KINETICS OF OXIDATION OF DEOXYBENZOIN BY CHROMIC ACID

KINETICS OF OXIDATION OF DEOXYBENZOIN BY CHROMIC ACID BY P. L. NAYAK AND N. C. KHANDUAL (Department of Chemistry, Ravenshaw College, Cuttack-3 and Department of Chemistry, G. M. College, Sambalpur) Received November 16, 1973 (Communicated by Prof. M. Santappa, r.a.ac.) ABSTRACT The kinetics of the oxidation of deoxybenzoin by chrcmic acid in 95% (vol/vol) aqueous acetic acid has been investigated. The reacticn rate is first order with respect to the oxidant as well as to the organic substrate. The reaction rate decreases in the presence of added Mn (II) ions. The presence of complexing agents like succinic acid, piperidine, etc. decreases the rate. The thermodynamic parameters for the oxidation have been computed. A mechanism proceeding through an enol intermediate has been suggested. INTRODUCTION Cm omic acid is one of the most versatile of the available oxidising agents reacting with all types of oxidisable groups.' Umeda and Tarama 2 as well as Best, Littler and Waters 3 have discussed the mechanism of oxidation of cyclohexanone by chromic acid. Rocek and Riehl 4 have reported that the aliphatic ketones are oxidised via their enol form. Very recently, we have reported5 some interesting features regarding chromic acid oxidation of aromatic ketones. This study is concerned with the kinetics and mechanism of the oxidation of deoxybenzoin by chromic acid, MATERIALS AND METHODS Deoxybenzoin was prepared and purified by the method of Billard and Dehn, m.p. 58 C. Acetic ac:d (BDH, AnalaR) was distilled over chromic oxide. Triple distilled water was used for dilution. All other chemicals were of AR grade. Kinetic measurements. The rate measurements were carried out at constant temperature (±O 10). The solvent used was 95% aqueous acetic 33 Acad.--A 3

34 P. L. NAYAK AND N. C. KHANDUAL acid (v/v) containing 0.1 M perchloric acid. The reactions were followed by withdrawing aliquots of the reaction mixture at known intervals of time arid quenching the reaction by adding slight excess of ferrous ammonium sulphate aid titrating the residual Fe (II) against standard K 2Cr 2O 7 using barium diphenylaminc sulphonate as an indicator. Diffused light did no affect the rate constants. The rate constants were computed with an accuracy of 1-2% in duplicate runs. Staichiometry and product analysis. The stoichiometry was determined by allowing reaction mixtures containing excess of chromic acid to star d for 3-4 days at 4Q 0 C. The unreacted oxidant was then estimated by the usual procedure. In the oxidation of deoxybenzoin to benzil three moles of the ketone undergo oxidation requiring four moles of chromic acid in accordance with the equation. 3 C6H5CH 2CO C 61-1 5 +4 H CrO4 + 16 H+ --+ 3 C 6H 5CO CO C 6H 5+ 4 Cr (III) + 13H 20. The TLC pure product isolated was characterised (IR, m.m.p) as benzil, m.p. 95 C. Rate laws. The reaction is first order with respect to the [ketone] as well as the [oxidant]( Tables I and II). The rate of the reaction increases with RESULTS TABLE I Dependence of rate on Chromium (VI) in 0.1 M Perchloric acid in 95 acetic acid at 40 C, [Deoxybenzoin] = 0.05M [Cr01] M kl x 105 sec-1 k2 x 10 2 M-' see-1 0.0015 6.2 4.133 0.0025 8.2 3.280 0.0030 8.8 2.934 0.0048 9.21 1.979 0.0056 9.21 1.644 0.0064 9.21 1.439 0.0120 9.30 0-775

