Abstract
A characteristic property of esters is that, they undergo both oxidation and hydrolysis under similar conditions. Furthermore, the product of each process is an aldehyde (the product of hydrolysis is alcohol, which in turn undergoes oxidation to give an aldehyde). It is, therefore, difficult to find out which process is operative. The kinetics of the reaction offers an opportunity to resolve this uncertainty. This method is illustrated in this article with two examples: oxidation of esters by Tl3+ and oxidation of esters by Co3+. The method described in this paper familiarizes students with the basic techniques involved in following the reaction, such as quenching the reaction, ensuring that the aliquots of reaction mixture are equal, taking the reaction mixture at all time intervals in an identical manner, taking infinite readings and the use of a thermostat. It also provides familiarity with the use of integrated rate equation, plotting graphs, the evaluation of slopes on graphs and the calculation of rate constants.
Highlights
The underlying principle of this method of using kinetics to find the reaction path, is to carry out the hydrolysis of esters under the same condition as the oxidation of esters, and to compare their rate constants
If the rate of the hydrolysis is negligibly small and the rate of oxidation of alcohols is very slow, it can be concluded that the ester is undergoing a “direct oxidation”,1 and the aldehyde formed in the reaction stems almost completely from the ester
Ethyl acetate and n-propyl acetate have been oxidized by aqueous bromine[2] and the reaction was believed to occur by direct oxidation
Summary
The underlying principle of this method of using kinetics to find the reaction path, is to carry out the hydrolysis of esters under the same condition as the oxidation of esters, and to compare their rate constants. These rate constants are ten times slower than those of the corresponding acetates (Table 1) This data indicates that the aldehyde formed in the oxidations is entirely attributed to direct oxidation, for which the reaction path is conceptualized in Scheme 1: CH3COOC. As can be seen in Scheme 1, direct oxidation envisages C-H bond breakage as the rate-determining step (the order of reaction is one in concentration of ester and the concentration of Tl3þ). This assumption is supported by the fact that the rate of oxidation of ethyl formate and ethyl acetate[1] are almost the same, and their activation parameters are comparable (Table 1).
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