INTRODUCTION:

Chemical kinetics deals with chemical interacting system as the interaction continues with observable macroscopic changes in the properties of the system which are time dependent. The factors that ordinarily affect the rate of reaction are concentration of the various species present in the reaction system, nature of the medium etc. The oxidation of Schiff bases in presence of acid ^2–10 has been studied but very few researcher^11–12 have studied the oxidation of Schiff bases by ceric sulphate in aqueous sulphuric acid medium.

In the present case, the study of kinetics of oxidation of Schiff bases by ce⁴⁺ ® ce³⁺ system has been carried out by using various concentration of different Schiff bases like 2 – Hydroxy –1- Naphthalidene anil (L₁) and its substituted anils such as 2’ – Methoxy – 2 – Hydroxy – 1 - Naphthalidene anil (L₂), 3’ – Methoxy – 2 – Hydroxy – 1 – Naphthalidene anil (L₃) and 4’ – Methoxy – 2 – Hydroxy, 1 – Naphthalidene anils (L₄) in aqueous sulphuric acid and also the effect of various concentrationof oxidant on different substrates. (L₁, L₂, L₃, L₄) is studied.

EXPERIMENTAL:

a) Effect of Various concentration of substrates:

The concentrations of substrate were varied between 0.6 × 10^{– 3}M to 1.0 × 10^{– 3}M and concentration of oxidant was kept constant i.e. 2.50 × 10^{– 2} M All other parameters were kept constant. The titration was carried out as per standard method³ by using ferroin indictor and titrated against FAS solution from the burette determining the amount of unreacted cerium (IV). The time interval for the follow up was 5 minutes for all experiments.

b) Effect of various concentration of oxidant:

To determine the order of reaction with respect to concentration of oxidant, the reaction was carried out at different concentration of oxidant keeping other parameters constant. The various concentration of ceric sulphate ranging from 1.5 × 10^{– 2} M to 2.5 × 10^{– 2}M were prepared and its ionic strength was maintained to µ = 0.3 by adding requisite amount of KCl and 1.00 × 10^{– 3} M solution of Schiff base in 2N H₂SO₄.

The order of the reaction with respect to the substrates and oxidant were determined. The rate constant were recorded with respect to oxidant as well as substrate.

RESULT AND DISCUSSION:

a) Effect of various concentration of substrate: The results are shown in Table-1.

[oxidant] = 2.5 ×10^{– 2}M Temperature = 303K.

[FAS] = 1.25 × 10^{– 2} M Ionic strength (µ) = 0.3.

[H₂ SO₄] = 2.0 N

Table-1

Concentration of Substrate× 10^{– 3}M	Rate Constant (K₁)
Concentration of Substrate× 10^{– 3}M	L₁ × 10^{– 2} min^{– 1}	L₂ × 10^{– 2} min^{– 1}	L₃ × 10^{– 2} min^{– 1}	L₄ × 10^{– 2} min^{– 1}
1.00	1.4212	1.8147	2.908	1.6593
0.90	1.2865	1.6315	2.642	1.4691
0.80	1.1286	1.4660	2.383	1.3396
0.70	1.017	1.2815	2.104	1.1630
0.60	0.866	1.1086	1.801	1.002
Mean K₂ Mol^{– 1} min^{– 1}	14.3148	18.2766	29.7440	16.5890

Fig. 1

Fig. 2

The titre values obtained for various concentration of substrates indicates that the reaction rate constant (K₁) values are directly proportional to the concentration of substrate. The values of second order rate constant (K₂) which is calculated from K₁/ [substrate] are practically constant for each substrate which also proves the first order dependence of reaction on substrate. The plot 1/K₁ against 1/[substrate] is linear and not passing through origin which indicates the formation of complex (fig. -1). The plot of log K₁ against log [substrate] are plotted for four substrates L₁, L₂, L₃, L₄ and slopes are determined which indicates the order of reaction (fig.-2). The slopes for four substrate L₁, L₂, L₃, L₄ are 0.98, 0.92, 0.88, 0.93 respectively which shows that the reaction follows the first order kinetics with respects to substrate.

b) Effect of various concentration of oxidant on substrates: The results are shown in Table-2

[Substrate] = 1.0 ×10^{– 3}M Temperature = 303K.

[FAS] = 1.25 × 10^{– 2} M Ionic strength (µ) = 0.3.

[H₂ SO₄] = 2.0 N

Table-2

Concentration of Oxidant× 10^{– 2}M	Rate Constant
Concentration of Oxidant× 10^{– 2}M	L₁ × 10^{– 2} min^{– 1}	L₂ × 10^{– 2} min^{– 1}	L₃ × 10^{– 2} min^{– 1}	L₄ × 10^{– 2} min^{– 1}
2.50	1.4212	1.8147	2.908	1.6593
2.25	1.533	1.9530	3.185	1.7430
2.00	1.645	2.1083	3.478	1.9061
1.75	1.872	2.2640	3.721	2.1344
1.50	2.038	2.4260	4.127	2.3016

Fig. 3

The titre values obtained for all the four different substrates for different concentration of oxidant were plotted against time in minutes. The average value of rate constant was calculated and it provides the evidence for the first order rate equation.

The rate constant value decreases with increasing concentrations of oxidant similar result were noted by J. shorter and C. Hinshelwod⁴ in their studies. This fact shows the formation of an equilibrium complex between the reactants prior to the rate determining steps^13-15

The plot of rate constant against 1/[oxidant] was linear with an intercept on Y- axis for all four substrate showing the evidence for the formation of a complex (Fig.-3).

Mechanism:

The species of cerium complexes in sulphuric acid has been widely investigate^{8 – 13}. The isolation of number of cerium (IV) complexes from sulphuric acid has also shown by G. Hargraves L.H.Sutcliffe⁷. the existence of ceric sulphate coupled with the fact that cerium in solution is present only as complex anion, suggests that ceric sulphate exists as the complex sulphatoceric acid⁹, H₄Ce(SO₄)₄ which must be reactive species in the reaction and the action of it on the substrate may be a rate determing step^13-14. The plausible mechanism for the oxidation of L₁ – L₄ will be as follows:

Where R = H, o – OCH₃, m – OH₃ or p – OCH₃ followed by fast reactions

REFRENCES : -

1) K.J. Laidler, Chemical Kinetics, 3^rd edition, New York (1987)

2) A. I. Vogel, Textbook of practical Chemistry, 4^th edition pg. – 269

3) A. I. Vogel, Textbook of Quantitative Inorganic Analysis, 3^rd edition pg. – 282

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8) P.S. Sankhla and R.N. Mehrotra J. Inorg. Nuclear Chemistry 34, 3781 (1972)

9) E.G. Jones and F.G. Soper, J. Chemistry Soc. 802 (1935).

10) R.L. Yadav and W.V. Bhagwat, J.Ind. Chemistry Soc. 41, 389 (1961)

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12) U.N.Pol, S.S. Dodwad and M.M. Shaikh Bull of Pure and App. Sci 13 (C), 31 (1994)

13) J.P. Singh, S.J.S. Sirohi and Miss Aishwati, J. Ind. Chemistry Soc. LXIV 440 (1987)

14) U.N.Pol, R.S.Yamgar and Dr. S.S.Dodwad. Asian Journal of Chem 58 – 62 (1997)

Received on 15.09.2011 Modified on 11.10.2011

Asian J. Research Chem. 4(12): Dec., 2011; Page 1861-1863