Kinetics of Oxidation of Formaldehyde, Acetaldehyde, Propionaldehyde & Butyraldehyde by Ditelluratocuprate(III) in Alkaline Medium

Jndlan Journal of Chemistry Vol. 17A, January 1979,pp. 48·51

C. P. MURTHY, B. SETHURAM & T. NAVANEETH RAO· Department of Chemistry, Osmania University, Hyderabad 500007

Received 8 June 1978; accepted 25 July 1978

Kinetics of oxidation oJ formaldehyde, acetaldehyde, proplonaldehyde and n-butyraldehyde by potasstum ditelluratocuprate(III) has been studied in alkaline medium spectrophoto- :metrically. The order in [aldeh;de] and [Cu(III)) are found to be one each and rates decreased with increase in [tellurate] and increase in [OH-]. There is no effect of addition of salts like Na, SO. and KNOa• The products of oxidation are identified as corresponding carboxylic acids. Under the experimental conditions the :monotelluratocuprate(III) species is assu:med as the active species. The ther:modynamic para:meters are also reported and a plausible mechanism has been suggested.

SE of Cu(III) as an oxidizing agent is well corrections made for any self-decomposition of known in analytical chemistry in the estimation Cu (III) during the reaction. U of sugars-, glycerols-, amino acids", proteinss, carboxylic acids", carbonyl compounds" and alcohols? Results The presence of Cu(III) as intermediate was also Under the conditions [Cu(III)] ~ [aldehyde] the reported in some Cu(II)-catalysed oxidation reactions plots of log (absorbance) versus time were linear by peroxydisulphate" and vanadium (V)9. The kine- (Fig. lA), indicating the order in [Cu(III)J to be tics of decomposition and formation of Cu(III) unity. From the slopes of the above plots the diperiodate and ditellurate complexes were also pseudo-first order rate constants (k') were evaluated. studied in detail by Rozovskii et a1.10-12• Though The plot of log k' versus log [aldehyde] Was also kinetics of oxidation of some alcohols by potassium linear (Fig. lA) with unit slope indicating the order diperiodatocuprate(III) has been investigatedt", in [aldehyde] to be unity. similar study on organic substrates using potassium Product analysis - Under the kinetic conditions ditelluratocuprateff l I) as an oxidant has not been the oxidation products were detected to be corres- made. The present study dealing with the kinetics ponding carboxylic acids by their characteristic of oxidation of aliphatic aldehydes by ditellurato- spot tests>. . cuprate(III) is a step in this direction. Stoichiometry - The stoichiometric studies were made by adding dropwise DTC of known concen- Materials and Methods trations to O·IM formaldehyde until no further All the chemicals used were of AR grade and decolorization Was observed. From the volume of purified wherever necessary by standard methods. DTC consumed it was observed that 1 mole of Potassium ditelluratocuprate(III) was prepared and aldehyde required 2 moles of Cu(III). Effect of varying [tellurate] - At constant [Cu(III)], standardized by the method reported by Chandra ( and Yadava-s. [aldehyde] and [OH-] increase in [tellurate] decreased The kinetics was followed in the temperature range the rate (Table 1) and the dependence with respect 5-40°. Reaction mixtures containing requisite quan- to [tellurate] Was found to be -0·4 indicating an tities of aqueous solutions of aldehyde, potassium inverse fractional order dependence of rate on tellurate, KOH were taken in a reaction vessel [tellurate]. while potassium direlluratocupratefl H) (DT~) was Effect of varying [OH-] - At constant [Cu(III)], taken in a separate flask. After thermostatmg the [aldehyde], and [tellurate], k' values increased with :solution at the desired temperature for 0·5 hr, increase in [OH-] and the order with respect to a known volume of Cu(III) solution was transferred [OH-] Was found to be fractional (Table 2). into the reaction vessel, stirred and the reaction Effect of varying ionic strength - The rate remained followed by estimating the amount of Cu(III) unaffected by added salts like KN03 and Na2S04 • .consumed spectrophotometrically at 405 nm. The Discussion molar absorption coefficient of Cu(III) under the conditions was 1 X 104 litre mol? ern. For all sets In the basic medium (0·001 to 2·00M KOH) blank reactions were carried out and necessary DTC exists in two forms in equilibrium, differing in

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MURTHY et al.: OXIDATION OF ALIPHATIC ALDEHYDES BY DITE,LLURATOCUPRATE(III)

III ~ ~ 14 0 .,. x 0 + -I'" ,., 13 t t 0:2 1'4 l2

I' 2

1'0 10

0'8 9

0'6 8

0'0 0·4 ~----~----~-----J------~----~-----r----~7 0'4 0·6 0·8 1'0 1·2 1'4 _ 4+ ItO C (A)

