6.18: E1 Reaction: Kinetics and Mechanism
Here, in contrast to the E2 reaction mechanism, we delve into the aspects of the E1 reaction mechanism, which has two steps: rate-limiting loss of the leaving group and abstraction of the beta hydrogen by a weak base. Typically, the experimental proof for the E1 mechanism is via kinetic studies or isotope studies. While the former demonstrates the first-order kinetics—the dependence of the reaction solely on substrate concentration—the latter proves the abstraction of hydrogen only in the second step.
Factors influencing E1 Reaction:
The three key factors that influence E1 elimination reactions are (a) the stability of the carbocation, (b) the nature of the leaving group, and (c) the solvent type. In this context, the mechanism of hyperconjugation that leads to the stabilization of carbocations is demonstrated. This is key to the rate-limiting step where the carbocation is formed, influencing the speed of the reaction of substituted alkyl halides. An interesting corollary is a 1,2-hydride shift in a primary carbocation to form a secondary carbocation or a 1,2-alkyl shift to give a more stable tertiary carbocation. Subsequently, as the carbon–halogen bond breaking is the rate-limiting step, E1 reactions are influenced mainly by the nature of the leaving halide groups as weak conjugate bases. Lastly, the polarity of protic solvents is elucidated, as they play a crucial role in stabilizing the intermediate carbocations/halides in the rate-limiting step.
Tertiary Halides: SN1 versus E1
At this stage, it is essential to compare SN1 versus E1 reactions, for both these reactions proceed via the formation of a common intermediate and, as a result, respond similarly to factors affecting reactivity. Typically, it is difficult to influence whether the formation of products proceeds via the SN1 or E1 route, for, in either case, the free energy of activation proceeding from the carbocation is very small. SN1 is often favorable as compared to E1 for unimolecular reactions when the temperatures are lower. However, in general, synthetic routes do not prefer substitution reactions for tertiary halides, as they undergo elimination very quickly. An increase in the temperature of the reaction condition shifts the mechanism to favor E1 instead. As an unwritten rule, when an elimination product is desired from such tertiary substrates, a strong base is used to promote the E2 mechanism against the competing E1 versus SN1 mechanisms.