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Indian Journal of ChemistryVol. 17A. January 1979. pp. 60-62

Oxidation of Ketones by Chloramine- TP. S. RADHAKRISHNAMURTl* & M. D. PRASADA RAO

Department of Chemistry, Berhampur University, Berharnpur 760007

Received 28 November 1977; revised and accepted 20 July 1978

Kinetics of oxidation of aliphatic, aryl aliphatic and cyclic ketones and of acetylpyridines bychloramlne-T in aqueous ethanol under alkaline Conditions are reported. Reactivity order ofthe ketones studied has been analysed. The ionic strength effect is negligible. Solvent effectsindicate the involvement of at least one neutral molecule in the rate-determining step. Effect ofOs(VIII) on the reaction rate is found to be negligible. Complete pH-rate profile has beenobtained for cyclohexanone. Halogenatlon has been postulated in the acidic pH and the reactionpath is found to change from halogenation to oxidation at pH 12'7. The reaction products havebeen identified to be 1,2-diketones in the case of aliphatic and cyclic ketones and phenacyl-dehyde in case of acetophenone.

OXIDATION of aliphatic, aryl aliphatic andcyclic ketones by oxidants like Ce(IV)1,2,TI(III)3, V(V)4, hexacyanoferratetl II)", per-

oxydisulphate" and Cr(VI)? have received consider-able attention. We have recently reported theoxidation of ketones by potassium permanganate"in acidic conditions. Chloramine-T (Eredox= 0·499at pH 12) serves as a good oxidizing agent.Oxidation of cyclohexanone", cyclopentanone'P, somealiphatic ketonesll•12 and acetophenone-" by chlora-mine-T in alkaline conditions was studied byMushran and coworkers. But these studies werenot aimed at correlating the structure with reactivity.Hence a detailed study of oxidation of aliphaticketones, aryl aliphatic ketones, cyclic ketones, andacetyl pyridines by chloramine-T (CAT) was takenup in aqueous ethanol medium under alkalineconditions and the results of the study are reportedin this paper.

Materials and MethodsAll the substrates (see Table 1) used were of BDH

grade and were distilled or crystallized before use.Chloramine-T (M & B, AR) was used as such andthe disappearance of CAT was estimated by standardiodometric method. The ionic strength was main-tained constant by adding 0·002M NaCr04 in allthe kinetic runs. The rate constants have beencalculated by the usual tangential method and arereproducible Within 3%. Oxidation of ethanolby CAT under the conditions studied was less than-6% over the range of the reaction studied and duecorrection was applied while computing the rateconstants.

Results and DiscussionConstancy in the first order rate constant for a

particular experiment indicates first order depend-ence with respect to CAT. The first order rateconstant increases with increasing concentration ofthe substrate and consequently k2 = kIt[S] remainsconstant showing the first order dependence on thesubstrate. The second order rate constant increases

-60

,.

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TABLE 1 - EFFECT OF VARYING SUBSTRATE CONCEN-TRATION ON THE OxIDATION OF KETONES BY CAT

[CAT = 0·00045M; 10% EtOH; O'lM NaClH; NaCIO,= 0'002M; temp = 45°]

[Substrate] hI X 10' hz X 103 [Substrate]M X 10z sec-t litre M xl oz

mol-!sec-I

». X 10· hz X 103

sec-I litremol-Isec-I

0·5081·0521·726

ACETONE

7'08415·0722·91

65'7766·5966'67

CYCLOPENTANONE

1·3791-4331·327

0·2550'5051·212

168·1336'5807·9

ETHYL METHYL KETTNE CYCLOHEXANONE

0·4580·8751·343

26'9642·4576·95

5·8826'1085'648

0·2440·4830'718

98·91201'6307·2

40·4741'6742·78

m-NOZ-ACETOPHENONE

0·099 78·81 78'890·145 115·2 79·240'217 172-5 79'1

p-Cl-ACETOPHENONE

0·058 12·12 20'730·132 25'52 19·250·188 33'83 17·97

2-ACETYLPYRIDINE

0·243 75'77 31-140'516 152'4 29'530·912 270·9 29-69

4-AcETYLPYRIDINE

0·268 425'0 158·40'541 960·7 177'61·057 1762·0 166·7

with increasing concentration of alkali (Table 2).A plot of log k2 vs log [OH-] is linear with unit slopeindicating the first order dependence on alkali.

