THEORETICAL ABINITIO STUDY OF THE REACTION OF FORMATION OF 2-FLUOROETHANOL FROM OXIRANE AND HF

The reaction of ring opening of epoxides under the action of a halogenic acid has been investigated using as model the system C2H4O + HF → CH2OHCH2F. The physical conditions chosen for the model more directly correspond to the gas phase reaction and under these conditions the preferred mechanism leads to the formation of a halohydrine having the same configuration at the C atom as the reagent. Parallel investigations have been performed on other mechanisms which postulate the preliminary formation of the conjugate acid of the oxirane (C2H4OH+) and proceed via the well known mechanisms A1 or A2. In this case the best mechanism corresponds to the so-called “borderline A2” mechanism. This last type of mechanism probably is the dominant one in protic solutions, but by coupling the present calculations with experimental conductometric measurements in anhydrous aprotic media one could consider the first concerted mechanism as a possible candidate also for the reaction in “inert” media. A qualitative analysis of the transition state indicates, in addition, that the propension for the retention path, is probably emphasized by the use of HF as reactant, and that with other acids, like HC1, or even by assuming the presence of dimers like HF·HF, the inversion path could be preferred. The investigations have been done by determining the geometry of the transition state and the reaction coordinate with ab initio SCF STO-3G calculations on the whole nuclear configuration space (21 dimensions). These calculations have been supplemented by a few CI calculations on the same basis set and by a few SCF calculations with a larger basis set. © 1979, Springer-Verlag. All rights reserved.