Indian Journal of ChemistryVol. 17A. February 1979. pp. 170-172

Kinetics of Chromic Acid Oxidation of CamphorP. S. SUBRAMANIAN· & D. R. NAGARAJ AN

Department of Chemistry, Madras Christian College, Tambaram, Madras 600059

Received 30 July 1977; revised 25 January 1978; rereuised 21 July 1978; accepted 21 September 1978

The kinetics of the Cr(VI) oxidation of camphor have been studied in aqueous acetic acidmedium. The reaction is of a total second order at constant acidity. being first order withrespect to each reactant. The reaction is acid-catalyzed and the order with respect to [H+] isunity. The reaction rate is decreased by the addition of Mn(II) and Co(II) ions. Addition ofethylenediamine increases the rate of oxidation. The rate of enolization of ketone has beencompared with that of oxidation of ketone. A suitable mechanism has been proposed.

"THE kinetics of chromic acid oxidation of organicsubstrates, including aliphatic, alicyclic andaromatic ketones have been studied extensive-

ly1-10. Chromic acid oxidation of camphor appearsto have received little attention. This therefore,forms the subject matter of the present paper.

Materials and MethodsCamphor was purified by sublimation. AR grade

manganese(II) acetate and cobalt (H) acetate wereused as such. The complexing agent, ethylene-diamine was distilled, collecting the middle cuts.Water and acetic acid were purified by standardprocedures. Ionic strength was maintained byadding Na2S04. Cr(VI) oxide (E. Merck) was twicerecrystallized from conductivity water and stocksolutions were prepared in conductivity water.

Kinetic method - The rate of the reaction wasfollowed by the usual titrimetric method-", thedisappearance of Cr(VI) being measured iodometri-cally. All the reactions were carried out in binarysolvent mixtures of acetic acid and water and atconstant ionic strength and pH. Hydroxycamphor,camphorquinone and camphoric acid were identifiedas the oxidation products.

Results and DiscussionThe reaction is of total second order at constant

acidity, being first order with respect to Cr(VI)and the substrate. The rate constants at varying[camphor] and constant [Cr(VI)] and also at vary-ing [Cr(VI)] and constant [camphor] are presentedin Tables 1 and 2 respectively. The rate of dis-appearance of Cr(VI) follows first order rate law,but the first order rate constant decreases withincrease in gross [Cr(VI)~. This clearly indicatesthat the effective oxidant is the acid chromate ionHCr04, whose concentration decreases with theincrease in gross [Cr(VI)] (refs 2, 6, 8). At higherconcentration, the monomeric species HCr04 isconverted bv a reversible reaction into dichromateion, which is known to be a less powerful oxidant-than HCr04. The [HCr04J cannot be calculatedwith certainty from the gross [Cr(VI)] in aqueousacetic acid media, though this has been achievedin aqueous medium=" .

170

(

TABLE 1 - EFFECT OF VARYING [CAMPHOR]ON THERATE OF OXIDATION

{[Cr03]=0'0008M; [H.SO.]=0·2M; !L=0'2M; solvent, 500/",acetic acid (v/v); temp. 300

}

[5] k, x 105 k,/[S] X 103 [5] k' x 105 k,/[S] X 10"Mxl0' see= Mxl0' sec!

2'0 2·98 1'49 3·5 5·46 1'582'5 3-84 1·54 4·0 6'34 1·583·0 4·70 1·57

5 = Camphor.

TABLE 2 - EFFECT OF VARYING [CHROMICACID] ON THERATE OF OXIDATION

{[Camphor]=0'02M; [H.SO.]=0·2M; !L=0'2M; solvent, 60%acetic acid (v/v); temp., 300

}

[Cr03]

Mxl03[Cr03]

Mxl03

1·3301'558

1·891·79

1'7731·997

1·701·67

TABLE 3 - DEPENDENCE OF RATE ON ACIDITY

{[Camphor]=0'025M; [Cr03]=0'0008M; !L=0'2M; solvent •.50% acetic acid (v/v); temp., 300

}

[H2SO.] k, x 105 k,/[H.SO.] [H.SO.] k, x 105 k,/[H.SO.]M see' xro- M see'< xio-0'20'40·5

3'837·909'75

1·921·971·95

0·60'8

11-1415'42

1·861·93

. The rate of the reaction increases with the increasein [H2S04]. Under the condition of constant ionicstrength, [camphor] and [Cr(VI)], the first order rateis found to be proportional to [H+] (Table 3).

Effect of changing the solvent composition - The-increase of the reaction rate in acetic acid-watermixtures of different compositions is moderater(Table ~). This is in agreement with the general-observatioriw-P, that increasing the acetic

SUBRAMANIAN & NAGARAJAN: OXIDATION OF CAMPHOR

TABLE 4 - EFFECT OF SOLVENT COMPOSITION ON THERATE OF OXIDATION

{[Camphor] = 0'03M; !L = 0'2M; [H2S04] = 0'2M; [CrO.]=0'0008M; ternp., 30°}

HOAc k2 x 103 HOAc k2 x 10'v/v (litre mol? sec ") v/v (litre mol'? secv)% %

50 1·56 70 4-1855 1'70 75 5·6260 2'57 80 8·2665 3·06

TABLE 5 - EFFECT OF Mn(II) AND Co(II) IONS ON THERATE OF OXIDATION OF CAMPHOR

Salt x 104

MSalt X 104

M

{[Cr03] = 0'0008M; [Camphor] = 0'03M; [H2S04] = 0'2M;iJ. = 0'2M; solvent, 60% acetic acid (v/v); temp. 30°; in

presence of Mn (II)}

nil1020

5·374·984·68

3040

4-083'63

{[Cr03] = 0'00067M; [Camphor] = 0'02M; [H2S04] = 0'2M;!L = 0'2M; solvent, 50% acetic acid (v/v); temp., 50° in

presence of Co(II)}

Nil1020

1·811·741'68

3040

1·371·31

concentration of the medium increases the rate ofoxidation by chromic acid. This is probably due tothe lowering of dielectric constant of the mediumwhich favours a less polar transition state comparedto more polar reactants. The pl?t of log kl ve~suslID is linear as expected. for .an ion-dipole re.actlOn.It is also probable that in binary solvent m~xtur~sof acetic acid and water most of the chromic acidis present as CH3COOCr020H or as its protonated

