Mechanistic investigation of SN2 dominated gas phase alkyl iodide reactions

The competition between substitution (SN2) 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 SN2 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 SN2 and E2 pathways exists for the isopropyl reactions. Steric and electronic effects, upon alkyl group substitution, produce looser and less stable SN2 transition states, however, they can favor the E2 process. These opposing effects on barrier heights produce E2/SN2 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.

Qualitative depiction of relative SN2 barrier heights for CH3I and E2 barrier heights for t-C4H9I estimated from reaction efficiencies.Figure optionsDownload high-quality image (187 K)Download as PowerPoint slideResearch highlights▶ Relative E2/SN2 barriers favor the SN2 pathway for methyl, ethyl, and isopropyl iodide reactions. ▶ Reactivity trends can be explained using electronegativity and looseness parameters. ▶ More normal isotope effects and looser SN2 transition states occur upon alkyl substitution. ▶ Results substantiates significantly tighter transition states in the gas phase versus solution.