Cyclic and open-chain carbonium ion intermediates in nucleophilic halogen displacement: 2-chloroethanol, 2-chloroethanethilol and their 1,3-propane homologues. A theoretical ab-initio mo approach

Abstract

The geometrical structure and thermodynamic properties of the open-chain and cyclic cations formally derived by halogen nucleophilic displacement in 2-chloroethanol, 2-chloroethanethiol and in the corresponding 1,3-propane derivatives were calculated in the MO ab-initio approach with the extended 6-31G ∗ basis set. To set up a comparison between the stability of these cations and that of starting materials, 2-chloroethanol and 2-chloroethanethiol were also examined in the same calculation scheme in order to have a homogeneous conformational description of these two molecules in their ground states. The results show that both in terms of total electronic energy and free-energy content the cyclic ion is more stable than the open form, and this occurs for both the three- and four-membered rings. This stability order is enhanced in the case of the sulphur derivatives. As regards entropy contributions, these do not show appreciable differences in three- and four-membered cyclic ions, at least in comparison with other energy contributions. According to these calculations, the energy required for generating the cations from the starting materials is quite large: of the order of 180 kcal mol −. These reactions thus seem very unlikely to occur, unless ion-pair association and solvation lower the intermediate and transition-state energies. Accordingly, it is rather difficult to give a proper description of the transition state for intra- and intermolecular nucleophilic displacements in these systems and an attempt has been made to obtain a number of approximate conclusions by comparing the stability of intermediate cations. Intramolecular displacements thus appear to be favoured almost to the same extent, when three- and four-membered cyclic intermediates are formed, both entropically and as regards the energy content. Minor effects should thus be involved in decreasing the energy of activation of the process in the case of 1,2-ethane derivatives with respect to the corresponding 1,3-propane derivatives.