0·0 0'1 0·2 0'3- ~(B) C 15 10 5 o ~S:f ~ (e) :Fig. 1- (A) Plot of log !?,' versus log [formaldehyde]. (B) plot of II!?,' versus 1/[formaldehyde] and (C) plot of f>.Ht versus f>.St {[Cu(III)] = 4'70 X 10-5111; [K2 TeOJ = 0'04111; [KOH] = 0'02111; temp. = 28°}

basicity of hydro tellurate ligand (Eq. 1). The DTC

TABLE 1- EFFECT OF VARYING[K2Te04] ON k' complex ion contains two hydroxyl groups as well as two tellurate groups in coordination sphere-", 5 {[Cu(III) = 4'00 x 10- 111;[ formaldehyde] = 12·5 X 10-4111; [Cu(OH)2(H Te0 )2]a-+20H- :? [KOH] = 0'002111; temp, = 32°} 4 6 I 2 2 k' X 10 [K2Te04]. 111 k' X 10 [CU(OH)2(HaTe06)2]5-+2H20 ... (1) min-! miri-! II

0·002 11·40 0·010 6·40 When the concentration of KOH is O·lM the concentration of both these species I and II Was 0'004 9'00 0'020 5-60 found to be equalw. At concentrations of OH- 0'02M used in the present study DTC would exist mainly as r. However, this cannot be the reactive TABLE 2 - EFFECT OF VARYING[KOH] ON k' species in the oxidation of aldehydes because increase in [OH-] increases the rate and increase in [tellurate] {[Cu(UI)] = 3·70 X 10-5111; [formaldehyde] = 12'5 X 10-4111; decreases the rate. So the dissociated form (III) [K2Te04] = 0'40111; temp. = 28°} of I as represented by the equilibrium (2) could be the active species. 2 2 [KOH].111 k' X 10 [KOH]111 k' X 10 K min-1 min-! [CU(OH)2(H4Te06)2]a- +20H- :? Cu(OH)~ +2H4TeO~- I III ... (2) 0·002 1-67 0'030 3090 However. the equilibrium constant for the equi- 0'006 2·37 0·040 4·70 librium (2) has been reported as 4 X 10-11 at 40° by Lister- '. Th~s. suggests that the species (III) is 0·010 3·00 present to a negligible extent. Hence the possibility of III being the reactive species is not reasonable.

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INDIAN J. CREM., VOL. 17A, JANUARY 1979

The [OH-] dependence on the reaction indicates an

equilibrium between a deprotonated re~ctive sI?ecies TABLE 3 - ORDER OF REACTIVITY in equilibrium with protonated unreactive specIes as shown by equilibrium (3) Kd Kd

[Cu(OH)2(H Te0 )2P-+OH- ~ [Cu(OH)2(H Te0 ) 4 6 3 6 (litre mole-' min-I) (litre mole-! min-') I IV (H4Te06)']4-+H20 ... (3) Formaldehyde Acetaldehyde The dependence of rate of oxidation on [tellurate] 34·4 'S'O x 10-' 26·0 0'70 indicates a dissociation equilibrium in which the Cu(III) moiety loses a tellurate ligand from its Propionaldeh yde n-Butyraldehyde coordination sphere forming a monotelluratocuprate- (III) complex. This can be represented by the 20·5 1·40 18·4 2·10 equilibrium (4)

[CU(OH)2(H3 Te0 ) (H4Te0 )]4- ~ [Cu (OH)2(H Te0 )J- 6 6 4 6 TABLE 4 - THERMODYNAMIC PARAMETERS IV V +H3TeO~- .. , (4) (~Et, ~Ht and ~Gt in kcal mol-t) Though the equilibrium (5) Aldehyde k" at ~Et ~Gt ~Ht -~St 29° [Cu(OH)2(H4Te06)2]3- ~ [Cu(OH)2(H4Te06)r (cal VI V (litre deg-t mol-t mol-')' +H4TeO~- ... (5) min)

