IJCA 18A(1) 53-55.Pdf

Indian Journal of Chemistry Vol. 18A, July 1979, pp. 53-55

Ru(III)-catalysed Oxidation of Ketones with Potassium Bromate In Acid Medium & Novel Behaviour of Acetone

P. S. RADHAKRISHNAMURTJ & D. K. MAHAPATRO Department of Chemistry, Berhampur University, Berhampur 760007

Received 14 August 1978; accepted 10 January 1979

The kinetics of Ru(llI)-catalysed oxidation of some cyclic ketones like cyc!opentanone, cyclohexanone, cycle- heptanone and cyclooctanone and of aliphatic ketones like acetone, methyl ethyl ketone, pentan-2-one and isobutyl methyl ketone by acid bromate have been studied. The reaction exhibits first order dependence on [substrate] and [acid] and zero order dependence on [oxidant] and is independent of [Ru(lll)], for all the ketones studied except acetone. Activation parameters have been computed. A mechanism involving enolisation of ketones in the rate-determining step followed by cleavage of the intermediate enol-Ru(llI) complex by bromate successively in fast steps yielding the products has been suggested. In contrast the reaction is first order with respect to [acetone] and [Ru(llI)] and independent of [acid] and [BrO- a] indicating the formation of a complex between acetone and Ru(llI) in a rate-determining step followed by cleavage of the complex to products by BrO; in a fast step.

HE kinetics of oxida.tion of labile substrates by of ketones by T1(IIJ) acetate" and Pb(OAc)'1 (ref. 6). potassium bromate in acid medium in the Moreover the values of kl = ko I[S] are constant, T absence of a catalyst have been studied>". for different concentrations of the substrate point However, this oxidant at low acidity and in the to the first order dependence on [substrate]. absence of catalyst is found to be ineffective in the Effect of varying concentration of H'Clt) 4 - It is case of ketones. The present investigation deals observed that the reactions are facilitated by an with the catalytic role of Ru(IlI) in effecting smooth increase in the concentration of HCI04 (Table 2). cleavage of ketones. This is the first kinetic report The plots of log ki versus log [H+] for different ketones on the catalysed oxidation of cyclic and aliphatic are linear with unit slopes, indicating first order ketones with acid bromate. dependence with respect to acid. Such a behaviour has been earlier observed in the T1(III) acetate oxidation of ketones by Venkatasubramanian et al,', Materials and Methods who postulated the rate-determining enolisation as an The ketones used were of either BDH(AR) or (GR) important step. grade and were purified by redistillation before use. Effect of varying concentration of Ru(IIl) - No Ru(III) chloride solution was prepared from RuCls reaction takes place in the absence of Ru(III); even as per standard procedures. Potassium bromate in the presence of HCI04 which facilitates enolisation (BDH, AR) was used as such. no reaction occurs in the absence of Ru(IlI): The The kinetics of the reaction was followed iodo- reaction rate is not dependent on Ru(III), indicating metrically by titrating the iodine liberated by KBr03 nonparticipation of Ru(III) in the rate-determining from acidic KI solution with standard sodium thio- step. This is analogous to the oxidation of anilines sulphate solution to a starch indicator end point .. by hexacyanoferratefffl)" catalysed by alkali. The All the experiments have been carried out under zero order dependence on oxidant and Ru(IlI) may pseudo-first order condition using ten-fold excess of be explained by invoking the formation of a weak substrates over the oxidant and the reactions were complex between Ru(III) and the intermediate of carried out in aqueous medium in order to vitiate the substrate species in a fast step, which is finally oxi- reaction from the attendant solvent influences. dised to the products by bromate in a fast step showing the independent nature with respect to Results and Discussion bromate and catalyst. Effect of varying oxidant concentration - The Effect of added salts - The first order rate linearity of the percentage-time plot indicates the constants (kl' min-I) in the absence and presence of reaction to be zero order with respect to oxidant. NaC104 (0.05M, 0.10M,0.15M) for cyclopentanone The constancy of the zero order rate constants (Table 1) are 0.000274 and 0.000267,. 0.000279, 0.000264 res- also supports this conclusion. pectively showing that an increase in [sodium per- Effect of varying the concentration of substrate- chlorate] has no effect on the reaction rate. The ionic The plots of'Iogx, versus log [S]for different substrates strength influence on the oxidation rate is thus negli- are linear with unit slopes indicating first order de- gible in this process. pendence on [substrate]. Such first order dependence Stoichiometry - It is observed that one mole of on [substrate] has also been observed in the oxidation substrate (cyclohexanone or acetone) consumes two

