Banerji, K. K.

Marigangaiah

Mathur, P. N.

English

NOPR

Indian Journal of ChemistryVol. 14A. February 1976. pp. 85-87Kinetics & Mechanism of Acid Permanganate Oxidation ofCyc1ohexanoneMARIGANGAIAH. P. N. MATHUR & K. K. BANERJIDepartment of Chemistry. University of Jodhpur, JodhpurReceived 1 October 1974; accepted 6 January 1975The oxidation of cyclohexanone by acid perrnanganate is of first order each with respect tocyclohexanone, the oxidant and the hydrogen ion. The increase in [acetic acid] increases therate sharply. The reaction exhibits a primary isotope effect kH/kn = 3·35 and a solvent isotopeeffect llD,O/kH,O = 5·5 at 25°. The oxidation is much slower than the acid-catalysed enolization.The magnitude of the solvent isotope effect suggests that oxidation proceeds through the enolintermediate. Tentative mechanisms have been proposed.LITTLERl reported that acid permanganateoxidation of cyclohexanone is of zero orderwith respect to the oxidant. Some preliminarystudies (unpublished results), however, showed thatthe oxidation of aliphatic ketones exhibits a firstorder depend ence on permanganate in acid conditions.Hence, it was considered worthwhile to reinvestigatethe oxidation of cyclohexanone more thoroughly.The results of such a study are reported in this paper.To our surprise, the results obtained are quite diffe-rent from those reported by Littler-.Materials and MethodsCyclohexanone (Fluka) was purified through thepreparation of its bisulphite adduct. Perchloricacid (Baker, 60%) was used as a source of hydrogenions. Acetic acid (BDH, an alar) was distilled overchromic oxide before use. 2,2,6,6- Tetradeuterio-cyclohexanone was prepared by the method of Bestet al». All other reagents used were of AR grade.Kinetic measurements - The reactions were carriedout at a constant temperature (± 0.1°). The kineticdata are reported in Tables 1 and 2. Aliquots ofthe reaction mixture were quenched with excess ofFeS04 solution and back titrated with standardizedKMn04• The data on isotope effect were obtainedspectrophotometrically at 532 nm. The rate con-stants reported are the mean values of duplicateruns and are reproducible within ± 4%. The rateof enolization was measured by the brominationmethods.ResultsThe acid permanganate oxidation of cyclohexanoneis autocatalytic. Such a phenomenon is well knownin permanganate oxidation and is attributed to thereactions of Mn(III)/Mn(IV). To suppress thesereactions, a large excess of NaF was added to eachreaction mixtures,Identification of the oxidation product - The oxida-tion of cyclohexanone (O·lM) by acid KMn04(O·OOlM) in the presence of HCI04 (1·0M) and NaF(0·05 M) resulted in the formation of mainly cyclo-hexane-1,2-dione which was isolated as its Ni(II)dioxime complex (85% yield) and identified.Rate laws - The reactions were studied underpsuedo first order conditions by keeping a largeexcess of cyclohexanone over permanganate. Underthese conditions, the oxidation of cyclohexanoneby acid permanganate gave first order plots. Werepeated carefully the experiments reported byLittlert but could not confirm his observations re-garding the zero order dependence of the reactionrate on the oxidant. Further, the first order rateconstant was independent of the initial [permanga-nate] thereby confirming that the oxidation is strictlyof first order with respect to the oxidant. Theoxidation was also first order with respect tocyclohexanone. Throughout the acidity range used,the reaction velocity was linearly dependent on the[H+] in solutions of constant ionic strength. Somerepresentative rate data are given in Table 1.Effect of varying solvent composition - The oxida-tion rate increased sharply with increasing [aceticacid]. This may be attributed to the lowering ofdielectric constant of the mediums, which favoursreactions involving protonation. Further, the enoli-zation of the ketone is catalysed by acetic acid andthis may also affect the rate of reaction. The valuesof k at 25° and [NaF] = 005 M were 8·28 X 10-2,9·89x10-2, 17·9xlO-2 and 22·1x10-2 litres mole-2see+ for 20, 30,40, and 50% acetic acid respectively.TABLE 1- OXIDATION OF CYCLOHEXANONE BY ACIDPERMANGANATE{[NaF] = 0'05M; solvent = acetic acid-water (20: 80, v/v);(.L = 2'OSM; (temp.