OXIDATION STUDIES. BY S. PADMA AND M. SANTAPPA, F.A.Sc.

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1 OXIDATION STUDIES VIII. Oxidation of Some Carboxylic Acids by Peroxydisulphate Catalysed by Metal Ions BY S. PADMA AND M. SANTAPPA, F.A.Sc. (Department of Physical Chemistry, University of Madras) Received March 25, 1968 INTRODUCTION RESULTS of Ag+ catalysed oxidation of a number of organic substrates, by peroxydisulphates (S 2O87) were reviewed by House' and Wilmarth and Haim. 2 Koishiroshinra, Kenichi, Saurai and Taayani 3 studied the Ag+ and Cu++ catalysed oxidation of mandelic acid by S 2O8r at 25 C. to 90 C. and reported that the relative amounts of products (benzene, phenol, etc.) depended on the ph of the reaction. Mishra and co-worers,4 Baore and Joshi5 and Venatasubramanyan 6 studied oxidation of lactic acid (LA) by S2O8 catalysed by Ag+. Kinetics of silver catalysed oxidation of citric acid' and formic acid 8 were also reported. The oxidation of o-phenoxy benzoic acid by S 2O8-' catalysed by Age- ions was studied by Thomson and Wylie.e In all these cases a two electron transfer resulting in the formation of Ag + 3 was suggested. We report in this paper results of Cu +2, Mn+2, Pb+2 and Hg2+ 2 catalysed oxidation of lactic, malic and succinic acids by S 2O8 ' in the temperature range C. in H 2SO 4 or HNO 3 media (depending on the catalyst) and at µ = 0.2. Our experimental results lead us to the conclusion that in all cases the total order is 2, one each with respect to [S 208j and [catalyst] and the order with respect to the substrate is zero in the rate equation for S 208 disappearance. The Pb+ 2 and Mn+ 2 catalysed oxidation of malic acid are directly proportional to [H+] while Cu +2 catalysed oxidation of malic and succinic acids bear inverse relation to [H+], the oxidation of all the other systems being independent of [H+]. Plausible mechanisms of oxidation are suggested and the inetic parameters determined. EXPERIMENTAL All the reagents used are A. R. grade. The experimental procedure was as given in the previous paper 1 except for the fact that a calculated quantity of the metal ion was added in these systems as catalysts. Rates, R [S 2081 were usually corrected for water oxidation which was negligible A4 189

2 190 S. P ADMA AND M. SANTAPPA (-1 % of the total rate) as found in the blan experiments. Oxidations were usually conducted for 30 min. to 3 hr., depending on the substrates and conversions of S 2O8 were ' 30 to 40%. Experiments under identical conditions with substrates + metal ions but without S 208 were also conducted for determining oxidation of substrates under these conditions. Rates of disappearance of the substrates were estimated by titration of the aliquots of the systems against standard Na (r N/50). Stoichiometries A (S 2O8 )/ A (Product substrate) were determined by running the oxidation of the systems, [S 2O8 ] = M, [Substrate] = 0.02 M and the (catalyst) [Cat.] = M for 4 hours. Product of oxidation of lactic and malic acids was acetaldehyde and that of succinic acid was formaldehyde. HCHO was identified qualitatively with chromotropic acid and all the aldehydes were estimated gravimetrically as the corresponding 2, 4-dinitro phenyl hydrozones. RESULTS AND DISCUSSION General. In the case of all the systems it was observed that the oxidation was independent of the [substrate] = (0.5 x 10-2 M to 22 x 10-2 M) [HSO4 ] (0.1 M to 1 M), [ionic strength] (01M to 1 M). The pseudo first order rate constants ( Ob,.) were calculated from the linear plots of log a/a-x vs. time and the second order rate constants () were evaluated from the plots of ob,, vs. [Cat.] (Plots A, B, C and D) (Fig. 2). The second -order rate constants were either independent of [H+] or of the forms (Plots E, F and G, Figure 3 and Plots A and B, Fig. 1 b). = l + 2 [H+] or =, + 2 ] / H+]. Intercepts in the plots of ob,, Vs. [Cat.] indicated the occurrence of an uncatalysed reaction though of negligible magnitude. (N 1% of total rate). It was observed that for all the systems increased when Cat. 3 x 10-4 M, but there was a gradual decrease of when Cat. > 3 x 10-4 M which may be attributed to the formation of substrate catalyst complex and the consequent decrease in the [Cat.], the reactive species. The values for the ratio of (catalysed/uncatalysed) oxidation rates for the system of succinic acid with Cu+ 2, Mn+ 2, Pb+ 2 and Hg2+ 2 respectively were 12, 17, 16 and 24; with malic acid the corresponding values were 2.6, 2.6, 2.3 and 1.5 and with lactic acid the values were 2.6, 2.6, 1.2 and 1.5. It was also found that no oxidation of the substrate too place with the metal ion alone but without persulphate. On the other hand the total order for the rate of decomposition of S 208 by the catalyst without the substrates was 2, first order each with respect to [S 2O8 ] and [Cat.]. The pseudo first order rate constants being 1 x 10--5, 1.3 x 10^b, 2.8 x 10-s and 5.7 x 10-b and A E values 7.8, 17 5, ]8 i aid 21.4, for Cu+ 2, Mn+ 2, Hg2 2 ard Pb}2 respectively. The foregoing

