AB-INITIO STUDY ON THE THERMAL DECARBOXYLATION OF BUT-3-ENOIC ACID AND ITS DERIVATIVES

The mechanism for thermal decarboxylation of but-3-enoic acid and its derivatives HXC=CYCH2COOH (X, Y = H, F, CH3, C2H5, and Cl) leading to carbon dioxide and olefins has been studied from the theoretical point of view by ah initio MO calculations. The transition states obtained by our ab initio calculations are completely consistent with the exper imental data, and support the ''synchronous'' mechanism for thermal de carboxylation of but-3-enoic acid and its derivatives via a ''twisted chair'' six-membered cyclic transition state. The effects of beta- and gamma-substituents on the activation energy (E(a)) can be explained i n terms of electronic charge distribution. beta-Substitution decreases the activation energy, while gamma-substitution increases it. Changes in the activation energy are related to changes in the charges at C-g amma(C1) and C-beta(C2) as the substituents are varied. The activation energy decreases with an increase of negative charge at C while It in creases with an increase of negative charge at Cg. The best estimate o f 156.8 kJ/mol for the activation energy with MP2/6-31G//HF/3-21G(*) is in reasonable agreement with the available experimental values of 1 64 +/- 7 kJ/mol and 160 kJ/mol for decarboxylation of but-3-enoic acid . The calculated primary k(H)/k(D), 2.86, and k(12C)/k(14C), 1.03, for the decarboxylation of but-3-enoic acid, are also in excellent agreem ent with the available experimental values of 2.7 and 1.035, respectiv ely, supporting the transition state structure obtained.