The E1 and E1cB Reactions

I Loss of a neighboring H in a fast step yields the neutral alkene product. The electron pair from the C-H bond goes to form the alkene tt bond. 

Mechanism of the El reaction. Two steps are involved, the first of which is rate-limiting, and a carbocation intermediate is present. 

El eliminations begin with the same unimolecular dissociation to give a carbocation that we saw in the S l reaction, but the dissociation is followed by loss of H from the adjacent carbon rather than by substitution. In fact, the El and S l reactions normally occur together whenever an alkyl halide is treated in a protic solvent with a nonbasic nucleophile. Thus, the best El substrates are also the best SnI substrates, and mixtures of substitution and elimination products are usually obtained. For example, when 2-chloro-2-methylpropane is warmed to 65 C in 80% aqueous ethanol, a 64 36 mixture of 2-methyl-2-propanol (SnI) and 2-methylpropene (El) results. 

Much evidence has been obtained in support of the El mechanism. For example, El reactions show first-order kinetics, consistent with a rate-limiting, unimolecular dissociation process. Furthermore, El reactions show no deuterium isotope effect because rupture of the C-H (or C-D) bond occurs after the rate-limiting step rather than during it. Thus, we can t measure a rate difference between a deuterated and nondeuterated substrate. 

O Spontaneous dissociation of the tertiary alkyl chloride yields an intermediate carbocation in a slow, rate-limiting step. 

Thomson MCW Click Organic Process to view an animation showing the mechanism of an E1 elimination reaction. 

---