Question

Which of the following sets of conditions most favors the E1 mechanisim?

1. When the alkyl halide is primary or secondary and the base is a strong base

2. When the alkyl halide is primary or secondary and the base is a weak base

3.when the alkyl halide is tertiary and the base is a strong base

4. When the alkyl halide is tertiary and the base is a weak base

Answer 1

Reactivity order of E1 mechanism is as follows:  

(CH₃)₃C- > (CH₃)₂CH-   > CH₃CH₂- > CH₃-

Since the base is not involved in the rate determining step, the nature of the base is unimportant in an E1 reaction. However, the more reactive the base, the more likely an E2 reaction becomes.

This E1 mechanistic pathway is most common with:

(i) Good leaving groups

(ii) Stable carbocations (Tertiary carbocation is most stable carbocation)

(iii) Weak bases.

Answer is (4)

Alkyl halide is tertiary and the base is a weak base.