Mechanistic investigation of S N2 dominated gas phase alkyl iodide reactions

Abstract

The competition between substitution (S N2) and elimination (E2) has been studied for the reactions of methyl, ethyl, isopropyl, and tert-butyl iodide with Cl −, CN −, and HS − in the gas phase. Previous studies have shown a dominance of the S N2 mechanism for sulfur anions and for some cyanide–alkyl iodide reactions. Although our results support this conclusion for the reactions studied, they reveal that competition between the S N2 and E2 pathways exists for the isopropyl reactions. Steric and electronic effects, upon alkyl group substitution, produce looser and less stable S N2 transition states, however, they can favor the E2 process. These opposing effects on barrier heights produce E2/S N2 competition as steric hindrance increases around the α-carbon, however the relative differences in intrinsic barrier heights lead to significantly different branching ratios. This interpretation is discussed in terms of reaction efficiencies, kinetic isotope effects, linear basicity–reactivity relationships, electrostatic models, and transition state looseness parameters.