Question

Following is the list of four halides. Select correct sequence of decreasing order of reactivity for $S_{N}1$ reaction using the codes given below.

I) $C_6{H}_5-\underset{C{H}_3}{\underset{|}{C}H}-Br$ II) $C_6{H}_5-C{H}_2-Br$

III) $C_6{H}_5-\underset{C{H}_3}{\underset{|}{C}H}-I$ IV) $C_6{H}_5-C{H}_2-I$

• A. III > I > IV > II
• B. III > I > II > IV
• C. I > III > IV > II
• D. I > III > II > IV

Answer 1

The correct option is A III > I > IV > II

$S{N}^1$ mechanism proceeds through carbocation formation, as the stability of carbocation increases, reaction favors $S{N}^1$ mechanism.

The benzylic carbocation is most stable due to more resonating structures and allylic carbocations are more stable than aliphatic carbocation due to resonance so the 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 > $3^0>2^0>1^0$.

Order of alkyl halide towards $S{N}^1$ mechanism is R-I > R-Br > R-Cl > R-F.