Increase of ethanol content of the solventmedium decreases the rate (Table 3) indicating theinvolvement of at least one neutral molecule in therate determining step.

Effect of ionic strength on the reaction rate wasstudied by varying the concentration of NaCl04and it is found that the reaction is not susceptibleto change in the ionic strength.

Structure and reactitniy - The second order rateconstants for all the ketones studied are reportedin Table 4. The observed order of reactivity inthe case of aliphatic ketones is: acetoacetic ester> benzoyl acetone > ethyl methyl ketone > iso-butyl methyl ketone > acetone f"'-.I diisopropyl

I

RADHAKRISHNAMURTI & RAO: OXIDATION OF KETONES BY CHLORAMINE-T

ketone, considering acetone as standard. The higherreactivity of acetoacetic ester and benzoylacetonecan be explained by comparing the reactivity ofacetone, acetoacetic ester and benzoylacetone. Itcan be commented that the introduction of carb-ethoxy and benzoyl groups increases the percentagecontent of the enol, thereby shifting the equilibriumtowards enolic form, The inductive effect of theelectronegative carbethoxy or benzoyl group reducesthe relative stability of the keto form and conjuga-tion effect of the unsaturated carbethoxy or benzoylgroup demands a suitably situated double bondand increases the relative stability of the enol.

In simple aliphatic ketones increasing the chainlength of alkyl groups increases the equilibriumenol contents. The observed reactivity ethyl methylketone > isobutyl methyl ketone > acetone ,.....,diisopropyl ketone is different from what has beenobserved in the oxidation of these ketones byKMn04 under acidic conditions. It is difficult tosay if the change in the medium from acid toalkaline results in a change of the order of reactivitydue to different percentages of enols present underdifferent conditions or whether factors of stericorigin also are partly responsible.

The observed reactivity in case of aryl aliphaticketones is p-N02-acetophenone > m-N02-aceto-phenone > p-Br-acetophenone > p-Cl-acetophenone> acetophenone. > p-CH3-acetophenone_ Thisreactivity order is quite in consonance with thefact that electron withdrawing groups acceleratethe oxidation process while the electron releasinggroups retard the process.

In the case of cyclic ketones the observed orderis cyclopentanone > cyclohexanone > cyclooctanone> cycloheptanone. This order shows that thereactivity is not dependent on the ring size as hasbeen reported in the oxidation of cyclanols by Br2(ref- 14) and in the oxidation of cyclanols byhexacyanoferratefl Iljw, In the case of acetyl pyri-dines the order of reactivity is 4-acetylpyridine> 2-acetylpyridine. It seems reasonable to arguethat factors of steric origin are called into play thusreducing the overall reactivity of the 2-isomer ascompared 4-isomer_

In the case of substituted acetophenones a plot of10g2 k2 vs (J is linear with a P value + 1-2 showingthat electron withdrawing groups are acceleratingthe process.

Effect of osmium tetroxide - Mushran and co-workers-" have shown that Os(VIII) catalyses theoxidation of acetaldehyde and formaldehyde andthey have indicated that a complex is formedbetween Os(VIII) and N-chlorotoluene-p-sulpho-narnide. They have also shown that the hydrideion abstracting capacity of N-chlorotoluene-p-sul-phonamide increases after complexation due todecrease in the electron density around nitrogenatom, thus weakening the N-Cl bond. So a fasthydride ion transfer from the hydrated form of thealdehyde was suggested in the subsequent step.The catalytic effect of Os(VIII) in the present studyhas been investigated. Table 5 shows the secondorder rate constants for cyclopentanone and cyclo-hexanone in pure aqueous medium in the presenceand absence of Os(VIII) _ It is clear that there is

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TABLE 2 - EFFECT OF VARYING ALKALI CONCENTRATIONON THE OXIDATION OF KETONES BY CAT

~[10% ethanol,' NaCI04 =' 0'002M, CAT = 0-00045M; sub-strate = 0-005M, temp, = 45°J

'[AlkaliJ, M 103 X k2litre mol-l sec!

