Nucleophilic identity substitution reactions. The reaction between hydrogen fluoride and protonated alkyl fluorides

Abstract

The gas phase reactions between HF and the protonated alkyl fluorides MeFH⁺, EtFH⁺, PrⁱFH⁺, and Bu^tFH⁺ have been studied using ab initio methods. The potential energy profiles for both nucleophilic substitution (S_N2) and elimination (E2) pathways have been investigated. Both backside Walden inversion and frontside nucleophilic substitution reaction profiles have been generated. Backside substitution is very favourable, but shows relatively little variation with the alkyl group. Frontside substitution reaction barriers are only slightly higher than the barrier for backside substitution for HF + MeFH⁺, and the difference in barrier heights for frontside and backside displacement seems negligible for the larger alkyl groups. Reaction barrier trends have been analysed and compared with the results of similar studies of the H₂O/ROH₂⁺ and NH₃/RNH₃⁺ systems (R = Me, Et, Prⁱ, and Bu^t). Compared to the two other classes, protonated fluorides have extreme structures which, with the exception of the Me substrate, are weakly bound complexes between an alkyl cation and HF. The results nourish the idea that nucleophilic substitution reactions are better understood in view of competition between frontside and backside substitution than from the traditional S_N1/S_N2 perspective.