( INDIAN J. CHEM., VOL. 18A, JULY 1979

TABLE 1 - EFFECT OF VARYING [KBrOa] AND [KETONES] ON THE TABLE 2 - EFFECT OF VARYING [HClO.] ON THE RATE OF REACTION RATE IN AQUEOUS MEDIUM OXIDATION OF KETONES IN AQUEOUS MEDIUM {[RuCla] = 9.285 X 1O-6M} {[Ketones] •• 5.0 x 10-a M; [RuCla] = 9.285 x 10-6M]!

2 6 Substrate loa [substrate] 10'[Br08-] 10"Ko lO'kl Substrate 10 [HCIO.lM 10 ko(mol mirr'") 10' kl (mirr<) l M M mol min"! min- [BrO~l = 5.0xl0-4 M; temp. = 35° [HCIO,]=5.0x 10-2M temp.=35° Cyclopantanone 2.50 0.76 1.50 5.00 1.38 2.74 Cyclopentanone 2.50 5.11 0.69 2.76 7.50 1.98 3.91 1.38 ". 5.04 5.11 2.74 10.00 2.63 5.21 7.54 5.11 2.13 2.82 10.08 5.11 2.87 2.85 Cyclohexanone 2.50 2.17 4.09 5.04 2.69 1.43 5.00 4.26 7.43 1.34 5.04 3.98 7.50 5.91 11.98 5.04 8.07 1.33 BrO-.] = 2.5x 10-'M; temp. = 60° Cyclohexanone 2.50 4.94 1.90 7.59 5.74 5.52 4.26 7.43 Methyl ethyl 2.50 1.73 3.47 7.52 4.94 5.49 7.31 ketone 5.00 3.41 6.78 10.02 4.94 7.50 7.49 7.50 5.84 11.62 5.55 2.77 4.13 10.00 7.04 14.08 5.55 8.70 4.34 11.48 5.55 3.92 Pentan-2-one 3.75 2.65 5.32 7.50 5.13 10.30 Cycloheptanone 1.01 5.12 1.74 0.18 10.00 7.94 15.54 1.89 5.02 0.35 1.86 5.52 3.79 0.72 1.90 Isobutyl methyl 2.50 1.37 1.92 2.66 4.74 5.52 0.91 ketone 5.00 2.79 5.41 5.41 1.16 10.00 10.53 Cyclooctanone 4.89 0.85 7.33 20.00 10.96 21.33 2.35 4.89 1.71 7.27 4.56 4.89 3.34 7.32 5.77 4.74 4.18 7.25 TABLE 3 - ARRHENIUS ACTIVATION PARAMETERS [HCIO,l=l0.0x 10-IM, temp. 60° Substrate Ea,kcal/ t-Ht 10gio PZ t-St T t-St Methyl ethyl ketone 2.49 3.36 3.50 14.0 mol kcal/mol e.u. kcal/rnol 4.97 2.96 7.04 14.1 7.46 3.00 10.42 13.9 Aliphatic ketones at 60· 9.95 2.74 13.72 13.8 Acetone 17.17 16.51 6.08 -37.57 -11.57 4.96 3.56 7.13 Methyl ethyl ketone 18.31 17.65 7.38 -31.56 - 9.72 4.96 4.89 6.91 Pentan-2-one 18.31 17.65 7.42 -31.38 - 9.67 4.96 7.77 7.06 Isobutyl methyl keytone 17.17 16.51 6.51 -35.58 - 10.96 Pentan-2-one 2.51 2.72 3.63 14.4 4.97 2.77 7.94 15.5 Cyclic ketone at 35· 7.54 2.72 10.21 13.5 Cyclopentanone 14.08 13.47 4.65 -39.38 -13.11 10.05 2.72 14.84 14.8 Cycloheptanone 21.75 21.14 9.94 -43.39 -14.45

