OXIDATION of ALDEHYDES by AQUEOUS Ce (IV) in NITRIC ACID

OXIDATION OF ALDEHYDES BY AQUEOUS Ce (IV) IN NITRIC ACID

BY M. RANGASWAMY, S. LAKSHMINARASIMHAN AND C. V. RAMADAS* (Department of Chemistry, A.M. Jain College, Madras-61, India) Received March 8, 1969

(Communicated by Prof. M. Santappa, F.A.SC.)

ABSTRACT The kinetics of oxidation of acetaldehyde (at 20°, 31° and 40° C.), propionaldehyde (at 15°, 25° and 35° C.) and chloral hydrate (40°, 50° and 60° C.) by ceric nitrate in nitric acid medium [(H+) 0.5-1 •5 M] were studied. The reactions were followed by determining rates of eerie disappearance for variations in [Ce4 ''], [RCHO], [H+], µ, temperature, etc. The stoichiometry, QCe 4+ / A CH3CHO . 2 was established. The reactions were found to be 2nd Order—first order each with respect to [Ce 4}] and. [RCHO]. No complex formation between Ce4 and aldehydes was observed. Neutral Ce (NO3) 4 (H20) 2 and diol of the aldehyde were assumed the active species. The rate and thermodynamic data were calculated and discussed. The rates of oxidation were correlated with the structures of the aldehydes.

INTRODUCTION HARGREAVES AND SUTELIFFE,' from their study on the oxidation of formal- dehyde by Ce 4+ in HC1O 4, concluded that the reaction occurred between Ce4+ and diol of aldehyde, CH 2 (OH) 2 . Rangaswamy and Santappa 2 also provided evidence for the diol forms of aldehydes as the active species from their study on the oxidation of HCHO, CH 3CHO, and CCI 3CHO by Ce4+ in HC1O 4 , and they extended work on HCHO 1 with regard to evaluation of thermodynamic parameters and also correlated rates of oxidation with structures of aldehydes, teased on conformational model of the aldehydes. The present work refers to kinetics of oxidation of acetaldehyde, propional_ dehyde (methyl acetaldehyde) and chloral hydrate by aq. CO-, in nitric acid medium [(H+) -. 0.5 to 1.5 M] in the temperature range, 15° C. to 60° C. Dukes's theory 3 of complex formation between oxidant and reductant,

* Department of Chemistry, College of Engineering, Madras-25, India. 292 Oxidation of Aldehydes by Aqueous Ce (IV) in Nitric Acid 293 common to many homogeneous oxidations of organic compounds by metal ions, was not applicable to aldehydes under consideration since no evidence for complex formation between Ce 4+ and aldehydes was forthcoming from kinetics or from an independent spectrophotometric method. 4 The infor- mation available on the state of cerium in nitric acid is meagre. A red hydroxy-nitrate [Ce. (OH) (NO3)3 . H 2O] has been isolated' and existence of tetranitrato hydroxy ceric ion [Ce (NO 3) 4 (OH) (H 2O)] - in low [HNO 3 ] was postulated by Duke and Forist,b during oxidation of 2 : 3 butanediol. Shorter' assumed the reactive species to be Ce.OH 3 '- in the oxidation of acetone by eerie nitrate in HNO 3 . The present work was undertaken with a view to throw some light on the state of Ce4+ in HNO 3 . The various rate and thermodynamic parameters for the system, Ce4 F + aldehyde, were evaluated and discussed, and the difference in rates of oxidation of the aldehydes studied were interpreted in terms of their structures.

EXPERIMENTAL METHODS All solutions for the kinetic studies were prepared in doubly-distilled water, once over alkaline permanganate. The chemicals, used in the present kinetic studies, were of AnalaR grade and were used as such without further purification. The reaction was followed (for Ce 4+ + acetaldehyde system for 50 minutes to '- conversion of Ce 4 F ; for Ce4+ + propionaldehyde system for 25 minutes to - 36% conversion of Ce 4+ and for Ce41 + chloral hydrate system for 50 minutes to — 34%) by measuring rates of eerie disappearance at 10 minutes intervals (5 minute intervals for Ce 4+ — propionaldehyde system)—aliquots (10 ml.) withdrawn being added to known excess of ferrous sulphate solution and the remaining (unused) Fee being titrated against standard eerie sulphate solution using ferroin indicator. The reaction was done in dark to avoid photochemical decomposition of Ce. Absorption measurements for the search of complex formation were made using u. v. spectrophotometer (H-700 type: Hilger and Watts).

