Question

From the given alkyl halides, choose the correct order of rate of reactivity toward $S_{N}1$ reaction.

• A. (I) > (II) > (III)
• B. (III) > (I) > (II)
• C. (III) > (II) > (I)
• D. (II) > (I) > (III)

Answer 1

The correct option is A (I) > (II) > (III)

Compound which forms most stable carbocation will undergo $S_{N}1$ reaction at faster rate.

Carbocation are stabilised by +I effect and hyperconjugation effect.

$\text{Stability of carbocation}\propto \text{Number of}\alpha -\text{Hydrogen}$

Order of stability of carbacation is
Tertiary > Secondary > Primary

Here, compound (I) forms stable tertiary carbocation with 9 $\alpha -hydrogens$
Compound (II) forms secondary carbocation with 6 $\alpha -hydrogens$
Compound (III) forms primary carbocation with 3 $\alpha -hydrogens$

Therefore, the order of rate of reactivity of $S_{N}1$ reaction is
(I) > (II) > (III)