I. Halide ion transfer reactions in the gas phase ion chemistry of haloalkanes

Abstract

Ion-molecule reactions have been studied in which a substituted methyl or ethyl carbonium ion reacts with a halogenated methane or ethane by halide ion abstraction to give another carbonium ion. The thermochemical control of these reactions has been interpreted in terms of a quantity, the halide affinity, which is a measure of the Lewis acidity of the carbonium ions. By studying competitive fluoride and chloride transfer reactions the relative ordering of fluoride affinities (CH 3 + ⩾ CF 3 + ⩾ CHF 2 + ⩾ CH 2F + ⩾ CHFCl + ⩾ CH 3CF 2 + ⩾ CH 3CHF + ⩾ CH 3CH 2 +) and chloride affinities (CF 3 + ⩾ CHF 2 + ⩾ CH 2CL + ⩾ CF 2Cl + ⩾ CHFCl + ⩾ CHCl 2 +) has been established. These orders are compared with values derived from available thermochemical data and used to establish new limits on ionic heats of formation. A qualitative discussion of relative Lewis acidity in terms of electronegativity, π-back donation, and polarizability of the substitutents is presented.