1. INTRODUCTION:

A survey of recent literature on kinetic study reveals that there is a lot of scope for the systematic study of oxidation processes involving various oxidants ^1-4 . There are various systems reported in the literature such as oxidation of pyridoxine by Mn (III); oxidation of aldehyde by Cr(VI), acid permanganate; N-Bromoacetamide, Os(VII) pyridinium hydrobromide and bis 2,2 (bi pyridyl), Cu(II) Permanganate, etc ^5-8 .

The present investigation reports the oxidation of pyridine-4-carbox aldehyde by potassium permanganate under pseudo first order conditions in acidic medium.

The oxidation state of Mn in MnO₄^- is (VII). Therefore, it can be represented as Mn (VII) which is a powerful oxidizing agent and usually reduced to Mn (II).

MATERIAL AND METHODS:

All chemical used for kinetic study were of A.R. grade. Kinetic investigations were performed under pseudo first order conditions with excess of the pyridine-4-carboxaldehyde over, the oxidant at 25⁰C. Requisite amount of solution of substrate, H₂SO₄, were equilibrated. A measured amount of KMnO₄ was added to the reaction mixture with stirring. The time of initiation of the reaction was recorded when half of the contents of pipette were released .The solution was taken in a curette and absorbance was measured at 526 nm using double beam spectrophotometer.

Pyridine -4-carboxaldehyde (0.1M), KMnO₄(0.2M), H₂SO₄(1M) and water (total volume to 100 ml) kept aside for 24 hours. The unconsumed KMnO₄was determined spectrophotometrically and the product pyridine-4-carboxaldehyde and was centired by TLC .The stochiometry is determined to be 1:1.

result and discussion :

1) Dependence of permanganate concentration:-

To study the effect of dependence of permanganate concentration. The concentration of KMnO₄ was varied from 1x10^-4M to 9x10^-4M keeping constant concentration of other reaction ingredients such as substrate and acid. Since reaction has been studied under pseudo first order condition. A plot of log [KMnO₄] verses time was made and pseudo first order rate constants were calculated.

The order of reaction was determined from log (<rate>) verses log (<C>) thus shows that rate of reaction varies linearly with concentration of KMnO₄. The graph between K(min)^-1 and [KMnO₄] gives good correlation (r= 0.617) .

2) Dependence of Substrate Concentration:

The concentration of substrate was varied from 1x10^-3 to 9x10^-3 M at fixed concentration of [KMnO₄] = 1 x10^-4 M and [H₂SO₄] = 1 M. The data obtained was used to calculate first order rate constant. From log (<rate>) verses log (<C>) graph, the order was found to be close to one first order dependence on substrate. The graph between K(min)^-1 and [substrate] gives good correlation (r²= 0.638) .

3) Dependence of Acid Concentration :

The hydrogen ion concentration dependence was studied by varying H₂SO₄ at fixed [KMnO₄] = 1x10^-4 M and [Substrate] = 1x10^-3. The pseudo first order rate constant were evaluated and the plot of these rate constant against [H⁺] shows direct proportionality. The graph between K(min)^-1 and [H₂SO₄] gives good correlation (r²= 0.642) and the log (K) against log <C> gives (r²=0.549) . (Table 1)

4) Effect of Salt:

The rate of reaction was studied by adding salts while keeping constant concentration of [KMnO₄], [Substrate] and [H₂SO₄] (Table 2). The result reveals that there is no regular trend for rate constant with change in concentration of added salt.

KMnO₄ is selected as an oxidizing agent for our present study because; it is a economically low cost material. It has high oxidation potential [E⁰=1.7V], it can oxidize wide variety of substances and it is effective over wide range of PH . There are various oxidation states of Mn like (+II, + III, +IV, +V, +VI and + VII). Hence it become very complicated to find out the exact species involved in it.

