Impact of Alkyl Substituents on the Gas-Phase Competition between Substitution and Elimination

The S(N)2 and E2 reactions of a series of alkyl bromides with varying substitution patterns at the α- and β-carbons have been studied in the gas phase using naphthoate and phenoxide-based nucleophiles. The experimental work is supported by calculations at the MP2/6-31+G(d,p)//MP2/6-31+G(d) level. The results parallel reactivity patterns observed in the condensed phase, but offer new insights into steric factors in S(N)2 processes. In the gas phase, polarizability is more important, and the highest S(N)2 reactivity is observed when the β-carbon is 2°. In addition, the data confirm that alkyl substituents at the β-carbon have a greater accelerating effect on E2 reactions than those at the α-carbon. Finally, computed data based on lowest enthalpy pathways provide poor descriptions of the reactions of the larger alkyl bromides and are skewed toward crowded systems that offer stabilizing, nonbonded interactions at the expense of conformational freedom.