is also possible, the equilibrium (4) is prefe.rred Formaldehyde 34-4 11'5 1% 10·9 15-4 as the loss of tellurate ion occurs preferentially from a species of higher negative charge. Hence Acetaldehyde 26·0 13·3 15·8 12·7 10·4 monotelluratocuprate(III) is assumed to be the reactive species. . n-Propionaldehyde 20'5 14·1 15'9 13-5 7·8Q.o The order with respect to [substrate] and [DTCJ are strictly one each and the plot of 11k' versus n-Butyraldehyde 18'4 15'2 16·0 14'6 4·30' 1/[aldehyde] is linear (Fig. IE) p~ssing .through origin indicating that the substrate IS not involved in any complex formati~n .with Cu(III). .The of acrylamide. The absence of salt effect suggests- plausible mechanism of oXldatlOn may be written that it is a ion-dipole type reaction. The above as in Scheme 1. mechanism also receives support from the stoichiometry of the reaction which was found to be 2: 1 K, [Cu(OR). \H.Te06l.]3- + OH- ~ [cu(OH).c;r3Te06) . [Cu(III): aldehyde]. The order of reactivity of (H.TeOs)] - + H20 ... (1) different aldehydes was found to be related to the K2 equilibrium constant for the dehydration of gem-diol" [Cu(OH). (HaTeOs) (H.TeOs)]·- ~ [CU(OH)2 (H.TeOa)]- to form aldehyde (Eq. 7) + H3TeO~- ... (ii) Kp OR HaTeO:- + H20 ~ H.TeO~- + OH- ... (iii) / Kd OH RCR ~ R-CHO+H20 k / *Cu(III)+RCH(OH)2_RCH + Cu(II) + H+ ... {iv) "'OH slow "- O' The values of Kd given in Table 3 are taken. OH from the Iiteraturets ,19. From these observations / RCH + Cu(III) --.. RCOOH + Cu{II) + H+ ... (v) it is clear that the hydrated form of aldehyde is. ( "- fast taking part in the reaction. O' The thermodynamic parameters were evaluated. * [Cu(OH) (H.Te06)]- is written for simplicity as Cu(III) and are listed in Table 4. The plots of !1Ht versus Scheme 1 !1St is linear (Fig. 1C) indicating that the reaction series follows isokinetic phenomenaw. The isokinetic- Eased on steady state treatment the rate law temperature (~) is found to be 190 K which is- can be expressed by Eq. (6) far below the experimental temperature range (275-315 K) suggesting that the reaction is entropy-. kK1K2[Cu(OH)2(H4Te06)] [RCHO] controlled. This also suggests that steric effects {Kp + [H4Te06] + [OH-]} playa dominant role in these reactions. The steric Rate [H Te0 J{1 +K1[OH-]} ... (6) effects in the straight chain compounds which, = 4 6 decrease the reactivity could arise due to the coiling In this mechanism the step (V) is evidently fast of the aldehyde molecule. The increasing order of as it involves free radicals. The evidence for the free reactivity is in accordance with the increasing order radical was obtained from induced polymerization of entropies (Table 4). so

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MURTHY et al.: OXIDATION OF ALIPHATIC ALDEHYDES BY DITELLURATOCUPRATE(III) 1 .References 9. MURTHY, G. S. S., SETHURAM, B."& NAVANEETH RAO, T., Indian J. cu-«, 15A (1977), 880. 1. (a) BECK, G., Mikrochemie,35 (1950), 169. (b) BECK, G., 10. ROZOVSKII, G. 1., PROKOPCHIK, A. Yu & YANKAUSKAS, Mikrochemie,38 (1951), 152. R. P., Kinet. Katal., 12 (1971), 82. 2. BECK, G., Mikrochemie, 40 (1953), 258. 11. ROZOVSKIS, G. & PROKOPCIKAS, A., LIETUVOS, T. S. R. MOKSLU, Akad. Darbai, Ser, B, 4 (1963), 11. 3. (a) BECK, G., Mikrochemie, 38 (1951), 1. (b) KOVAT, Z., 12. ROZOVSKII, G. 1., MISYAVICHYAS, A. K. & PROKOPCHIK, Acta cbim, hung., 21 (1959),247. (e) KOVAT, Z., Acta. chim. hung., 22 (1960), 313. A. Yu., Kinet. Katal., 16 (1975), 402. 13. MOV'IUS, W. G., Inorg. Chem., 12 (1973), 31. -4. (a) BECK, G., Mikrochemie, 39 (1952), 313. (b) BECK, G., Mikrochem. Acta, (1956), 977. 14. CHANDRA, S. & YADAVA, K. L., Talanta, 15 (1968), 349. 15. FEIGL, F., Spot tests in organic analysis (Elsevier, Amster- 5. ]AISWAL, P. K. & YADAVA, K. L., Indian J. cu«, 11 dam}, 1966. (1973), 837 . 16. ROZOVSKII, G. 1., MISYAVICHYUS, A. K. & PROKOPCHIK, .~. CHANDRA, S. & YADAVA, K. L., Microchem. J., 13 (1968), A. Yu., Zh. neorg, Khim., 16 (1971), 3625. 491. 17. LISTER, M. W., Can. J. Chem., 31 (1953), 638. I. CHANDRA, S. & YADAVA, K. L., Microchem. ].,13 (1968), 18. GRUEN, L. C. & McTIGUE, P. T., J. chem, Soc., (1963), 589. 5217. ·8. RAM REDDY, M. G., SETHURAM, B. & NAVANEETH RAO, 19. BELL, R. P., Adv. Phy. Org. Chem., 4 (1966), 1. T., Indian J. Chem., 16A (1978), 31. 20. LEFFLER, J. E., J. org. Chem., 20 (1955), 1202.