In acidic media , MnO^-₄ gets converted into MnO₂

4 MnO₄^-+ 4 H⁺ → 3 O₂+2 H₂O + 4 MnO₂

In acidic media, MnO₄^2-is converted to Mn²⁺⁺

MnO₄^2-+ 8 H⁺+ 6e^- →Mn²⁺+ 4H₂O

The Mn²⁺ may react with MnO^-₄ and the product is MnO₂

2 MnO^-₄ + 3 Mn²⁺+ 2 H₂O → 5 MnO₂+4 H⁺.

It is assumed that during the oxidation of aldehyde, positively charged species attack a lone pair of electron of the reductant at centre of high electron density.

The formation of oxo-bridge in intermediate compound indicates the the oxygen passage of one electron from the substrate to bonded Mn⁺⁷

This bridge due to protonation, rupture and gives Mn⁺³ species. Since the solution does not indicate any presence of Mn( III )or precipitated MnO₂it is quite logical to state that Mn( III ) react or its dispropenated product Mn (IV) instantaneously react with substrate giving final end product Mn⁺².

2 Mn (III) → Mn (II) + Mn (IV)

Mn (IV) + Substrate → Mn (II) + Product.

Considering the following steps the kinetic expression can be given,

The probable mechanism can be depicted as:-

+ H₂O

Unstable.

Mn(І І І) + 2H⁺

Product

The Mn ( ІІІ ) undergoes disproportionation to give final Mn( ІІ ).

2 Mn( ІІІ ) → Mn( ІІ )+Mn(ІV)

The Mn (ІV) obtained further reacts with substrate to give final product. The mechanism involved oxo-bridge formation and abstraction of hydrogen from substrate.

Table1.-Effect of varying concentration of reactants at 25⁰C

[PYRIDINE-4-CARBOX ALDEHYDE]10^-3M	[KMnO₄] X 10^-4M	[H₂SO₄]M	K(min)^-1
1	1	1	2.0449
1	2	1	2.1823
1	3	1	2.1561
1	4	1	2.14
1	5	1	2.2798
1	6	1	2.2409
1	7	1	2.3071
1	8	1	2.1942
1	9	1	2.5587
1	1	0.1	0.016366
1	1	0.2	0.017481
1	1	0.3	0.017727
1	1	0.4	0.017839
1	1	0.5	0.019065
1	1	0.6	0.019642
1	1	0.7	0.019865
1	1	0.8	0.020521
1	1	0.9	0.029209
1	1	1	1.6632
2	1	1	1.6762
3	1	1	1.6632
4	1	1	1.712
5	1	1	1.7367
6	1	1	1.7324
7	1	1	1.8201
8	1	1	1.8426
9	1	1	2.2143

Table 2 .Effect of salts on reaction rate .

M [Salt ]	KBr	KCl	KI	K₂SO₄	ALCl₃	MnSO₄	CaCl₂
1 X 10^-2	0.031142	0.01675	0.032381	0.021132	0.028943	0.024855	0.023983
2 X 10^-2	0.016305	0.014537	0.037095	0.018073	0.029227	0.031918	0.02393
3 X 10^-2	0.017204	0.017536	0.027963	0.013951	0.024986	0.031603	0.022843
4 X 10^-2	0.026647	0.014171	0.038624	0.015793	0.024327	0.035305	0.020287
5 X 10^-2	0.077732	0.018503	0.037115	0.028992	0.021281	0.038015	0.016817
6 X 10^-2	0.043181	0.016137	0.037036	0.014327	0.022551	0.038212	0.01626
7 X 10^-2	0.058213	0.013183	0.042589	0.020388	0.026042	0.037522	0.016816
8 X 10^-2	0.071594	0.016233	0.056682	0.016304	0.025877	0.04149	0.0205
9 X 10^-2	0.076887	0.010364	0.047342	0.02612	0.030226	0.057484	0.018217

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Received on 24.07.2013 Modified on 07.10.2013

Asian J. Research Chem. 6(12): December 2013; Page 1092-1094