RESULTS

(i) Orders with respect to [Ce 41-] and [Aldehyde]

At constant [aldehyde] ( 0 . 007 M for acetaldehyde, .., 0.11 M for chloral hydrate and ,. 0.1 M for propionaldehyde), [H+] 1 M, µ ( 1.2 M), temperature (31° C. for acetaldehyde, 50° C. for chloral hydrate and 25° C. for propionaldehyde) and varying the [Ce4+] (3.5 x 10-3 M to 6 x 10-3 M), a first-order dependence with respect to [Ce 4* ] was observed (Fig. 1). From linear plots of log [CO - '] vs. time, pseudo uni- 6-A6 294 M. RANGASWAMY AND OTHERS +] molecular rate constants, k„ b ,., were evaluated. At constant [Ce4 (4 x 10-3 M), [nitric acid] (1 M), µ (1.2 M) and temperature, variations of [ aldehydes] (. 0 . 005 to 0 . 009 M for acetaldehyde, 0.002 to 0.008 M for propionaldehyde, and 0.068 to 0.160 M for chloral hydrate) gave rates increas-

ing with increasing [aldehydes] and by plotting k ob; , vs. [ aldehyde], a first order dependence with respect to [aldehyde] was observed (Fig. 2). That the total order of reaction is two was proved by linear plots of 1/a — x vs. time (mts.) with an intercept of 1/a on the ordinate, under conditions of [Ce4+] = [RCHO] = 0.004 M.

A CERIC -ACETALDEHYDE SYSTEM A : CERIC - ACETALDEHYDE SYSTEM B CERIC-PROPIONALDEHYDE SYSTEM B CERIC - PROPIONALQEHYDE SYSTEM C CERIC -CHLORALHYDRATE SYSTEM C CERIC - -CNLORAL HYDRATE SYSTEM V m < urn -^ 11 O O O ^ ^ r 0 + u 0 '^ On O O 0 O ,o 0 iii V zt i V O V a y 0 0 0 _.V 9

O n O m ry u 0 0 0 \ 06 IS 0 0 y s^ m 00 N 0 • 0 0 0

p - O O - - O 0 0 0 O n

o 0 0 00 . 0.5 0.55iii: 0.60 04S 0.70 —1.9 0 1.2 2.4 3.6 4.8 +B; QICHO) %10 0 0.15 030 045 0.60 —...073 C 0 0.002 0.004 0006 0.000—A 0 0.04 008 0.12 0.16 —C LOG[C. 4* )

FIG. 1 FIG. 2

(ii) Effect of [H+]

At constant µ, [Ce4+], [R-CHO] and temperature, the rates of oxidation of various aldehydes were found to be independent of [H+], [(H+) 0.05M to I5 MI.

(iii) Effect of Ionic Strength, µ Keeping constant [H+] and varying µ (from I •2 to 2) the rates of oxidation of the aldehydes were almost unaffected showing absence of any salt effects. Oxidation of Aldehydes by Aqueous Ce (IV) in Nitric Acid 295

(iv) Effect of Initially Added Products

(a) Ce3+.—At constant [Ce4+], [aldehyde], [H+], µ, temperature, etc., initial addition of [Ce 3+] {[Ce 3+]/[Ce4+] > 2} did not affect the rates of oxidation of aldehydes. (b) Organic acids.—With the aldehydes, under study, the corresponding acids formed were not further oxidised under our experimental conditions. This was also proved by the rate of Ce 4++ aldehyde reaction being slightly decreased by initial addition of the corresponding acids, under conditions [Ce4+] _ [(organic) acid] . 4 x 10-3 M. (v) Stoichiometry The stoichiometry, ACe4+/ A acetaldehyde . 2 (under conditions of initial [Ce4+] . 1.6 x 10 -2 M; [aldehyde] 2 x 10-3 M; [HNO 3 ] = 1.0 M) was established by allowing the reaction to go for completion (left overnight) and then estimating the unused [Ce4+] from which the concentration of Ce 4^ consumed per mole of aldehyde w. s calculated.

