Question

Which is the least reactive alkyl halide in $E1$ reaction?

• A.
• B.
• C.
• D.

Answer 1

The correct option is C

E1 reaction is a two step process.
In step 1, leaving group leaves and form a carbocation. Thus, carbocation intermediate is formed.
In step 2, the base will attack the proton and proton abstraction takes place.
Slowest step in the reaction is the rate determining step.

In E1, formation of carbocation is the slowest step. Hence, it is the rate determining step.
$\text{Rate of reaction}\propto \text{stability of carbocation}$

Carbocation at bridge head carbon of bicyclic compound is highly unstable due to lack of planarity thus, compound (c) is the least reactive alkyl halide for $E1$ reaction.

Compound (a), (b) and (d) forms secondary carbocation which is relatively stable than the carbocation at bridge head carbon.