Which of the following alkyl halides will undergo S_N1 reaction most readily?

(CH₃)₃ C—I

The answer is (4) \((CH_3)_3C–I\).

The SN1 reaction is a two-step process that involves the formation of a carbocation intermediate. The stability of the carbocation intermediate is important for the rate of the reaction. The carbocation intermediate in \((CH_3)_3C–I\) is the most stable because the iodine atom is the least electronegative of the halogens, so the \(C–I\) bond is the weakest. This means that the carbocation intermediate is formed more easily, and the reaction proceeds more quickly.

The other three alkyl halides, \((CH_3)_3C–Cl\), \((CH_3)_3C–Br\), and \((CH_3)_3C–F\), have carbocation intermediates that are less stable because the \(C–Cl\), \(C–Br\), and \(C–F\) bonds are stronger than the \(C–I\) bond. This means that these reactions proceed more slowly than the reaction with \((CH_3)_3C–I\).