(vi) Search for Complex Formation between Ce 4+ and Aldehydes In the present study, as well as in oxidation of aldehydes by Ce 4+

in HC10 41, 2 no kinetic and/or absorptiometric evidence was obtained for complex formation between Ce 4+ and diol forms of aldehydes, under the experimental conditions employed; plots of 1 /k. bs . vs. 1 / [RCHO] being linear with no intercept on the ordinates (Fig. 4: cf. Michaelis and Menten 8 and others 9).

DISCUSSION Mechanism of Oxidation Under our experimental conditions, oxidation of water by Ce 4+ was negligible as shown by blank experiments. The rates of oxidation of aldehydes by Ce4+ were unaffected by initial addition of Ce 3+ to the system and therefore possibility of oxidation by OH radicals produced in a possible equilibrium

Ce4+ + H 2O ^ Ce 3+ + OH + H+ (1) might be ruled out. The species existing mainly in nitric acid solution of Ce (IV) are Ce (NO 3) 6 2-, Ce (NO3), and Ce (NO 3) 4 and Ce (OH) (NO3)4 which arise from the equilibria, Ce (NO3)4 + NO3 -- Ce (NO3)5 (2) Ce (NO3)5 + NO3 - Ce (NO3)6 2 (3)

Ce (NO 3) 4 + H 2O -_ Ce (OH) (NO 3)4 H- H+, (4) 296 M.. RANGASWAMY AND OTHERS

Our observation that the rates of oxidation were unaffected by changes in [HNO 3] and [NO3- ] (µ) would mean that equilibria (2), (3) and (4) were not operative and possibility of higher nitrato species of cerium as active species may also be neglected. Therefore, neutral Ce (NO 3) 4 (H 20) 2 may be taken as active species, for which symbol Ce 4+ may be assigned for simplicity. Results reported here may be satisfactorily explained on the basis of a mechanism involving Ce 4+ and diol of aldehyde, RCH (OH) 2 . The possibility of RCH (OH) as active aldehyde species, possibly due to an equililbrium,

RCH (OH) 2 + H+ -_ RCH (OH) + H 2O (5)

may also be remote since the rates of oxidation were independent of [Hq - ]. Therefore, for Ce4+— acetaldehyde system, assuming the equilibrium

K' CH 3CHO + H 2O CH 3 CH (OH) 2 (6)

A : CERIC - ACETALDEHYDE SYSTEM A : CEPIC - ACETALDEHYDE SYSTEM 8 : CERIC - PROPIONALDEHYDE SYSTEM 8 CE8 C - PROPIONALDEHYDE• SYSTEM C CHLORAL-HYDRATE SYSTEM C CERIC-CHLORALHYDRATE SYSTEM U

f, C p m ^O O V$ ( ' , 100 0 O O O O a *ii J -

L a O V 000 OR YO n e O m 000 . O i O O ^ O m 000 m D a ^ O d Q N 0 0 D 0 Q t 0 O M O O 000 1 EEE O O.1 0.2 0.3 0.4 0.5—B;1/[RCHO^X 0 3.0 3.I 3,2 33 10 4.0 8.0 1 0 12.0 16.0 —C / T %103 FIG 3 FIG. 4

the mechanism suggested is:

CH3 OH CH$ O \ / k (i) Ce4+- + C ----^ C + H+ + Ce3-r- (7) /\\ slow /\ H OH H OH Oxidation of Aldehydes by Aqueous Ce (IV) in Nitric Acid 297

CH, 0 CH, fast e Ce4+ + C ---> C =0 + HL .+ C s+ (8) /\ H OH HO Acetic acid

(ii) Since acetaldehyde is only hydrated to 52.8% at 20° C. 10 another possible mechanism, explaining the observed kinetics, would be slow Ce4+ + CH,CHO ----- CH 3CO + Ce 3+ + H+ (9) fast CH3CO + Ce4+ + H 2O ----^ CH,COOH + Ce 3+ + H (10) (iii) Trahanovsky and Young's mechanism," for oxidation of organic substrates like toluenes by Ce 4+, involving a two electron transfer with possible Ce 2+ intermediate was also assumed to be not operative in the present study since reactions occur via formation of free radicals. At constant [H+], the rate law would be

d [Ce41 ] = k, [Ce4+]T [CH3 CHO]T = k [Ce4+] [CH3CH (OH) 2 dt ] (11)

where k, and k are observed and theoretical rate constants; [Ce4+]T and [CH3CHO] T are total [Ce4+] and [CH,CHO]. Taking into consideration, the following equalities,