3 Oxidation Studies.VIII 191 observations emphasise that the rate determining step in the oxidation of the substrates must involve the interaction of catalyst and subsequent reactions involving the substrates must be fast. The second order rate- constants for catalysed lactic acid oxidations were of the order: Hg$ > Cu^ 2 > Pb+ 2 > Mn 2+ and all were independent of [H+]. In the case of malic acid the trend was Cu+ 2 > Pb+ 2 > Hg2 2 > Mn+ 2, the magnitudes for Cu+ 2 and Mnt 2 differed considerably from each other and the magnitude for Pb+ 2 and Hg2 2 catalysed reactions of this substrate being independent of [H+] while the Cu+ 2 catalysed reaction was inversely proportional to [H+] and Mn+ 2 catalysed one directly proportional to [H+]. In the case of succinic acid though Mn+ 2 and Pb catalysed reactions were dependent directly on -[H+], unexpectedly their rate constants were found to be widely different at the same [H+]. The A E values for the oxidation of substrate acids and water with different catalysts under identical conditions being generally in agreement lends support to our assumption that S 208` + catalyst reaction must indeed be the rate determining step. Lactic acid (LA). The oxidation of LA catalysed by Cu+2, Mn^ 2, Pb+ 2 and Hg2 2 exhibit common characteristics, the rates being independent of [Sub], [HSO9 ], [NO3 ], [H+] and [ionic strength] and first order each with respect to [S $O8 ] and [Cat.]. The decreasing trend in the rate for LA 0.35 M observed in the case of uncatalysed and Ag+ catalysed reaction Was absent here. The rate law for all the lactic acid-metal ion systems are of the form: Rte, = [S20871 [Cat.] and the Arrhenius equations obtained were: = x 10 exp. ( 13, 450/Rx 333 3) lit. mole' sec.-'; AS# = e.u. = x 1014 exp. ( 20, 820/Rx 333.3) lit, mole-' sec.-'; AS+ = e.u. = x 10 2 exp. ( 4645/Rx 333.3) lit, mole-1 sec.-'; AS+ = e.u. = x 1013 exp. ( 19, 720/Rx 333-3) lit, mole-' sec.-1 ; AS+ = +2' 38 e.u. with Cu+Q, Mn+ 2, Hg2 2 and Pb+2 catalysts, respectively. The stoichiometries. A [S 2081 / ± A [CH,CHO], 1 for catalysis by Hg+ 2 and Pb+ 2 and 2 for catalysis by Cu 2 and Mn+ 2 ions. Our results may be explained satisfactorily on the basis of the following mechanisms.

4 192 S. PADMA AND M. SANTAPPA Hgti 2 Catalysis: S 2O87 + Hg SO42 + H g +3 (rate determining) followed by fast reactions, Hg+3 + CH3CH CO + H 2O ---> CH3CH + CO 2 ' + 2H+ + Hg; CH 8CH^ ---- CH3CHO + H 2O '- Pb+2 Catalysis: S2O87 + Pb +2 > Pb+4 + 2SO4 2 (rate determining) followed by fast reactions, Pb4 + CH3CH CO + H 2O CH3CH' + CO2 + 2H+ + Pb+ 2 ; CH3CH' ----b CH3CHO + H 2O Cu+ 2 and Mn+2 Catalysis: S208 + Cu+ 2 (or Mn+ 2) --* Cu+ 3 or (Mn+3) + SO4 + SO4 followed by fast reactions. CH3CH CO + Cu+s + SO4 HSO4 + Cu+ 2 + CH3CH COO CH3CH COO + S 208 }H BO ----p CH 3CH( CO + 2HSO4.

