E2 Reaction: Kinetics and Mechanism

Analogous to S_N2 substitution reactions, E2 eliminations of alkyl halides also proceed via a concerted pathway.

However, unlike S_N2 reactions where the nucleophile attacks the α carbon, in E2, it functions as a strong base and abstracts a β hydrogen.

Loss of the β hydrogen and the halide occur simultaneously through a rate-limiting transition state characterized by a partially broken carbon-hydrogen and carbon-halogen bond and a partially formed π bond between the α and β carbons. Completion of the reaction yields an alkene.

E2 reactions are bimolecular and follow second-order kinetics, where the reaction rate depends on the concentrations of the alkyl halide and the base, supporting a concerted mechanism.

Deuterium isotope studies further confirm this mechanism based on the fact that the carbon-hydrogen bond is weaker than a carbon-deuterium bond.

For example, in the base-induced dehydrohalogenation of propyl bromide, the rate constant for the hydrogenated substrate, k_H, is 6.7 times higher than its deuterated counterpart, k_D, confirming that the rate-limiting step involves the breaking of a carbon-hydrogen bond at the transition state.

Factors influencing the E2 mechanism include