6.16: Reacción E2: Cinética y Mecanismo
SN2 substitutions and E2 eliminations of alkyl halides proceed via a concerted pathway. While the nucleophile attacks the alpha carbon in SN2 reactions, it functions as a strong base and abstracts a beta hydrogen in the E2 mechanism. The rate-limiting transition state in E2 elimination reactions is characterized by partially broken carbon–hydrogen and carbon–halogen bonds and a partially formed pi bond between the alpha and beta carbons. The beta hydrogen and halide are eliminated simultaneously, yielding an alkene along with the conjugate acid and the halide.
E2 reactions are bimolecular and follow second-order kinetics, where the reaction rate depends on the concentrations of the alkyl halide and the base. The concerted mechanism is further confirmed by deuterium isotope studies, which are based on the higher energy required to break the C–D bond compared to the C–H bond. Thus, when the hydrogen that is transferred in the rate-determining step (beta hydrogen) is substituted by deuterium, a primary kinetic deuterium isotope effect is observed.
Studies of various dehydrohalogenation reactions reveal that the rate constant for the hydrogenated substrate (kH) is 2.5–8 times higher than that for the deuterated counterpart (kD), confirming that the rate-limiting step involves the breaking of a carbon–hydrogen bond at the transition state.
The E2 mechanism is affected by the strength of the base, nature of the substrate and leaving group, and the type of solvent. Because the base appears in the E2 rate equation, the rate of the reaction increases with the strength of the base. Strong bases like hydroxide, alkoxide, and amide anions promote E2 reactions.
Since the stability of the carbon–carbon double bond increases with substitution, more substituted tertiary haloalkanes undergo E2 eliminations faster than primary and secondary haloalkanes. Ideally, a good leaving group is a weak conjugate base. The iodide group is the least basic and the best leaving group among the halides.
The base can be solvated via strong hydrogen bonds in polar protic solvents (such as water), making them less effective. Thus, polar aprotic solvents (such as acetone), where the base is only weakly solvated, favor E2 reactions.