[AlkaIiJ, M 103 X k2litre mol-l sec-l

ACETONE CYCLOPENTANONE

0-10-150-20

1-3791-9712-679

0-050-100-15

34-4966-59

122-9

:ETHYLMETHYLKETONE CYCLOHEXANONE

0-050-10-15

15-3741-6756-67

0-10-150-20

5-8827-502

11-04

-m-N02-ACETOPHENONE 2-ACETYLPYRIDINE

0-050-10-15

31-0878-89

101-0

0-10-150-2

29-5343-9167-94

p-CI-ACETOPHENONE 4- ACETYLPYRIDINE

0-10-150-2

19-2528-5734-37

0-10-150-2

177-6270-7395-4

TABLE 3 - EFFECT .OF VARYING THE DIELECTRICCONSTANTOF THE,MEDIUM

[Substrate = O-OOSM; NaOH = 0-lM; N,:~I04 = 0-002M;CAT = 0-00045M; temp.e- 4.) J

Substrate 108 X k2' litre mol-l sec-l

10% EtOH 20% EtOH 30% EtOH

CyclopentanoneCyc1ohexanoneCyc1oheptanoneCyc1ooctanone

66-5941-674-114

24-41

53-0434-223-473

18-02

35'6726-36

3-00914-37

TABLE 4 - SECOND ORDER RATE CONSTANTSFOR THEOXIDATION OF KETONES BY CAT

110% EtOH; NaOH = 0-lM; CAT = 0-00045M; NaCI04= 0-002M; temp.= 45°J

Substrate 103 X k2 Substrate 10' X k2litre mol-l litre molr+

sec-l see=

Acetone 1'379 p-Br-aceto- 30-40Ethyl methyl 5-882 phenone

78-89ketone m-N02-aceto-Isobutyl methyl 1-549 phenone

20-73ketone p-Cl-aceto-Diisopropyl 1-366 phenone

ketone Cyclopentanone 66-59Eenzoylacetone 115-4 Cyc1ohexanone 41-67Acetoacetic 1630-0 Cvcloheptancne 4-114

ester Cyc1ooctanone 24'41Acetophenone 6-743 2-Acetylpyri- 29-53p-CH.-aceto- 6-264 dine

phenone 4-Acetylpyri- 177-6p-N02-aceto- 95·21 dine

phenone

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61.

rINDIAN J. CHEM., VOL. 17A, JANUARY 1979 •

TABLE 5 - EFFECT OF Os(VIII) ON THE REACTION R.UEIN AQUEOUSMEDIUM

[Aqueous medium, NaClO. = 0'002M; NaOH = 0·1M;substrate = 0'005M; CAT = 0'00045M; temp. = 45°]

106 [Os(VIII)] 103 x k2M litre mol"! sec-1

3·935 100·77·870 84'0

15·74 88·8279·43*54·7147'32*

Cyclopentanone

Cyclohexanone 3·935

*Extrapolated values in pure aqueous medium withoutOs(VlII).

no abnormal catalysis as observed by Mushran andcoworkers for acetaldehyde and formaldehyde.Hence such a complex may not be formed at allin the present study. Such abse;nce of O~(VI.n)catalysis has also bee;n obser~ed m ~he l~xIdabonof anilines with CAT m alkaline medlU.~ .

Mechanism and rate law - The negligible effectof ionic strength and the effect of dielectric constantpoint to the involvement of at east one neutralmolecule in the rate limiting step. . .

