Oxidation studies—III. Oxidation of lactic acid by peroxydisulphate catalysed by Cu(II) ion

932

Notes

lanthanide perchlorates have been reported: one with a ligand/metal ratio decreasing from 8ll to 7/117] and another in which this ratio remains constant and equal to 8/1 [8]. Several factors (e.g. the nature of the rare earth--ligand bonding, predominantly ionic and non-directional, the relatively great radius of the metal ion, the existence of conformational equilibrium in the ligand itself) influence the establishement of a compromise in obtaining the maximum shielding for the metal ion and the minimum ligand-ligand repulsion. There may be several possibilities of arrangement for the metal, anions and neutral ligands, with small energy differences between them. Depending on the conditions of precipitation, a particular species separates out. A mixture of species can also be obtained and this is a reasonable explanation for the non-stoichiometric compounds mentioned above.

Acknowledgements--The authors are grateful to A. A. de Andrade for valuable help during the early stages of this research and to the Funda~to de Amparo h Pesquisa do Estado de S,~o Paulo, for a fellowship to A. de O. Thanks are also due to Dr. M. Kawashita Kuya for her comments. V. K. LAKATOS OSORIO A. de OLIVEIRA E. GIESBRECHT

Departamento de Quimica Fundamental lnstituto de Qufmica da Universidade de S~o Paulo C.P. 20780, S(w Paulo 05508 Brasil

REFERENCES 1. J. O. Edwards, R. J. Goetsch and J. A. Stritar, lnorg. Chim. Acta 1, 360 (1967). 2. O. A. Serra, M. Perrier, V. K. Lakatos Osorio and Y. Kawano, lnorg. Chim. Acts 17, 135 (1976). 3. Y. Hase and Y. Kawano, Spectrosc. Lett. 11, 161 (1978). 4. W. J. Geary, Coord. Chem. Rev. 7, 81 (1971). 5. L. C. Thompson, Complexes, in K. A. Gschneider and L. Eyring, Editors, Handbook on the Physics and Chemistry of Rare Earths, North Holland Publ. Co., New York (1978). 6. D. K. Koppikar, P. V. Sivapullaiah, L. Ramakrishnan and S. Soundararajan, Struct. Bonding (Berlin) 34, 135 (1978). 7. L. B. Zinner and G. Vicentini, lnorg. Nucl. Chem. Lett. 7, 967 (1971). 8. K. Nagase, H. Yokobayashi, A. Iwase and K. Sone, Thermochim. Acta 17, 335 (1976).

1.inorg,nucl.Chem.Vol.42,pp. 932-935 PergamonPressLtd.,1950. PrintedinGreatBritain

Oxidation studies--HI. Oxidation of lactic acid by peroxydisulphate catalysed by Cu(lI) ion (Received 8 February 1979; received .for publication 7 September (1979) Cu(II) has been reported as a catalyst in a few peroxydisulphate oxidations[I-7]. Kinetic studies suggest that catalysis occurs where the organic substrate can serve as a ligand (loc. tit). Lactic acid is known to form complexes with copper(II)[8-12]. Therefore, this reaction was selected for a kinetic study to determine the mechanism operative in copper(lI) catalysed S2Os2- oxidations. The uncatalysed and the Ag(I)-catalysed oxidation of lactic acid has already been reported [13, 14].

EXPERIMENTAL All chemicals were either "AnalaR" or E. Merck G.R. Kinetics were followed by estimating residual peroxydisulphate iodometrically, at different intervals of time [13]. The $2032- equivalents of Cu(II) concentration (I>5× 10-4 moles/litre) in an experiment, in terms of titre volume (of 0.04 N), have been substracted from the observed titre volume values to obtain ( a - x). At lower Cu(lI) concentrations it was not necessary. The rate (R) and first order rate constant (k) were evaluated as previously [6]. RESULTS Stoichiometry. The stoichiometry was verified by allowing a solution containing 0.01 M lactic acid, 0.04 M K2S208 and 2.0 × 10-4 M CuSO4 to react for 3 hr at 55°C. The decrease in peroxydisulphate concentration was ascertained and a blank was run simultaneously. The results conform to the equation, CH3CH(OH)COOH + 2K2S208 + H20 = CH3COOH + KHSO4 +CO~. The main reaction product, acetaldehyde, was identified in the reaction mixture by conversion into 2,4-dinitropbenylhydrazine by a spot test[15]. The rate law. The results of the kinetic run at 55°C are given in Table I.

