Kinetic Studies on Permanganate Oxidation of Acetophenones Under

Indi an Journal of Chemistry Vol. 40A, June 2001, pp. 610-612

Kinetic studies on permanganate oxidation of The ketones were further purified by vacuum acetophenones under phase transfer catalysis distillation. The solvents employed were purified by standard methods and doubly distilled water was P S Sheeba & T D Radhakrishnan Nair* always used. The catalysts used were tricapryl­ Department of Chemistry, Calicut University methylammonium chloride (TCMAC) and tetrabutyl­ Calicut University P.O., Kerala 673 635. India ammonium bromide (TBAB). The former has Received 18 October 2000; revised 8 March 2001 relatively larger organic structure (C 10H21 )JN+CH 3Cr compared to the latter (C4 H9) 4N+B(. Kinetic studies on the permanganate oxidation of The solutions containing the permanganate ions in acetopheno_ne and some of its substituents in organic media using the organic solvents were prepared by shaking the techn1que of phase transfer catalysis are reported. aqueous potassium permanganate solution with the Tncaprylmethylammonium chloride and tetrabutylammonium bromide have been used as the phase transfer catalysts. The organic solvents contammg the phase transfer 4 reaction shows first order dependence each on [ketones] and the catalysts as reported elsewhere . The solutions of the [permanganate ions] respectively . The rate coefficients fit well oxidant thus prepared in the organic solvent and the with the Hammett equation and the p-value calculated aorees well substrate in the organic solvent were thermally with the reaction requirements. "' equilibrated (± 0.05°C) at the desired temperature for Potassium permanganate is a powerful oxidizing about half an hour before mixing. Kinetic runs were carried out under pseudo-first order conditions. The agent both for organic as well as inorganic oxidation reactions. It can be conveniently used as a selective progress of the reaction was followed by measuring oxidant for removing small quantities of organic the changes in the optical density of the reaction matter in water bodies and the reaction assumes mixture employing a Shimadzu 1601 UV spectro­ special significance when carried out under phase photometer at Amax 526 nm. transfer catalytic conditions, as water insoluble organic materials can be oxidized with ease. In the Results and discussion present study acetophenone and its p-nitro, p-bromo, The percentage extraction of the permanganate ions was found to be 97 to 100 with 0.02 mol dm-3 of the p -chloro, p-methyl, p-methoxy and m-nitro deriva­ catalysts employed. The transfer of the oxidant was ti ves have been subjected to oxidation reactions under more with TCMAC, as expected in accordance with phase transfer catalysis. Tricaprylmethylammonium 5 its greater organic structure . The extraction of Mn0 - chloride (Aliquat 336) and tetrabutylammonium 4 bromide were used as catalysts in various organic ions into benzene was 100% with TCMAC and 97% with TBAB in about 30 min for a catalyst solvents. 3 concentration of 0.02 mol dm- . The solutions Traditionally the organic oxidation reactions in remained stable for a fairly long duration of time. The aqueous medium were facilitated by the use of co­ loss in the concentration of Mn04 ions durina a solvents especially the dipolar aprotic ones. This • b penod of about 2 h was only about 2%. The could partially solve the situation through cation stoichiometry of the reaction was investigated by solvation. However, with the development of the equilibrating known amounts of acetoohenone with an phase transfer techniques it has become quite easy to excess of permanganate in the of the catalyst carry out oxidation reactions of organic compounds in presen~e and by estimating the unreacted permanganate. One organic media using permanganate, dichromate, 1 3 mole of Q+Mn04- consumed one mole of th e ketone to hypochlorite, etc - • Such oxidations have been carried produce the carboxylic acids. out in heterogeneous system of aqueous and organic media. Reports on the kinetics of oxidation of The kinetic rate data with various oxidant carbonyl compounds using phase-transferred oxidants concentrations and substrate concentrations are are scanty and hence the present work. presented in Table l. Under th e conditions when [Acph] >> [Mn04-], th e plots of log [Mn04-] versus Experimental time were found to be linear indicating first order Analar grade potassi um permanganate was used. dependence on Mn04- ions. This was further confirmed ----- NOTES 611 from the constancy of the specific rate values (kobs) for investigated. The solvents employed were benzene, various concentrations of Mn04- ions. This is given in toluene, carbon tetrachloride, chloroform and the set of k obs values in the first and second rows in dichloromethane. The specific rates with both the Table 1 for the two different catalysts. The observed catalysts showed similar trends and these values are in rate constants with different [substrate] and the same the decreasing order in the solvents as benzene> [oxidant] with both the catalysts (set of k obs values carbon tetrachloride> toluene> dichloromethane > given in the third and fourth rows of Table I) increase chloroform (Table 2). The observed decrease in rate linearly with increase in [substrate]. Plots of log k obs 0.6 .------, 0.-!0 versus log[acetophenone] was linear with a slope of unity showing that the reaction is first order with P'lC respect to the ketone also. This is further confirmed TBAB 0.3!1 from the reasonably constant values for the second ~ 0.5 order rate constant, k i.e kobsf[AcPh] obtained from 2 ...... the values in Table l. It was also observed that the + + 0 .30 plots of log kobs versus log [PTC] (Fig. 1) for both the ~ ~ ~ -=. catalysts were linear with small fractional order 0 .9 0.4 .... dependence. The rate constants obtained also shows 0.:

