What is the increasing order of reactivity of the following compounds towards $S_N1$ reactions?

(A). $C_6H_5CH_2Br$
(B). $C_6H_5CH(C_6H_5)Br$
(C). $C_6H_5C(CH_3)(C_6H_5)Br$
(D). $C_6H_5CH(CH_3)Br$

Choose the correct answer from the options given below:

(A), (D), (B), (C)

The correct answer is Option (4) → (A), (D), (B), (C)

For SN1 reactions, the rate depends on the stability of the carbocation formed.

Let’s examine each compound:

• (A) C₆H₅CH₂Br → Primary benzylic carbocation
• (D) C₆H₅CH(CH₃)Br → Secondary benzylic carbocation
• (B) C₆H₅CH(C₆H₅)Br → Diphenylmethyl carbocation (stabilized by two phenyl rings)
• (C) C₆H₅C(CH₃)(C₆H₅)Br → Tertiary benzylic carbocation (most stable: resonance + hyperconjugation)

Stability order of carbocations:

Primary benzylic < Secondary benzylic < Diphenylmethyl < Tertiary benzylic

So, increasing order of SN1 reactivity is: (A) < (D) < (B) < (C)