Question

The order of reactivity of halides towards ${S_N}1$ mechanism is :
A.$benzyl > allyl > 1^\circ > 2^\circ > 3^\circ > Me$
B.$Me > 1^\circ > 2^\circ > 3^\circ > allyl > benzyl$
C.$3^\circ > 2^\circ > 1^\circ > Me > allyl > benzyl$
D.$Benzyl > allyl > 3^\circ > 2^\circ > 1^\circ > Me$

Answer 1

The reactivity of halides towards${S_N}1$ reaction depends on the carbocation stability. More stable is the carbocation faster the nucleophile will attack the carbocation to proceed the reaction.

Complete step by step answer:
The nucleophilic substitution reaction is a type of reaction where a nucleophile attacks the positive charge carbocation and gets attached to it to form the product.
In this reaction one nucleophile displaces another nucleophile.
The reaction is illustrated below.
$R - LG + N{u^ - } \to R - Nu + L{G^ - }$
Where
R is the alkyl group
LG is the leaving group (less)
$N{u^ - }$ is the nucleophile (strong)
The nucleophilic substitution reaction is divided into two forms
(1) ${S_N}1$ reaction: This reaction is unimolecular which means the reaction rate depends on only single reactant species.
(2) ${S_N}2$ reaction: This reaction is bimolecular which means the reaction rate depends on both the reactant species.
The reactivity of halides towards the ${S_N}1$ mechanism depends on the stability of the carbocation.
The stability of carbocation increases with the main three factors:
(1) The stability increases with an increase in the number of carbon adjacent to the carbocation.
The order of stability is
Tertiary > secondary > primary
(2) The stability of the carbocation increases with the delocalization of pi electrons through resonance.
(3) The stability of carbocation also increases with atoms carrying lone pairs that are adjacent to the carbocation.

Thus, the order of reactivity of halides towards ${S_N}1$ mechanism is :
$Benzyl > allyl > 3^\circ > 2^\circ > 1^\circ > Me$

Hence, option D is correct .

Note:
Any atom bearing positive charge or negative charge is stable as long the charge is delocalized. The tertiary carbocation requires less energy for its formation as compared to secondary carbocation and primary carbocation.