Kinetics of Oxidation of Deoxybenzoin by Chromic Acid 35 TABLE II Deoxybenzoin dependence for the oxidation at 40 C. in 95% acetic acid [Chromic acid] = 0.004 (M): [HC1O4] = 01(M) [Deoxybenzoin] M.. 0.05 0.06 0.07 0.08 0.09 0.11 0.21 15 0 k1 jx 10 5 in sec-1.. 9.21 9.97 10.75 11.51 12.57 14.20 14.97 16.92 increase in [HC1O 4] and the first order rate constant is proportional to [I-I+]. Umcda and Tarama as well as Waters and coworkers 3 have investigated the kinetics of the chromic acid oxidation of cyclo-hexanone and found the rate to be first order with respect to the ketone, chromic acid and hydrogen ion. Very recently, similar observations have been found by Rocek and Riehl4 and Bakore and coworkers' for aliphatic ketone oxidaions and Nayak and coworkers on aromatic ketone oxidations. Effect of added Mn (II). The reaction rate decreases in the presence of addcd Mn (II) ions (Table VI). This is possibly due to the fact that Mn (II) zions catalyse the disproportionation of intermediate valence states of chromium. This also indicates that Cr (IV) is the active oxidising species. Since Mn0 2 does not accumulate in the reaction mixture, it may be reasonable to propose the following reaction in the presence of Mn (I1) ions. 3Cr (IV) + Mn (II) +HCrO4 + 2Cr (III) + Mn (III). The same argument has also been advanced by Radhakrishnamurty's Sengupta 9 and Bakore 1 while discussing the effect of Mn + 2 in these oxidation reactions. Influence of certain dibasic acids on rate The rate constants in the presence of added dibasic acids and bases are given in Table V. The rate of the reaction decreases in the presence of dibasic acids and organic bases. This can be explained if we consider that Cr (VI) gets reduced in the rate determining step to Cr (IV), assuming that most favourable geometry of the transition state will be a two-electron transfer. Any system which inhibits such a transfer is expected to decrease the rate of reaction. The same argument has also been advanced by Anantakrjshnan and coworkers."

36 P. L. NAYAK AND N. C. KHANDUAL Influence of temperature. The plot of logk vs. in-verse of temperature is a straight line. Arrherius equation is, therefore, valid for this reaction. The data on the effect of temperature on the rate of oxidation are given in Table III. TABLE III Influence of temperature on reaction velocity Temperature in 0 C. 30 35 40 45 k1 x 105 sec 1. 2.878 4.620 8.20 16.140 E in K. Cal/mole 21 70 AS" e.0 9.90 AH" K.Cal/mole 21.1 L F" K.Cal/deg mole 24 20 The enol intermediate. The rate equation for the oxidation of a ketona through an enol may be represented as follows: k g CrO3 Ketone + H+ Enol ---^ products k K k Thus d [CrO3] _ ke k [Ketone] [CrO3 ]' dt kk + k [Cr03 ] Where ke and kk represent the rate of enolisation and ketonisation respectively. The experimentally determined rate constants kl and k3 may be represented as follows: d [CrO. ]kl [ketone] dt = k2 [CrO3] [ketone] (2) kl = ke k [CrO3]/ kx + k [CrOs ] (31

Kinetics of Oxidation of Deoxybenzoin by Chromic Acid 37 and or ka = ke k^ [kk -f- kk (CrO3)] (4) 1 kk [CrO3] k 2 k ke (5) At low chromic acid concentration, the oxidation of enol will be the ratelimiting step and the reaction w;ll be first order both in ketone and chromic acid and at high chromic acid concentration enolisation will become rate limiting step and ki = ke. Thus the enolisation mechanism to be true, (i) Plot of kl vs. [Cr03] should give a curve approaching a constant value at high chromic acid concentration, and (ii) Plot of I lk 2 vs. [Cr03 ] should give a straight line. This has been verified by plotting kl vs. [Cr03] and 1/k 2 vs. [CrO3]. TABLE IV Acid dependence of the oxidation of deoxybenzoin by chromic acid at 40 C. [Deoxybenzoin] = 0.05 (M) [Chromic acid] = 0.004 (M) [HC1O 4] kl x 105 sec' H 0.150 9.96 0.46 0.200 13.05 0.60 0.225 16.12 0.66 0.250 19.19 0.730 0.275 22 26 0.78 0.300 25.33 0.83 0.325 28 24 0.90 0.350 30 24 0.97

