Question

The correct order of reactivity of following compounds in $S_{N}1$ reaction is :

a) $C_6{H}_{5}C{H}_{2}Br$

b) $C_6{H}_{5}CH\left(C_6{H}_5\right)Br$

c) $C_6{H}_{5}CH\left(C{H}_3\right)Br$

d) $C_6{H}_{5}C\left(C{H}_3\right)\left(C_6{H}_5\right)Br$

• A. $d>b>c>a$
• B. $d>c>b>a$
• C. $a>b>c>d$
• D. $a>c>d>b$

Answer 1

The correct option is A $d>b>c>a$

$S{N}^1$ mechanism proceeds through carbocation formation, as stability of carbocation increases, reaction favors $S{N}^1$ mechanism.Benzylic carbocation is most stable due to more resonating structures and allylic carbocations are more stable than aliphatic carbocation due to resonance so order of stability of carbocations is benzyl > allyl > $3^0>2^0>1^0$.
($3^0$ carbocation are more stable than others or stability order of carbocations is $3^0>2^0>1^0$ so order of alkyl halides towards $S{N}^1$ reaction is $3^0>2^0>1^0$.)
So order of reactivity for $S{N}^1$ mechanism is
benzyl > allyl > $3^0>2^0>1^0$.