+form CH3COOCrOpH2 with an oxidizing powerconsiderably greater than that of Cr03 (ref. 16).It is evident that replacement of hydroxyl groupby acetyl group would considerably decrease theelectron density on the central chromium atom andconsequently increase its electrophilic characterand hence its oxidizing power.

Temperature dependence of reaction rate - Thetemperature dependence of rate of reaction hasbeen studied in the temperature range 30-50°of [camphor] = 0'025M; [Cr03] = 0·0008M; [H2S0~]= 0·2M; fL = 0'2M and solvent = 50% <l;q. aceticacid (v/v). The activation parameters denved fromtemperature dependent rate data are: E« = 12·5kcal mol<: 6..Ht = 11·9 kcal mol=": 6..Gt = 21·0kcal mor+: 6..St = -30·1 e.u.; and A = 1·706xl06•

Effect of added Mn(II) and Co(II) salts - Thereaction rate decreases III the presence of addedMn (II) ions and Co(II) ions (Table 5), an observationalso recorded by earlier workers. This may be dueto the fact that Mn(II) and Co(II) ions interactwith Cr(IV) (Scheme 1), thereby reducing the

(

Cr(VI) -l-enol form -7 Hydroxy ketone-j-Crfl V)Cr(IV) -l-enol form -+ Hydroxy ketone-j-Cr'(H)Cr(VI) +Cr(II) "" Cr(V) +Cr(III)Cr(V) -l-enol form -7 Hydroxy ketone-j-Cr (Ll I]Cr(IV) + Mn(II) "" Mn\III) +Cr\III)Cr(IV) +Cc (II) "" Co (III) +Cr(III)

... (1)

... \2)

... (3)

... (4)

... (5)

... (6)

Scheme 1

effective concentrations of Cr(IV) and Cr(V) whichare also good oxidizing agents.

The Mn(II) ions and Co(II) ions are convertedinto Mn(III) and Co(III) ions respectively and theoxidant which accomplishes this must be Cr(IV)formed by the initial Cr(VI) and enol reaction.Since Mn(II) and Co(II) ions consume Cr(IV)(reactions 5 and 6). reactions (2) and (3) are prevent-ed and Cr(V) which in the absence of Mn(II) orCo(II) ion, oxidizes the substrate, is not formed.It is assumed that manganous ion or cobaltous ionwill react with Cr(IV) in a reversible manner andas more and more Mn(II) or Co(II) ions are addedto the reaction medium, the rate of the reaction isdecreased progressively.

Effect of complexing agent on the reaction rate-The effect of addition of ethylenediamine on therate of chromic acid oxidation of camphor has beeninvestigated. It should be noted that the com-plexing agent itself is resistant to oxidation underthe conditions employed. Ethylenediamine is foundto accelerate the rate of the reaction. However,added pyridine has no influence on the reactionrate showing thereby that the reaction is notsusceptible to base catalysis'<. The rate enhance-ment observed in the present. study is probablydue to the stabilization of the end product Cr(III)which pushes reaction (3) to the right.

Enolization of camphor - Enolization of camphorhas been studied under the experimental conditionsemployed for its oxidation and the results are givenin Table 6. The order with respect to camphorat constant acidity is unity.

Mechanism - The oxidation of camphor by Cr03shows that (a) the reaction rate is first order withrespect to the substrate, (b) the rate at whichCr(VI) disappears also follows the first order ratelaw, (c) the reaction is acid-catalysed and (d) therate of enolization of camphor is faster than the rateof oxidation.

TABLE 6 - RATE OF ENOLIZATION OF CAMPHOR

{[Camphor] = 0'025M; [H2S04] = 0'2M; !L = 0'2M; solvent,50% acetic acid (v/v); temp., 30°}

[Br2] hI X 105 [Br2] hI X 105M see"! M see"!

[Cr03] = 0'0008M

nil 3·84

[Cr031 = 0·00

0·0008 6·35 0·0032 6'190·0016 6'72 0'0040 6'93

171

r~----------INDIAN J. CREM., VOL. 17A, FEBRUARY 1979

J:ro ~OH

~~\.I" ~o=Cr =0

'-"IOH

~Cr~+H+J

~O+H20 '''',

'O-Cr-OH1\o Schern e 2

According to Best et al», the rate of chromicacid oxidation of cyclohexanone is slower than thatof enolization at the same acidity. But it is ratherdifficult to say whether the ketone molecule isdirectly attacked or its enol form is being attackedby chromic acid. Recently Rocek et al" have givensufficient evidence to show that the oxidation ofketone proceeds through the enol intermediate.The formation of enol can not be rate-determiningsince the rate of reaction depends on the chromicacid concentration and the rate of enolization isfaster than that of oxidation. It could be possiblethat at higher concentration of chromic acid, the

/

172

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rate of reaction might show zero order dependenceon Cr(VI) and hence be equal to the rate of enoliza-tion.

The mechanism which explains the results ob-tained is shown in Scheme 2.

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