INTRODUCTION:
The Kinetics provides the useful information about the mechanism and rate of chemical reaction, which helps to run a chemical reaction successfully by a way of selecting optimum condition as to get maximum yield. The kinetic study also helps us to study the factors which influence the rate of reaction like temperature, pressure, substrate concentration, oxidatant concentration, composition of reaction mixture and catalyst. The reaction kinetics plays a very important role in the investigation of the reaction mechanism. Oxidation of organic compound carried out by oxidising agent like potassium dichromate Cr (VI) 1,2,3.
The update literature survey shows that, though the considerable amount of work has been done on the oxidation of organic compounds4,5,6,7,8,15 by potassium permagnate, but only a few studies is found on the kinetics of oxidation of ester by potassium permagnate 9,10. The object of present investigation is to formulate the reaction mechanism from the data gathered from kinetic measurement. It is found that the oxidation of ester occurs by two ways, hydrolysis followed by the oxidation of alcohol, direct oxidation of esters. But no conclusive evidence was provided in support of either of the two pathways, hence it is decided to undertake the systematic investigation kinetic of ethyl cynoacetate. The kinetic of oxidation of ester by potassium permagnet in moderately concentrate sulphuric acid medium has been investigated.
The rate law
The result obtained shows that the direct oxidation is the only process occurring under the applied conditions of experiments.
MATERIAL AND METHODS:-
All the chemicals used were of AR grade, specially potassium permagnate used were of AR grade and was prepared and estimated by standard method. Esters are of Zobo Chem. Ltd. and the boiling point of both esters was confirmed. The ethyl cynoacetate were always freshly distilled before use for the kinetic measurement permagnate and sulphuric acid solution were taken in two different flasks and covered with black cloth and placed in a thermostat for 1 hour to attain constant temperature by both the flask. In order to prevent the hydrolysis, required volume of given ester was directly added to acid solution with micro pipette just before mixing it with permagnate solution.
The course of reaction was followed by measuring the absorbance (optical density) of unreacted permagnate ions from time to time at 520 nm using Carl - Zeiss spectrophotometer. The reaction were followed upto 70 to 85% completion and the product were identified as acid i.e. acetic acid and aldehyde by 2, 4, DNP test11. The aldehydes were obtained in 90% yield as estimated from their 2.4 DNP derivative. The addition of mercuric chloride to the reaction system did not induce the precipitation of mercuric chloride showing that no free radicals are formed in the system 12.
RESULTS AND DISCUSSION:-
Under the conditions [ester] > [KMnO4] in 3.20 M. H2SO4. The plot of log absorbance (O.D.) Vs time were linear indicating the first order dependence of rate on [KMnO4] Fig. 1, A Oxidation of esters depends on the concentration of potassium permagnate. This was also confirmed by verifying [KMnO4] which did not show any change in Pseudo First order constant (k1) value (Table No.1). The reaction was also found to be first order in [ester] Fig. 1B (Table 2 [A]. The rate of reaction increases with increases in [H2SO4]. Table No. 3.
Fig. 1- The plot of log absorbance (O.D.) Vs time
Table No.1 Shows effect of oxidant (KMnO4)
[Ethyle cyanoacetate] = 4.7x10-7m
Temp = 30Oc 
max = 520 nm
|
Sr. No |
[KMnO4] x 10-4 M |
kx10-2/min |
B |
B |
|
|
1 |
3.14 |
0.3454 |
|
2 |
3.57 |
0.6909 |
|
3 |
3.9 |
0.8060 |
|
4 |
4.3 |
0.8060 |
|
5 |
4.6 |
0.9212 |
|
6 |
4.8 |
0.8060 |
|
7 |
5.4 |
0.9212 |
|
8 |
5.3 |
1.381 |
Table 2 : (Oxidation of Ehyle cyanoacetate)
|
Sr. No. |
[Ester] x 10-3 |
log [Sub] |
k x 10-2/min |
logkI |
|
1 |
3.125 |
-2.5052 |
0.6909 |
3.8394 |
|
2 |
3.529 |
-2.4524 |
0.8060 |
3.9063 |
|
3 |
3.888 |
-2.4103 |
0.9212 |
3.9643 |
|
4 |
4.210 |
-2.3758 |
1.036 |
2.0153 |
|
5 |
4.5 |
-2.3468 |
1.048 |
2.0203 |
|
6 |
4.76 |
2.3224 |
1.151 |
2.0610 |
|
7 |
5.00 |
-2.3011 |
1.166 |
2.0666 |
|
8 |
5.21 |
-2.2826 |
1.182 |
2.0726 |
Zucker and Hammett proposed that for acid catalysed reaction in strong acid the rate correlates with Ho of activated complex does not contain a water molecule. For (A-1) unimoleculor mechanism the rate is correlated with Hammett acidity function Ho and a linear plot between log Kabs Vs Ho is obtained. The A-2-mechanism (Ingold's terminology) gives correlation with H+ ion and a linear plot is obtained between log Kabs against log [Hx] that is the molecular concentration of an both the plots gives unit slope. This hypothesis is called Zuckker- Hammett hypothesis, hold good for large number of reaction. For example hydrolysis of benzylidene diacetate, ethyl acetate and propidlactone completing with A-1 mechanism. The reaction consistent with the A-2 process is hydrolysis of ester ethyl cynoacetate. The addition of sulphate and pyrophosphate ions has negligible effect on the rate. For reactions in solution the nature of solvent plays an important role which has been discussed in detail by Aims . In present investigation, effect of solvent could not be studies because of reactivity of solvent such as alcohols, ketones, dioxine, etc with KMnO4.