Hence it may be possible that enolate am~n whichis in equilibrium with the enol may react wIth. CATin a slow and rate-limiting step to ~orm an inter-mediate which subsequently reacts in a fast stepwith another molecule of CAT to give the product~.In keeping with the observations rate law (1) ISproposed

_ d[CATJ = klk2 [CAT] [S][OH-] ... (1)dt k.,

which predicts first order. dependence on t?e sub-strate, CAT and ORo, m agreement WIth theexperimental observations. . . . ..

Further, the fact that the oxidizing specI~s ISthe CAT itself is proved by the fact that (1) p-toluenesulphochloramide and .dichloram~~e-T do nO.texist in highly alkaline SOlutIOI~.sand (11). the POSSI-bility of OCl- is ruled ou~ ':S It should myolve aninteraction between two similarly charged IOns andthus would correspond to positiv~ ionic strengtheffect which is contrary to expenmental o?~erva-tion. Hence it is the CAT itself that participatesin these oxidations.

Reaction mixtures containing excess of chlor-amine-T over ketones were allowed to equilibrateat 45° for 48 hr. The stoichiometry (1 :2) wasobserv~d for all the ketones studied. The productsin the case of aliphatic and cyclic ~etones were thecorresponding 1,2-diketones and in the case ofacetophenone the product was found to be phenacyl-dehyde. . .

pH -raie profile for cyclohexano11~':-' As a con~mumgstudy it was thought worthwhile to examine theeffect' of pH on the reaction rate of cyclohexan0l!-eand to establish a complete pH-rate profile In'aqueous medium. The second order rate constantsare recorded in Table 6.

It is clear that in the pH range studied thereactivity is maximum at pH 1·3 and decreases

62

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TABLE 6 - EFFECT OF pH ON THE RATE OFOXIDATION OF CVCLOHEXA.NONE

[Aqueous medium, cyclohexanone = 0'005J11I;CAT = 0'00045M; NaCIO. = 0'002M; temp. = 45°]

pH 103 Xs, PH 103 Xk.litre mol"! see"! litre mol"! ·sec-lo·

1'3 101'4 7·6 No reaction1·9 50·08 10·0 No reaction4'5 '3'43 12·7 56'456·85 2'36

linearly up to pH 6·85 (the rate studies below pH1·3 were not carried out). Surprisingly there isno reaction at pH 7·6 and 10 and the reactivityappears again above pH 10 and the reactivity is.maximum at pH 12·7. The product study andbreakdown in the reactivity at pH 7·6 indicatesthat the reaction path up to PH 7·6 is different fromthat observed at pH 12·7.

Evidently the sulphochloramide is the reactivespecies up to PH 3 and HOCl is the reactive speciesin the pH range 3·0-4·5. The enhanced reactivitydue to HOCI as observed in halogenation of amines-Pis masked here by the overall pH dependence. Thedirect dependence on [H+] in the acidic pH anddirect dependence on [OR] in the alkaline pH also-establishes that the reactions in acidic pH arehalogenations and are different from the oxidation.process in alkaline medium.

Thus the reaction path is a Iunction of pH andthis change over of the path from halogenation tooxidation is an Important observation.

References1. PANIGRAHI, G. P. & MISRA, P. K., Proc. Indian Acad.

su.. 5 (1976), 211.2. PANIGR.'\.HI,G. P. & MISRA, P. K., Indian J. cu«; 14A.

(1976), 579.3. SANTAPPA, M. & MEENAKSHI, A., Indian J. Chem., 11

(1973). 393.4. RADHAKRISHNAMURTI, P. S. & SUSHILA DEVI, Indian

J. cu,«, 14A (1976), 399.5. RADHAKRISHNAMURTI, P. S. & SUSHILA DEVI, Indian.

J. Chem., 10 (1972), 496.6. SANTAPPA, M. & MURUTHAMUTH,P., Indian J. Chem.~

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chem. Soc., (1964), 3263.15. RADHAKRISHNAMURTI, P. S. & MAHANTI,M. K .• Indian.

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J. Chem., 12 (1974), 311.17. RADHAKRISHNAMURTI, P. S. & PRASAD RAO, M. D.,.

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