),1154.22. KITCHING, W., Rev. Pure appl. Chem., 19 (1969), 1.

Kinetics & Mechanism of Oxidation of Benzoin & Its Derivatives by Tl(III) in Aqueous Acetic Acid

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Kinetics & Mechanism of Oxidation of Benzoin & Its Derivatives by Tl(III) in Aqueous Acetic Acid

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