Isobutyl methyl 2.54 2.76 2.77 10.9 ketone 5.15 2.71 5.41 10.5 7.61 2.76 8.29 10.9 of the carbonyl oxygen by the equatorial hydrogens 10.04 2.76 11.06 11.0 at C-2 and C-6 appears to be relieved when the hybridisation of the carbonyl oxygen is changed from Sp2 to Sp3. moles of oxidant to give the products. The products The enolisation of ketones depends upon two steps: isolated are pyruvaldehyde, diacetyl and cyclohexane- (i) equilibrium protonation of carbonyl group, and 1,2-dione in case of acetone, methyl ethyl ketone and (ii) deprotonation of a.-carbon of the conjugate acid. cyclohexanone respectively. These products have These two steps effect the rate of enolisation depend- been characterised by di 2,4-dinitrophenylhydrazones ing on the structural factors. (m.ps 296° and 315°) in cases of pyruvaldehyde and Effect of temperature - To investigate the effect diacetyl respectively. The cyclohexane-l,2-dione has of temperature and to evaluate the various activation been identified as the dioxime (m.p. 188°). The pro- parameters experiments have been carried out at ducts have also been confirmed by usual spot tests", 30°, 35° and 40° for cyclic ketones and at 50°, 60° and Structure and reactivity - The order of reactivity 70° for aliphatic ketones under identical conditions. of aliphatic ketones is methyl ethyl ketone ,...pentan- The plots of log kobs versus liT are linear. The 2-one > isobutyl methyl ketone. This is in conso- activation parameters are listed in Table 3. nance with the stability of enols. The low reaction Mechanism of the reaction -.The observed orders rate in the case of isobutyl methyl ketone may be with respect to substrate, oxidant, catalyst and acid due to steric factor. clearly indicate that the protonated form of the sub- The order of reactivity among the cyclic ketones strate species participates in the reaction with a rate studied is cyclohexanone ,..., cyclooctanone > cyclo- determining enolisation followed by fast decomposi- pentanone > cycloheptanone. Cyclohexanone is tion to the final products. Further the plot of log essentially free of strain but the strain due to eclipsing kl [Br03- and Ru(III)] versus log k, (LTA) for

( RADHAKRISHNAMURTI & MAHAPATRO : Ru(III)-CATALYSED OXIDATION OF KETONES WITH KBrO;