Table 1. First order rate constant in Cu(II)---catalysed oxidation of lactic acid Time sec 0 600 1200 1800 2250 2700 3600 4500 5400 6300 7200 8100 9000 Mean Slope value

Volume of 0.04 N Na2S203 (S'zOs2-) ml moles 1-t 4.95 4.63 4.39 4.00 3.08 2.80 2.11 1.67 !.29 1.10 0.94 0.71 0.62

0.0198 0.0185 0.0176 0.0160 0.0123 0.0112 0.0084 0.0067 0.0052 0.0044 0.0038 0.0028 0.0025

tMean calculated hereafter. (Lactic acid)o = (K2S2Os)o = 0.02 M, Temp. 55°C.

107R moles 1-1 sec -~ -21.3 18.7 21.0 33.2 31.8 31.5 29.2 27.2 24.5 22.3 20.7 19.2 25.0

10Sk sec -~ -ll.I 10.0 I 1.8t 21.2 21.2 23.7 24.3 24.3 23.7 23.2 24.0 23.2 23.2 24.0

(CuSO~)o = 0.0001 M,

It appears that the first order peroxydisulphate decomposition is disturbed initially for a period from 600 to 1800 sec, whereafter a fair constancy in k is obtained. The concentration-time curve is initially S-shaped, typical of autocatalytic reactions. Such kinetic features are reproduced at other concentrations. It is presumed that the reaction starts as an uncatalysed one and after a lapse of time a catalysed reaction starts; after about 1800 sec both the reactions occur simultaneously. This view is supported by the results obtained with an uncatalysed oxidation under

Notes

933

identical conditions. The value of the first order rate constant for the uncatalysed reaction was 10.6 × 10-5 sec -t, which is in good agreement with the value 11.1 × 10 5 sec ~ obtained in the initial stage of the catalysed reaction. Peroxydisulphate dependence. The rate (R) and the first order constants ~k) at different initial concentrations of peroxydisulphate are summarised in Table 2. The rate is not strictly first order in (S2Os2-) and the plot of R against (S:Ox2 )~ is linear (Fig. I) satisfying the equation, -

Table 3. Rate dependence on (Lactic acid)

(Lactic aeid)o M

R × 10-7 moles 1 ~ sec-~

lOSR (Lactic acid) °5

0.005 0.010 0.015 0.020 0.025 0.030 0.040

13.1 18.5 21.9 25.0 28.3 30.7 34.5

1.85 1.85 1.79 1.77 1.79 1.77 1.72

dtSzO82 )/dt = k'(S2Os2-)H

where k' is a function of the reducible substrate and catalyst concentrations. Thus the order in preoxydisulphate is I.I, which can be considered as unity. Lactic" acid dependence. The order with respect to (lactic acid) determined from the slope of the linear plot of log R vs log (lactic acid1 (Fig. 2) was found to be fractional (Table 3). Copperlll) dependence. A plot of rate constant against ICuOI))°~ (Fig. 3) gives a straight line not passing through the origin, indicating that the uncatalysed reaction is occurring simultaneously with the catalysed one. The first order rate constant obtained experimentally is given by the expression,

(K2S2Os)o = 0.02 M, (CuSO4)o= 0.0901 M, Temp. 55°C. initially uncatalysed the second term is absent in the initial stage. Only when the catalysed reaction occurs are first order constant values obtained. Temperature dependence. The effect of temperature on k was studied in the range 45--60°C; the results are shown in Table 4. The Arrhenius plot is linear and conforms to the equation,

k - 120 × 10 ~sec ~+ 9.5 × 10 ~(Catalyst)°'5 at 55°C.

k = 5.25 × 1024exp (- 42.82 RT) sec ~.