TCMAC, [(C,oH21hN+CH3Cr] can be seen to be a o.s '-----=-o.'=s---o-:-~_...,..4 ___o,.....c::-o----:-'o.6°' 20 better catalyst than TBAB, [(C4H9)4N+BrT The effect of dielectric constants of the solvents loi[PTC] + 3 used, on the rate of these oxidations were also Fig. 1-Plot of log kobs versus log[PTC]

Table ! -Effect of [subst rate], [oxidant] and [catalyst] on the rate of oxidation of acetophenone

[Organic solvent : benzene; temp. : 303 K]

3 2 [Mn04' ] X 10 [AcPh] x 10 mol [TCMAC] x [TBAB] x 4 k obsX 10 s·l mol dm-3 dm-3 103 mol dm- 3 103 mol dm-3

0.50 1.00 2.00 2.694 1.00 1.00 2.00 2.755 1.50 1.00 2.00 2.663 2.00 1.00 2.00 2.740 0.50 1.00 2.00 1.846 1.00 1.00 2.00 1.850 1.50 1.00 2.00 1.896 2.00 1.00 2.00 1.811 1.00 1.00 2.00 2.755 1.00 1.50 2.00 4.133 1.00 2.00 2.00 5.323 1.00 2.50 2.00 5.964 1.00 1.00 2.00 1. 850 1.00 1.50 2.00 2.790 1.00 2.00 2.00 3.573 1.00 2.50 2.00 4.118 1.00 1.00 2.00 2.755 1.00 1.00 2.50 2.909 1.00 1.00 3.00 3. 101 1.00 1.00 3.50 3.216 1.00 1.00 2.00 1. 850 1.00 1.00 2.50 2.003 1.00 1.00 3.00 2.118 1.00 1.00 3.50 2.279 612 INDIAN 1 CHEM, SEC. A, JUNE 2001

2.1 Table 2-Effect of solvent polarity on the oxidation of 1.2 p acetophenone in organic solvents PTC ;r-p-NO, 3 3 2 3 1.0 {[Q•Mno4·]= LO x 10' mol dm- , [AcPh]== LOx 10· mol dm- , ,...NO, 1.8 'lBAB temp. -303 K} ~ 0.8 Organic solvent Intrinsic U5 dielectric TCMAC TBAB 0.1 + 0.6 "'+ constant .J.' .J.' ~ 1.2 ~ Benzene 2.27 2.755 L850 .s .s 0 .4 Toluene 2.40 2.406 1.731 Carbon tetrachloride 2.22 2.575 1.784 0 .9 Chloroform 4.70 1.992 1.539 0.2 Dichloromethane 8.90 2.203 1.638 • p-OCH, Table 3-Effect of substi tuents on the oxidation of 0 0.6 acetophenone in organi c solvents '~0.4 -0.1 0.2 0.~ 0.8

3 cr [Q+Mn04·] =LO X 10-J mol dm' ; organic solvent- benzene 2 3 Fig. 2-Hammett plot for the oxidation of acetophenone [substrate]= LOX 10' mol dm' ; temp=- 303 K Substituted acetophenone with maximum speed and the para- methoxy TCMAC TBAB derivative is the least reactive one. The linear free energy relationships as seen in the Hammett plots for p-N02 11.215 IL062 m-N02 10.432 9.181 both the catalysts gave positive p values indicating p-Cl 4.153 2.729 that electron deficiency at the seat of reaction favours p- Br 3.703 2.533 the reaction. The p values found are +0.8451 when Unsubstituted 2.755 L850 TCMAC was used as the catalyst and +0.9208 when p-CH3 1.400 0.6870 p-OCH3 Ll82 0.6060 TBAB was used as the catalyst. This observation supports the fact that the Mn04- attacks the polarisable carbonyl carbon atom of the of the oxidation in solvents of higher dielectric ketone and the speed of the reaction is enhanced more constant can be attributed to the larger stabilization by by the better electron-withdrawing para-nitro group solvation extended to the quaternary permanganate in the series of compounds chosen. ion pair by these solvents. The ion-dipole interaction more strongly solvates the ion-pair formed by Acknowledgement quaternary ammonium cation and permanganate anion PSS gratefully thank the CSIR, New Delhi for the in solvents of larger dielectric constants making the award of the CSIR (JRF and SRF) fe llowships. Q•Mn04- ion-pair Jess reactive. The specific rate values for the various para- and References meta- substituted acetophenones for both the catalysts I Starks C M & Liotta C, Phase transfer catalysis, principles and techniques, (Academic Press, New York), 1978. are given in Table 3. The Jog k obs values gave 2 Sam D 1 & Simmons H E, JAm chem Soc. 94 ( 1972) 4024. excellent linear fits with the respective substituent 3 Dehmlow C V & Dehmlow S S, Phase transfer catalysis constants according to the Hammett equation (Fig. 2). (Verlag Chemie, Weinheim), 1993. The data in Table 3 show that the reactivities of the 4 Herriot A W & Picker D, Tetrahedron Lett, (1974) 1511. 5 Brandstrom A, Advances in physical organic chemistry, edited different substituted acetophenones are in the by V. Gold Vol 15 (Academic Press, London and New York) decreasing order p- N02 > m-N02 > p- Br > p-CI >-H > (1977), 267. p-CH3 > p-OCH3. The para- nitro derivative reacts