Question

In the following pair of halogen compounds, which compound undergoes a faster S_N1 reaction?

In the following pair of halogen compounds, which compound undergoes S_N1 reaction faster and why?

Answer 1

The activity of halogen compounds in S_N1 reaction depends on the stability of the carbocation formed due to ionization. The order of stability is tertiary > secondary > primary. Hence, 3° alkyl chloride is more active than 2° alkyl chloride. Hence, 2° alkyl chloride is more active in S_N1 reaction.

will react faster. The carbocation  will be more stable and hence the reaction will be faster.

Answer 2

An S_N1 reaction occurs through the formation of a carbocation intermediate. Alkyl halide (I) is tertiary (3°), whereas (II) is secondary (2°). As a result, (I) forms a tertiary carbocation, while (II) forms a secondary carbocation. Since the rate of an S_N1 reaction increases with carbocation stability, and a tertiary carbocation is more stable than a secondary carbocation, compound (I), i.e., 2-chloro-2-methylpropane, undergoes the S_N1 reaction faster than compound (II), i.e., 3-chloropentane.