This expression supports the view that if the reaction is Table 2. Rate dependence on (Peroxydisulphate)

(S2Os2 ) moles 1 ~

10~k sec ~

107R moles 1-I sec-'

104R (S~O82-)H

(I.005 0.010 0.015 0.020 0.030 0.040

18.4 20.1 22.5 23.2 26.0 26.5

5.32 11.4 17.7 25.0 42. I 58.3

1.81 1.81 1.80 1.83 1.99 2.01

Effect of salts and gases and (H+). Allyl acetate, sodium acetate, chloride ions and oxygen inhibit the reaction rate while K + ions and CO: have catalysing effects. The reaction was also studied in the presence of different amounts of H2SO4 (0.005 to 0.05 M) (Table 5). The rate constant decreased from k=25.0×10~sec ~ at (H*)=0.00M to k= 10.9 × 10-5 sec ~at (H +) = 0.05 M. Table 4. Temperature dependence Temp. °C 10~k (sec -~) (Lactic 0.0001 M.

tLactic acid) = 0.02 M, (CuSO4)o = 0.0001M, Temp. 55°C.

45 2.42

o

50

-/ 40

U'}

Z ~ 3o o

fi

v

~

2o

10

S

10

15

20

55 23.2

acidh = (K2S:O8)o= 0.02 M,

60

u

50 7.30

25

103 [ $ 2 0 ~ - ] ' ' ' Fig. 1. 1:1 Order dependence in [$2Os2-].

30

60 58.7 (CuSO4)o =

934

Notes Table 5. Effect of varying (H +)

By the steady state treatment, one can derive

(H*) M

107R moles 1-1 sec -1

lOSk sec -~

--

23.2

25.0

0.005 0.01 0.02 0.03 0.04 0.05

22.2 20.0 16.9 13.8 11.9 9.8

24.2 22.4 18.1 15.0 13.2 10.9

(K2S2Os)o = (Lactic acid)o = 0.0001 M, Temp. 55°C.

0.02 M,

- dCs2o~2-/dt = ksCszo~2-[kl_+(k 2 + 4klk2ks/kdCu(ll)L)°'512ks] (I) which simplifies to, - d C s ~ 2 / d t = (4klk2ks/k6)°S(Cu(II)L)°5(S~Os 2-) if kl is small. The half-order dependence in lactic acid (Table 3, Fig. 2) indicates that all of the Cu 2+ is not complexed. An equilibrium of the type III could exist in solution, for which the equilibrium constant (K) could be written as in eqn (IV).

(CuSO4)o =

DISCUSSION

On the basis of the results, the mechanism shown in scheme 1 is suggested; this is consistent with the earlier views [2, 4, 6, 7, 16].

Cu2* + lactic acid~Cu(II) lactate + H +

(III)

K = (Cu(II)lactate)(H+)/(Cu 2+) (lactic acid)

(IV)

Substituting Cu(II) lactate = K(Cu 2+) (lactic acid)/H +) in eqn

S20s2- = 2SO4":

(1)

Cu(II)(C3H403)22- + SO4"-= Cu(III)(C3H403)2 + S042-

(2)

Cu(III)(C3H403)2 = Cu(II)(C3H403) + CH3CHO + C02 ~" Cu(II)(C3H403) + CH3CHOHCOO-

=

Cu(II)(C3H403)22- + H +

(5)

CO2~ + S04~ = C02 + SO42-

The pH variation during the oxidation in presence of Cu(II) shows a steady decrease for an initial period of ca. 1800 sec, unlike that obtained without copper (Fig. 4). This period coincides with the disturbed first order kinetics of S:Ose- decomposition.

(3) (4)

CO2~ + S~Os2- = CO2 + SO4- + SO42-

(6)

III, we get the rate eqn (A): - d(S2Os2-)/dt = (4klk2ks/k6)O.5KO.5(S20s 2-) (Cu2+)°.~(lactic acid)°~/(H+) °'5

0"4 I n..

