Journal of M olecular Structure, 239 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGF (1990) 193-203 Elsevier Science Publishers B.V., Amsterdam 193 MONTE CARLO STUDY OF LACTONE FORMATION KINETICS PERE ALEMANY and EUDALD VILASECA Departament de Quimica Fisica, Facultat de Quimica, Universitat de Barcelona, C/Marti i Franquks I,08028 Barcelona (Spain) (Received 8 January 1990) ABSTRACT The Monte-Carlo method is applied to the study of the conformational behaviour of the O- bromoalkanoate anions in order to analyse the dependence on the molecular chain length of the experimental rate constants for the lactone formation. The results show that the probability of a favourable contact between the reactive atoms to undergo a S,2 reaction is extremely dependent on the conformational features of each molecule. INTRODUCTION Lactonization is a ring-closure reaction characteristic of w-substituted al- kanoic acids. In a basic medium, it consists of a nucleophilic substitution that can be represented as: cl--c P X-CH2-(CH,),-COO - I I +x. H,C-_(CWn Although nucleophilic substitutions occur in a single step with a transition state having the following geometry, I x_______c_______y in the case of lactonization the whole mechanism is controlled by the geometric features of the molecule. The reaction rate depends on the feasibility of contact between the reacting atoms. Moreover, the differences observed in the lacton- ization rate constants of a o+substituted alkanoic acid series can be mainly interpreted in terms of the molecular chain lengths. As in all cyclization reactions, we can consider the whole process of lactone formation as divided into two steps: first, the approximation process between the reactive atoms; and second, the proper nucleophilic substitution process. The present work was centred on the analysis of the first step, specifically in the calculation of the probability of contact between the reactive atoms. zyxwvutsrqpon To zyxwvutsrqpo 0022-2860/90/$03.50 0 1990 - Elsevier Science Publishers B.V.