3CH (OH)2] [CH3CHO]T = [CH3CH(OH) 2] -}- [CH

from equilibrium (6) and [Ce]T being only [Ce4+] (since neutral Ce (NO3) 4 (H20) 2 is assumed as active species), the rate equation (11) would become

(12) ki k + kK) From equation (12) it is obvious that the rate of oxidation of the aldehydes should be independent of [H+] and it is experimentally realised ([H+] .. 0.5 to 1.5M]. Assuming K' values 2 (K' . 0 . 99 at 20°C. and 0.88 at 25° C.), 298 A RANGASWAMY AND OTHERS

the value of k ,. 4.6 at 20° C. and 16 . 9 at 31° C. were computed and AE, AS and AF$ for k and kl have been evaluated for Ce 4+-acetaldehyde system.

Propionaldehyde and Chloral Hydrate Since both these aldehydes exist as purely hydrates in aqueous acidic medium, 12-14 sequence of reactions (7) and (8) suggested for acetaldehyde would apply to these systems, neglecting the possibility of equilibrium for diol formation.

Rate and Thermodynamic Data From kinetic experiments carried out at three temperatures (20 °, 31° and 40° C. for acetaldehyde, 40°, 50° and 60° C. for chloral hydrate and 15°, 25° and 35° C. for propionaldehyde) QE, AS and p F$ values for k, (and k for acetaldehyde) have been computed for the aldehydes under consideration (Table I) :

TABLE I Rate and thermodynamic parameters

k1 1.mole-1AE ASt AFt Temp. sec.-1 K.cals./mole e.u. K.cals./mole Substrate °C. at 1.0 M k (H'') k, k k1 k k1 k

Acetaldehyde 31 7.2 16.9 18.3 20.8 3.50 13.6 17.2 16.7 Propional- 25 dchyde 24.0 .. 22.4 .. 20.5 .. 16.3 Chloral 50 0.08 .. 21.4 .. 0.37 .. 20.2 hydrate

No rational explanation is possible at present for AS$ values of CH 3CHO and CH3CH 2CHO excepting perhaps that high positive ASt and AE values mean smooth reaction with faster rate as is realised by us experimentally with regard to the latter. But the change in A St values for chloral hydrate as compared to acetaldehyde or propionaldehyde shows drastic differences in the structures of their activated states. The near constancy in the AFt values for acetaldehyde and propionaldehyde may be understood in terms of isokinetic relationship.'5

Oxidation of Aldehydes by Aqueous Ce (IV) in Nitric Acid 299

Structure and Reactivity The rate of oxidation of propionaldehyde was faster than that of acetaldehyde, while chloral hydrate was oxidised at a very much slower rate so that the order of reactivity towards oxidation by Ce 4+ is propionaldehyde > acetaldehyde > chloral hydrate. Since oxidation depends on the electron release from the reductant to oxidant, the presence of—CH 3 group in propionaldehyde, being strongly electron releasing in nature, favours faster rate of oxidation. The slow rate of oxidation of chloral hydrate as compared to either acetaldehyde or propior- aldehyde may be possibly due to its greater stability.'

ACKNOWLEDGEMENTS

Grateful acknowledgements are made to Prof. Dr. M. Santappa for his valuable suggestions and encouragements towards completion of this work and the authors thank Prof. Dr. V. Venkatasubramanian, T. Thothadri, P. S. Jayaraman and S. P. Srinivasan for their active part in the present study.

REFERENCES

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12. Herold, W. and Wolf, Z. Physik. Chem. Aht., 1931, 128, 165. K. L. 13. Bezzi, S. .. Gazz Chim. (tal., 1933, 63, 345. 14. Jack Hine .. Physical Organic Chemistry, 2nd Ed., (McGraw-Hill Book Co., Inc., N.Y. and Kogaksuha Co., Ltd., Tokyo), p. 50, 15. Leffler, J. .. J. Org. Chem., 1955, 20, 1202.

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