5 Oxidation Studies VIII 193 Succinic acid. The total order in all the systems studied was 2 ore each With respect to [S 208 ] and [Cat.] (Plot A, Fig. 1 a) and Plots. Fto. 1. (a) RS,Oj- vs. [S,08 =] at 60 C. [Sub.] 2 x 10-E M, [Cat.] = ix 10-` M,-µ =_ 0'1 2 M. (A) Succinic acid - - Pb'; (B) Malic acid - - Mn ` 2 ; (C) Malic acid - - Pb}2 (D) Malic acid Cu++. (b) Kobs. X 10' vs. [1/H+] ; T= 60'C., µ M, [Cat.] = 1 x 10-'M, [Sub.] -2 x 10-9 M, [S20815 x 10-EM. (A) Succinic acid - - Cu++; (B) Malic acid - - Cu++. The rate laws are of the forms: Rsaoa= = i [S2081 [Cu+2] + 2' [S208 ] [Cu+2]/[HTI, with Cu 2 catalyst. A5 Rs,O8,. = i [S208*] [Pb+$] + 2 [S208-] [Pb+2] [H+] with Pb+ 2 and Mn 2 catalysts. Rs,o,= = [S209 ] [Hg2 2] with Hg2+ 2 catalyst.

6 194 S. PADMA AND M. SANTAPPA The Arrhenius equations obtained were:, = x 101 exp. ( 8226/Rx 333.3) lit, mole-1 sec.-1 ; AS e.u. 8 = x 10' exp. ( 22,820/Rx 333.3) sec.-'; AS+ = 15.8 e.u. with Cu+$ catalyst. l = x 105 exp. ( 12,810/Rx 333.3) lit. mole-' sec.-'; AS+ _ 31.6 e.u. E = x 10' exp. ( 18,780/Rx 333.3) lit.'. mole -' sec.-1 ; AS+ = 15.9 e.u. with Mn' catalyst., = x 101 exp. ( 7940/Rx 333.3) lit, mole-' see.'; = 43.1 e.u. with Pb' catalyst. = x 10' exp. ( 18,910/Rx 333.3) lit, mole`' sec -' ; AS# = 17.1 e.u. with Hg,' catalyst. The stoichiometries. A [S a08 ]/ A [HCHO] = 1.5 for Hga 2, Pb+' and Cu+$ catalysed reactions and unity for Mn+a catalysed reaction. The above results may be satisfactorily explained on the basis of the following mechanisms: H92+2 Catalysis: S,O$ + Hg' ----o Hg4' + 2SO4 - f.,ollnwed by fast steps, CH,CO ^ - + Hg 4 + 2H2O --^ Hg* -{- CHa000H CH2 CH2 + 2CO3 + 4H+; I a 2HCHO +2H4 CH2 CH$ Pb' Catalysis: Pb+' + S,O, Pb+4 + 2SO 47

7 Oxidation Studies VIII 195 at Ti r 44 Gl 1 ^t u No Id 1 O O a o " P..S LL W Tr 00 0 q 1f1 In W C ^ D ^ 0 0 K O '.0 o s 0 6 ' o 3.. (AlgI M FIo. 2. K 0), vs. Catalyst, T 60 C., p 0.2 M, [Sub.] 2 X 10-' M, [S,OO`i 5x10-' M. (A) Lactic acid - - Mn*'; (B) Lactic acid - - Pb; (C) Lactic acid -- Hg,'; (D) Lactic acid - - Cu'. Koer vs. [H]; T 60 C., µ-175m, [Sub.] 2 x 10-' M, [S'O.'] = 5 x 1 P4 J4 [Catalyst] 1 x 10-` M. (E) Malic acid- - Mn'; (F) Succinic acid - - Mn'; (G) Succinic acid - - followed by the fast reactions CH2CO 2H2O CH2 I + Pb+d ; ( + 2CO 2 + 4H+ CH$CO CH= CH2 CH2 2HCHO + 2H+