= 25°)}103[KMnOJ 103[Ketone] [H+] kl X 103M M M min"!2'0 6'15 1·0 30'61'0 6"15 1'0 30·74·0 6'15 1·0 30-71·0 3'07 1·0 15·41·0 7'70 1·0 38'41·0 12-3 1·0 61'41'0 6'15 0'5 17'81·0 6'15 1·5 56'31·0 6'15 2'0 75·585INDIAN J. CHEM., VOL. 14A, FEBRUARY 1976TABLE 2 - OXIDATION AND BROMINATION RATES OFCYCLOHEXANONE{Solvent, acetic acid-water (20: 80, v/v)}Temp. Oxidation BrominationoK 10'k (litre' 104k. (litremole-' sec'<) mole"? sec-l)293 5·20 7·33298 8'26 10'9303 12'4 15'8308 18'7 22'8Solvent isotope effoct - The oxidation of cyclo-hexanone was carried out in 95% D20. The rateconstants in D20 and H20 at 25° were 27·92x 10-3litres mole= see'< and 5·12 X 10-3 litre-2 mole+ see'<respectively. Thus the solvent isotope effect kD oikH,o = 5·5 at 25°. •Kinetic isotope effect - The oxidation rates ofcyclohexanone and 2,2,6,6-tetradeuteriocyclohexa-none at 25° were 103k = 5·12 and 1·52 litresmole= see= respectively. Thus the kinetic isotopeeffect kH/kD = 3·35 at 25°.Effect of temperature - The effect of temperatureon the reaction rate is given in Table 2. Thespecific rate constant (k) was obtained from therelation k = kl/[ketone][H+] litres mole-2 sec+.Rates of bromination - The bromination of cycle-hexanone was found to be of first order each withrespect to cyclohexanone and H+ ion, but independentof [bromine]. The rate of. brornination divided by[cyclohexanone] and [H+] grves k2 (litre mole< sec<)(Table 2).Activation parameters - The values of IlRt, IlS+and 11Ft for the oxidation and bromination o~cyclohexanone were found to be 15·2 + 0·5 and12'2±0'5 kcal mole'<. -13·3±1·5 and ~27·0±1·5e.u., and 19·3±1·5 and 20·4±1·5 kcal mole-trespectively.DiscussionThe observation of Littler! that the rate ofoxidation of cyclohexanone is equal to its rateof enolization is not borne out by the presentstudy. The results indicate that the rate of oxida-tion of cyclohexanone by permanganate is muchslower than its rate of enolization, and hence thekinetic relations cannot show whether the ketonemolecule or its enol form is attacked. Howeverit merely indicates that the transition state of therate determining process involves the elements of am?lecule of ketone and a molecule of permanganicacid. The active oxidizing species is likely to bepermanganic acid (HMn04)6,7.The primary isotope effect (kH/kD = 3·35) showsthat the fission of an 1X:-C-H bond is rate con-trolling but this does not necessarily mean that theoxidation step involves the ketone form and notthe enol form. The acid-catalysed enolization in-volves the reactions (1) and (2).RCO.CH,R'+HaO+ ~ R+C(OH)CH2R'+H20RC+(OH)CH,R'+H.O ~ RC(OH)=CHR'+HaO+86Mechanism 1R- C-CHR"\I I + HOHo OMn02H201 (A)R-C-CH(Oli)H'/I + HMn03°R-C-C-R'/I IIo 0R-C-CH(OHIR' HMn04II fast •° Mechanism 2..• (1)..• (2)The forward reaction in Eq. (2) is slow since itinvolves the fission of a C-H bond". Thus the for-mation of an enol should also exhibit the primaryisotope effect.A comparison of acid-base equilibria" in waterand D20 indicates that for any proton-catalysed~eaction the expected. solvent isotope effect kD,o/kH,oIS about 2·0-2·5. ThIS should favour the oxidationby HMn04 as the equilibrium (3) is shifted by D20so as to yield higher concentration of HMnO, thanwould be present in water.Mn04+H30+~HMn04+H20 ... (3)However, the observed solvent isotope effect (kD 0/kH,o = 5'5) is almost double of that reported for a~yproton-catalysed reaction. This cannot be due tothe part~cipation of a second proton as any suchprotonation would have been revealed by kinetic~easurements. The only other conclusion possibleIS that the pre-equilibrium involved in producingrate-det errn'ning reaction complex of the oxidationi~clude two different acid-catalysed processes;VIZ: ~h~se represented. by Eqs. (2) and (3).ThIS IS III agreement WIth the conclusion reachedby Best et al.2 in the chromic acid oxidation ofcyclohexanone.The rate laws obtained for the oxidation ofcyclohexanone are similar to those for the oxidationof alcohols by acid permanganatet". Since the per-manganate oxidation of alcohols involves formationof a carbonium ion4•10, the formation of suchan intermediate is also likely in the case of enols(Mechanism 1).Another possibility is that 1he attack on the~nol is. a concerted process (Mechanism 2) givingimmediately an ce-ketomang anate ester (A) whichis hydrolysed to give an oc-ketol and Mn(V)species.MARIGANGAIAH el al.: PERMANGANATE OXIDATION OF CYCLOHEXANONEReferences1. LITTLER, J. 5., j. chem. s«., (1962), 827.2. BEST, P. A., LITTLER, J. S. & WATERS, W. A., j. c"'em.Soc., (1962), 822.3. BANERJI, K. K., NATH, P. & BAKORE, G. V., Indian I-Chem.,8 (1970), 337.4. BARBER, R.M.&LITTLER, J.S.,j.chem.50c. (B), (1967),205.5..COHEN, M. & WESTHEIMER, F. H., j. Am. chem. Soc., 74(1952), 4387.6. BANERJI, K. K. & NATH, P., Bull. c"'em. Soc. jajJafJ,41 (1969), 2038.7. BANERJI, K. K., Indian j. cu«; 11 (1973).242.8. BELL, R. P., Tke proton in chemistl'y (Methuen, London).1959, 140.9. RULE, C. K. & LAMER, V. K.,j. Am. chem, Soc .• 60 (1938).1974.10. BANERJI, K. K., j. chem, Soc., Perkin II, (1973),435.87

Kinetics & Mechanism of Acid Permanganate Oxidation of Cyclohexanone

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Kinetics & Mechanism of Acid Permanganate Oxidation of Cyclohexanone

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