0"3

o=

+

U9

0'2 SLOPE: 0.47. 0.1 I

0"/.

(II)

0"8 1"2 1'6 3 -I- Log (Lactic Qcid)

!

2.0

Fig. 2. Demonstration of half order dependence in lactic acid by Log-Log plot.

(A)

Notes

935

50

I 40

"Q 30 6/3

M

o

20

L, p

:,, c ;12ox,o-'

10 OPE I

I

0.02

[c-so~ ]o.5

1

= 9'5 XlO-3

1

0-01

1

0.03

•

Fig. 3. Plot of k vs [Cu(lI)] °5.

Acknowledgement--We are grateful to Dr. O. Chandra and Dr. G. K. Chaturvedi for their keen interest and the State ('SIR for ~ contingent grant.

30 tL

25

Chemical Laboratories Agra College Agra- 282002 India

20

S.C. AGARWAL* 1,. K. SAXENA

REFERENCES

10

~AcT,c AC,D] : 002

M

[K 2S208]

M

:

0"02

" IM1

O [LACTIC ACID] = 0'00 FK2 $2 08] = 0.02 [cu so~] : o.oool

05

1

1200

I 2/.,00

I

I

I

3600

4800

6000

SECONDS Fig. 4. pH Variation. Application of the steady state treatment to both catalysed and uincatalysed reactions gives the following expression for the overall rate of disappearance of peroxydisulphate: - d(S20~~ )/dt = k'k~ S2Os2 )+ (4klk2ks/k6)°~K°S(S~082 ) (Cu2+)°'S(LA)°'S/(H+)°'~

(B)

The rate law (B) is in conformity with the experimental facts.

*Present address: Department of Biochemistry, S,N. Medical College, Agra, India.

I. D. A. House, Chem. Rev. 62, 185 (1%2). 2. Ben-Zvi, Ephraim and T. L. Allen, J. Am. ('hem. Soc. 83, 4352 (1%1). 3. O. A. Chaltykyan, A. N. Mamyon and R. V. Mov~esyn. Ser Khim. Nauk 60, 135 (1957). 4. Misaru Kimura, J. Phys. Chem. 77, 1265 11973). 5. S. P. Bhargava and Y. K. Gupta. Bull. Chem. Soc. ,lapan 41, 843 (1%8). 6. S. C. Agarwal and L. K. Saxena, Ind. J. ('hem. 16A(7), 602 (1978). 7. M. G. Ram Reddy, B. Sethuram and T. Navaneeth Rao, Ind. J. Chem. 16A, 31 (1978). 8. C. K, Prout, R. A. Armstron, J. R. Carrunthens, J. (k Foresl, A. P. Murray and J. C. Rossottifrancis, J. Chem. ,%('. 11, 2791 (1%8). 9. O. N. Srivastava and C. M. Gupta, Ind..L Chem. 8, 1007 ( 1970t. 10. J. S. Savic and 1. Fillipovic, Croat. Chem. Acta 37, 91 (1%5k I1. Const. Gh. Macarovici and G. Schmidty, Studia Unit'. Babes Balyai Ser 1, 103 (1%0). 12. H. Leopold and Zd. Valtr, Naturwissenschafler ~ , 558 (1957). 13. K. Kumar and L K. Saxena, J. lnorg. Nuel. Chem. 31, 1053 (1%9); ibid. 33, 1050 (19701. 14. G. V. Bakore and S. N. Josh(, Z Physik Chem. 229, 25(I (1%5); D. D. Misra and S. Ghosh, Ind. Chem. Soc, 41. 397 (1964); N. Venkatasubramanian and A. Sabesan, Tetrahedron Lett. 40. 4919 (1966); Ind. J. Chem. 9, 942 09711. 15. F. Feigl, Spot test, p. 352. Elsevier, New York I I%0L 16. Z. Kovates, Magyar Kern Folyoirat 66, 181 [1%01,