Overview

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: S_N1 versus E1

At this stage, it is essential to compare S_N1 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 S_N1 or E1 route, for, in either case, the free energy of activation proceeding from the carbocation is very small. S_N1 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 S_N1 mechanisms.

Procedure

Elimination reactions of alkyl halides through an E1 mechanism occur in two steps analogous to its S_N1 counterpart.

The first step in both reactions is a slow rate-limiting step which proceeds with the loss of the halide leaving group forming a carbocation intermediate.

The second step in E1 reactions involves the abstraction of a beta hydrogen by a weak base forming an alkene. In contrast, an S_N1 reaction yields a substitution product.

Evidence supporting the E1 mechanism comes from kinetic studies, which show that E1 reactions are unimolecular and follow first-order kinetics. Meaning, the reaction rate depends only on the concentration of the substrate.

Deuterium isotope studies provide further evidence. For the dehydrohalogenation of tert-butyl chloride, the ratio of the rate constants of the non-deuterated and deuterated analogs, k_H/k_D is 1.1.

Since the rate constants are almost identical, this suggests that a carbon-hydrogen bond is not broken in the rate-limiting step but in the second step, consistent with the proposed mechanism.

Factors that influence E1 elimination reactions include the stability of the carbocation, the nature of the leaving group, and the type of solvent.

In an E1 reaction, the rate-limiting step involves the formation of a carbocation intermediate. Electron donating alkyl groups can stabilize the positive charge by delocalizing the carbon-hydrogen sigma electrons into the empty p-orbital of the positively charged carbon.

This stabilizing interaction, called hyperconjugation, increases with the number of alkyl groups. Therefore, E1 reactions are fastest with tertiary alkyl halides.

However, a primary carbocation can also undergo a 1,2-hydride shift to form a secondary carbocation, or a 1,2-alkyl shift, to give a more stable tertiary carbocation.

Since the carbon-halogen bond breaks in the rate-limiting step, E1 reactions further depend on the quality of the leaving group. Compared to bromides and chlorides, iodides are weak conjugate bases, and therefore, better leaving groups.

Finally, polar protic solvents, like ethanol, stabilize the carbocation and the halide ions formed in the rate-limiting step, thereby favoring E1 reactions.