Portal de Periódicos da CAPES

Reaction Kinetics of the Model Reaction of Benzoic Acid with Ethylene Carbonate

1984; Marcel Dekker; Volume: 21; Issue: 2 Linguagem: Inglês

10.1080/00222338408056544

2332-4082

David R. Fagerburg,

Chemical Reaction Mechanisms

Abstract As a model for several polymer-related reactions, we have reexamined the reaction kinetics of benzoic acid with ethylene carbonate in which various quaternary ammonium salts were used as catalysts. The salt anion appeared to have little or no effect on the rate of reaction. At a 1:1 mole ratio of acid to carbonate, the reaction was zero-order to about 95% reaction. With tetraethyl-ammonium hydroxide catalyst at 0.4 mol% (based on carbonate moles), the activation energy was 20.8 kcal/mol with a preexponential factor of In A = 17.1 (activation entropy of -26 cal/deg). The activation energy for decomposition of the carbonate alone was 33.0 kcal/mol with a preexponential factor of In A = 27.3. This rules out previously suggested mechanisms that essentially involved decomposition of the carbonate prior to esterification. The proposed mechanism for the reaction involves the attack of the quaternary salt carboxylate on the methylene carbon of the carbonate. The attack causes ring opening and is followed by proton transfer and carbon dioxide loss from the carbonate half-ester intermediate. Ether linkages (such as diethylene glycol) are postulated to arise from the resultant alkoxide intermediate prior to protonation to give the hydroxyethyl ester rather than by separate postreaction of carbonate with the hydroxyethyl ester.