TABLE4 - EFFECT OF VARYING [KBr03] AND [ACETONE]ON TABLE 5 - EFFECTOF VARYING[RuC1al AND [HCIO,] ON THE THE RATE OF OXIDATIONOF ACETONEIN AQUEOUS MEDIUM RATE OF OXIDATIONOF ACETONEIN AQUEOUS MEDIUM {[HCIO,]=10.0X 1O-6M; [RuCI.]=9.285 X 1O-6M, temp=600} ([Acetone) = 5.0x lO-'M; [BrO.-] =2.5x lO-'M; temp.-600} loa [Acetone] 104[BrO.-l 106 », lq' kl 10s [HCIO,] 106[RuCI.] 10' kl min-1 M M mol min"! min"! M M 5.12 2.63 1.99 1.76 10.00 4.64 2.20 5.12 3.70 10.00 5.26 1.79 9.28 3.89 5.12 10.00 13.92 5.12 7.81 1.82 5.80 10.00 18.57 8.67 10.00 27.85 2.55 2.64 1.06 4.15 10.92 5.12 2.63 1.92 3.89 2.50 9.28 3.41 7.73 2.64 3.16 4.27 5.00 9.28 10.39 2.64 4.13 4.24 3.39 10.00 9.28 3.89 20.00 9.28 3.88 30.00 9.28 3.69 different cyclic ketones is linear indicating the opera- 40.00 9.28 4.02 tion of identical mechanisms in both the oxidations. 50.00 9.28 3.75 In the present study, enolisation of the protonated ketone occurs in a rate-determining step. The enol complexes with Ru(Hl) in a fast step which breaks catalysed oxidation of alcohols. by hexacyanoferrate with bromate in a fast step to the final products. (IH)ll in alkaline medium. Thus it is an evidence for This explains the zero order dependence with respect the change over of mechanism from rate determining to bromate and Ru(IH). Below the optimum enolisation catalysed by H+ in the ketones studied except acetone to rate- determining compl exation (4.64 X 1O-6M) concentration of Ru(HI) there is no perceptible reaction which may be because of the low between acetone and Ru(HI). This is also substan- concentration of the Ru(IH) to react in the proposed tiated by the fact that the enol content of acetone is mechanism. Hence the detailed mechanism can be substantially less compared to the other ketones envisaged as shown in Scheme 1. studied. The plausible mechanism of oxidation of acetone o OH+ can be envisaged as shown in Scheme 2. 11 II o R-C- 3+ + :;::!: CH H R-C-CH3 II slow CHs-C- CH Ru(HI) :;::!: Complex OH OH s+ fast 11 slow I Complex + BrOs- ) Products. R-C-CH3 ---+ R-C=CH2+H+ Scheme 2 OH I fast BrOa- References R-C=CH2+Ru(HI) -- Complex --~ Pro- 1. VIJAYALAXMI& SUNDARAM,E. V" Indian J. Chem .• 15A ducts. (1977), 612. Scheme 1 2. NATARAJAN,R. & VENKATASUBRAMANIANN., Tetrahedron Lett., 51 (1969), 5021. Behaviour of Acetone - Among the different 3. NATARAJAN,R. & VENKATASUBRAMANIAN,N., Tetrahedron. ketones studied acetone shows a novel behaviour. 30 (1974), 2785. It differs from other ketones in that the dependence 4. PADHI, S. c., Mechanism and structure of some organic on [Acetone] and [Ru(III)] is unit and the reaction reactions, Ph.D. Thesis, Berhampur University, 1978. 5. RADHAKRTSHNAMURTI,P. S. & PATI, S. N., Indian J. Chem., rate is independent of [H+] and [BrO~] (Tables 4 17A (979), 57. and 5). This anomalous behaviour may be attributed 6. RADHAKRTSHNAMURTI,P. S. & PIoTI,S. N., Indian J. Chem .• to the fact that in the case of acetone it is the keto 16A (1978), 319. form which is involved in complex formation with 7. SRINIVASAN,V. S. & VENKATASUBRAMANIAN,N., Indian J. the Ru(IH) hexaaqua species'? in a rate-determining Chem., 14A (1976), 488. step, which breaks with BrO~ in a fast step. Moreover 8. RADHAKRISHNAMURTI,P. S. & PANDA, R. K., Indian J. the double reciprocal plot of ko and [S] is linear with Chem., 9 (1971), 1247. 9. FEIGL, F., Spot tests in organic analysis (Elsevier Publishing a finite intercept indicating the complexation between Co., New York), 1960, 219. Ru(III) and substrate, which is further confirmed 10. COTTON, F. A. & WILKINSON, G., Advanced inorganic from the finite intercept obtained from the double chemistry (Wiley Eastern, New Delhi), 1972, 999. reciprocal plot of ko and [Ru(HI)]. This behaviour 11. RADHAKRISHNAMURTI,P. S. & SAHU, B., Indian J. Chem., of acetone is analogous to the observation of Ru(IH)- 17A (1979),95.