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 the strength of the base, the nature of the substrate and leaving group, and the type of solvent.

Since the base appears in the E2 rate equation, increasing the strength of the base increases the rate of the reaction. Strong bases like hydroxide, alkoxide, and amide ions favor E2 reactions.

The stability of the carbon-carbon double bond increases with substitution. Therefore, with strong bases, the more substituted tertiary haloalkanes undergo E2 eliminations faster than the secondary and primary counterparts.

Ideally, a good leaving group is a weak conjugate base. In alkyl halides, the iodide group is the least basic and favored over bromide and chloride.

In E2 reactions, polar protic solvents, like water, solvate the base via strong hydrogen bonds, making them less reactive towards the substrate. In contrast, in polar aprotic solvents, like acetone, the base is only weakly solvated, making them more suitable for E2 reactions.