8 196 S. PADMA AND M. SANTAPPA Cu+2, Mn+ 2 Catalysis: Mn^ 2 + S Mn+s + SO4 2 + SO4 CH 2CO CH2CO ^ + Mn+s ± SO4 --^ I + Mn+ 2 + HSQC CH2CO CH2CO CH 2CO H2O CH 2CO _ I + S 2O ---p + HSOq + SO 4 + CH2CO CH2CO CH 2CO CH 2CO + SO4 + H+ -+ 2CH 2CO + SO4- + ISO 2CH 2CO + 2 H 2O - - -> 2CH 2 CO + 2H+ 2CH 2 CO y 2HCHO + 2CO 2 + 4H+. Malic acid. The rate laws for the malic acid metal ion systems are of the forms: and - Rs,o.= = l [S208`l [Cu+2l + 2' [S208 ] [Cu+2]/[H+l - Rs,08- = [S2087] [Pb+2l, - = [S208`I [Hga 2] - Rs,o8- = i [S IMn+ 2l + 2 [S20r] [Mn+2] [H+J The Arrheriius equations' obtained were: Cu+ 2 Catalysis: l = x exp. (- 39,760/Rx 333.3) lit. mole -1 sec.-'; 2' = x 10' exp. ( /Rx 333.3) sec. -1 QSl and AS2' = e.u. Mn+ 2 Catalysis: l = x exp. (- 22,230/Rx 333.3) lit. mole-' sec.-1 ; AS =-17.9e.u.

9 Oxidation Studies VIII = x exp. ( 40,650/Rx 333.3) lit. 2 mole-2 sec.-1 ; AS+ = 17.9 e.u. Pb+2 Catalysis: = x 108 exp. ( 12,930/Rx 333.3) lit, mole -1 sec.-1 ; AS+= 20.9e.u. Hgz 2 Catalysis: = x 1011 exp. ( 18,270/Rx 333.3) lit, mole-1 sec.-1 ; AS# = 4.1 e.u. The stoichiometries. p [S208 ]/ Q [CH3CHO] were unity in all the 4 cases. The observed results may be explained on the basis of the following mechanisms: Pb+2 Catalysis: Pb+ 2 + S Pb+4 + 2SO4 CH CO H2O CH Pb+4 + I Pb CO2 CH2CO CH2 CH CH2 1 CHS --'.^ I +0H CH 2 CHO J CHO Mn+2 Catalysis: Mn+ 2 + S Mn SO 4` CH CO H2O CH +'' --; Mn^ H4 CH2000H + Mn + CH2 + 2CO CH CH$ +H +H20 CH2 CHO

10 198 S. PADMA AND M. SANTAPPA Hga 2, Catalysis: Cu+s Catalysis: Hg S20$ -+ Hg SO4 CH CO H2O CH Hg+3 + I ----* Hg I + 2COs CH2 CO CH2 CH CH2 1 CH3 i - I-- I +0H CH 2 CHO j CHO Cu+2 + S2Oe -+ Cu+g + SOa + SO 4 CH CO CH COO I + Cu+$ -- Cu+2 + I + H+ CH 2CO CH2CO CH COO CH COO I ± SO,' - I -+- HSOi CH2CO CH2COO CH COO I CH3CHO + 2CO2 CH2COO REFERENCES 1. House, D. A... Chem. Rev., 1962, 62, Wilmarth, W. H. and "Mechanism of oxidation by peroxy disulphate ions," in Haim, A. Peroxide Reaction Mechanism, J. O. Edwards, Editor, John Wiley, N.Y., 1962, pp , K oichiroshinra, Kenichi, Ishibashi Sci. Rept. (Osaa Univ.), (Eng.). 1963, 12, Saurai and Taayani 4. Mishra, D. D. and J. Ind. Chem. Soc., 1964, 41 (6). Ghosh, S. 5. Baore, G. V. and Z. Phisi. Chem. (Leipzig), 1965, 229 (3/4), Joshi, S. N.

11 Oxidation Studies VIII Venatasubramanyam, N. and Sabesan, A. 7. Mhala, M. M. and Iyer, R. V. 8. Saxena, L. K. and Singhal, C. P. 9. Thomson, R. H. and Wylie, A. G. 10. Padma, S. and Santappa, M. Tetrahedron Letters, 1966, No. 4, pp Ind. J. Chem., 1965, 3, J. W. Chem. Soc., 1961, 38, 346. J. C. S., 1966, p Unpublished results.