38 P. L. NAYAK AND N. C. KHANDUAL Mechanism of the reaction. The rate of enolisation of deoxybenqoin was measured by bromination method. The rate of bromination was found TABLE V Effrct of complexing agent on rate Temperature = 40 C. [Chromic acid] = 00048(M); [Deoxybenzoin] = 0.05 (M) [Complexing agent] = 0.005 (M) ; [HCIO4] = 0.1 (M) Catalyst used kl x 10 5 sec -1 Nil.. 9.210 Succinic acid.. 7.658 Adipic acid.. 6.817 Diethyl amine.. 7.283 Pyridine.. 8.060 Piperidine 6 908 TABLE VI Effect of manganous ions on the oxidation of deoxybenzoin Temperature = 40 C. [Chromic acid] = 0'004(M); [Deoxybenzoin] = 005(M); [HCIO 4] = 01M [MnSO,] M k1 x 105 sec-' Solvent 95N acetic acid Nil 9.2 0.005 4.222 0.001 3.664 0.0015 3.453 0 002 3.262

Kinetics of Oxidation of Deoxybenzoin by Chromic Acid 39 to be first order w;th respect to the ketone and zero order with respect to bromine. The rate constant at 40 C. was computed to be kedo = 12.74 X 10-5 sec-1. By comparison, it can be concluded that the rate of oxidation is very much similar to the rate of enolisation. The correlation of the rate of oxidation with the rate of enolisation suggests that the mechanism of oxidation reaction proceeds through enolisation as pointed out by earlier workers. Enols are hydroxy compounds. Recently, Wiberg and Schafer 12 showed that alcohols are oxidised via the rapid reversible formation of a chromate ester. Such a mode of oxidation may be conceived for ketones also. However, enols are acids of pk comparable to phenols and their esterification would be small. Hence attack of chromic acid, H 2CrO 4 or its conjugated acid, H.CrO4 on the enols is best represented as a concerted process. Thus the - mechanism may be represented as follows: MECHANISM + OH C6H5 CO., CH2 - C6H5 ` o C6H5 - C = elf - C6H5 H dh H 1 C6Ii5-C = Cr+ - OH slow C6H5 - I - 0 Cr+ 02H2 U C O H^: ^H.0 0 + H2O 1 C6H5 C6H5 0 Cr 02H2 + C6H5 - CH - CO - C6H5 + H2O _._.p H3Cr 03 + CrHS_CH (OH ). CO. C6HS Cr (VI) C6H5 CH (OH).CO.C OHs --- CgH S CO.CO.C6H5. fast Very recently, Bakore' has proposed similar mechanism for fhe chromic acid oxidation of aliphatic kitones and a diketone has been isolated as the product of oxidation. Similar mechanism has also been proposed by Corey and Schaefer 13 for the SeO, oxidation of deoxybenzoin where the enol form

40 P. L. NAYAK - AND N. C. I(HANDUAL has been directly attacked by seleneous acid and benzil has been identified as the product of oxidation. The authors are grateful to Prof. M. Santappa, F.N.A., Director, Central Leather Research Institute, Madras, for many valuable suggestions. 1. Wiberg, K. B. 2., Umeda, K. and Tarama, K. 3. Best, P.A. and Waters, W. A. 4. Rocek, J. and Riehl, A. 5. Khandual, N. C., Satpathy, K. K. and Nayak, P. L. 6. Billard, D. A. and Deh, W. M. 7. Tandon, S. K., Banerjee, K. K. and Bakore, G. V. 8. Radhakrishnamurty, P. S. and Behera, T. C. 9. Sengupta, K. K. 10. Bakore, G. V. and Narain, S. 11. Anantakrishnan, S. V. and Varadarajan, R. REFERENCES Oxidation in Organic Chemistry, Academic Press, 1965, pp. 69. Nippon Kagaku Zasshi, 1962, 83, 1216. J. Chem. Soc., 1962, 822. J. Amer. Chem. Soc., 1967, 89, 6691. ` Kinetic and Mechanism of chromic acid oxidation of Aromatic ketones," Ind. J. Chem., 1973, 11, 770, J. Amer. Chem. Soc., 1932, 54, 3969. Ind. J. Chem., 1971, 9, 677. Ibid., 1971, 9, 41. Bull. Chem. Soc. (Japan), 1970, 43, 590. J. Chem. Soc., 1963, p. 3419. Ind. J. Chem., 1970, 8, 423. 12. Wiberg, K. B. and J. Amer. Chem. Soc., 1969, 91, 923. Schafer, H. 13. Corey, J. E. and Ibid., 1960, 82, 918. Schaefer, J. P.