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+, Pr(i)FH+, and Bu(t)FH+ have been studied using ab initio methods. The potential energy profiles for both nucleophilic substitution (S(N)2) 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, Pr(i), 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(N)1/S(N)2 perspective.

Steric and electronic effects in SN2 reactions

“…In other words, increasing steric repulsion seems not to be the determining factor explaining reactivity in this particular example. For the analogous reactions of protonated amines the normal trend is observed [17], while for protonated fluorides the barriers are low and close to constant throughout the series [18]. With regard to the protonated alcohols, it appears that gas-phase and solution reactivity are diametrically opposite.…”

“…Quite surprisingly, the experimentally found reactivity order [CH 3 > CH 3 CH 2 < (CH 3 ) 2 CH < (CH 3 ) 3 C] for these gas-phase reactions is almost the opposite of the normal trend [15]. In addition, high-level quantum chemical calculations reproduce the experimental results very accurately [15,16]. In other words, increasing steric repulsion seems not to be the determining factor explaining reactivity in this particular example.…”

On the Gas‐Phase Reactivity of Complexed OH+ with Halogenated Alkanes

“…Since many of the optimized critical point structures deviated substantially on those levels, we modified the G3 scheme such that structures were optimized on the B3LYP/6-31(G) level and ZPVE corrections were obtained from scaled (scaling factor = 0.9434 [31] ) MP2/6-31(G) frequencies. For a further description see reference [32]. We use G3 m* as notation for the modified G3 scheme.…”

Carbocations

“…The degenerate gas-phase reactions between HF and protonated alkyl fluorides RFH + (R = Me, Et, i-Pr, t-Bu) were investigated by ab initio methods. 15 With the exception of MeFH + , the protonated alkyl fluorides can be viewed as weak complexes of the carbocation R + and HF. Both frontside and backside substitutions occur, supporting the proposal (see Introduction) 8 that nucleophilic substitution reactions are better understood through such a competition as opposed to the traditional S N 1/S N 2 competition.…”