Arrange the following compounds in the decreasing order of reactivity towards S _ N 1 reaction.

Step-by-step solution

The reactivity of alkyl halides towards the S_ N 1 reaction depends on the stability of the carbocation intermediate formed after the departure of the leaving group (chloride ion). The more stable the carbocation, the faster the S_ N 1 reaction. Analyzing the structures: [A] is 2-chloropentane, which forms a secondary ( 2^ ) carbocation. [B] is 1-chloropentane, which forms a primary ( 1^ ) carbocation. [C] is 2-chloro-2-methylbutane, which forms a tertiary ( 3^ ) carbocation. [D] is 1-chloro-2-methylpropane (isobut