Question

Which is the incorrect order of relative reactivity of Alkyl halide?

• A. $S_{N}1\to$ tertiary > secondary > primary
• B. $E1\to$ tertiary > secondary > primary
• C. $E2\to$ primary > secondary > tertiary
• D. $S_{N}2\to$ primary > secondary > tertiary

Answer 1

The correct option is C $E2\to$ primary > secondary > tertiary

(a). The reactivity order of alkyl halide for $S_{N}1$ reaction is depends on the stability of carbocation formed because the reaction goes through the carbocation intermediate.
Order of stability of carbocation is,
tertiary > secondary > primary
Therefore, the relative reactivity of alkyl halide for $S_{N}1$ is,
tertiary > secondary > primary

(b). The reactivity of alkyl halide for E1 reaction is based on the stability of alkene formed. Tertiary alkyl halide form more stable alkene followed by secondary and at last primary.
So the correct order of reactivity for $E1$ reaction is,
tertiary > secondary > primary

(c). The reactivity of alkyl halide for E2 reaction is based on the stability of alkene formed. Tertiary alkyl halide form more stable alkene followed by secondary and at last primary.
So the correct order of reactivity for $E2$ reaction is,
tertiary > secondary > primary

(d). The $S_{N}2$ reaction proceeds through transition state where the nucleophile will attact the akly halide at back position. Hence, less steric alkyl halide will undergo $S_{N}2$ reaction fastly. Therefore, the relative reactivity of alkyl halide for $S_{N}2$ is,
primary > secondary > tertiary

Thus, option (c) is the incorrect order.