Question

The most reactive towards $S{N_1}$ is :
( A ) $PhC{H_2}Cl$
( B ) $PhCl$
( C )$PhCHCl(C{H_3})$
( D )$P - N{O_2}{C_6}{H_4}C{H_2}Cl$

Answer 1

Order of reactivity towards $S{N_1}$ reaction depends on the formation of intermediate carbocation and another factor is degree of carbocation formed.

Complete step-by-step answer: $S{N_1}$reaction is the nucleophilic substitution reaction . They are the unimolecular reaction because the rate of $S{N_1}$ reaction depends only on the concentration of one reactant.
Order of reactivity of $S{N_1}$ nucleophilic substitution reaction depends on the degree of carbocation i.e; $3^\circ > 2^\circ > 1^\circ$.
Let's check the above given options :
( A ) $PhC{H_2}Cl$ $\to$
The carbocation of this structure is $1^\circ$ . As we know that $1^\circ$ carbocation is less reactive towards $S{N_1}$ nucleophilic reaction .
( B ) $PhCl$ $\to$
The carbocation of this structure is $2^\circ$. So as we know that $2^\circ$ carbocation is more reactive towards $S{N_1}$ nucleophilic reaction than the $1^\circ$ carbocation .
( C ) $PhCHCl(C{H_3})$ $\to$
The carbocation of this structure is $2^\circ$ . So $2^\circ$ carbocation is more reactive towards $S{N_1}$ nucleophilic reaction than the $1^\circ$ carbocation . The presence of methyl $- C{H_3}$ group also increases the positive charge present on the central carbon atom .
( D ) $p - N{O_2}{C_6}{H_4}C{H_2}Cl$ $\to$
The carbocation of this structure is $1^\circ$. As we know that $1^\circ$ carbocation is least reactive towards $S{N_1}$ nucleophilic substitution reaction . The presence of an electron withdrawing group at the para position of the phenyl group also decreases the positive charge on carbocation .

So, from the above explanations we can conclude that the CORRECT answer of the above question is OPTION ( C ) .

Note: We have to remember that the order of reactivity of $S{N_1}$ nucleophilic substitution reaction depends on the degree of carbocation . Also presence of electron withdrawing group and electron donating group contributes in the reactivity towards $S{N_1}$ nucleophilic substitution reaction .