Which one of each of the following pairs (shown in the figure) undergoes the SN1 substitution reaction faster, and why? i) 2-chloro-2-methylpropane or 2-chloropentane ii) 3-chlorohexane or 1-chloropentanePair (i): first compoundPair (i): second compoundPair (ii): first compoundPair (ii): second compound
Solution
SN1 rate follows carbocation stability (3° > 2° > 1°). (i) (CH3)3CCl ionises to the \textbf{tertiary} carbocation (CH3)3C+ — far more stable than the secondary cation from 2-chloropentane ⇒ \textbf{2-chloro-2-methylpropane is faster}. (ii) 3-Chlorohexane gives a \textbf{secondary} carbocation; 1-chloropentane only a primary one ⇒ \textbf{3-chlorohexane is faster}.
