Order of reactivities of alkyl halides depends upon \({\rm{C}} - {\rm{X}}\) bond length. As, on going down the group, the size of halogen atom increases. Therefore, \({\rm{C}} - {\rm{X}}\) bond becomes weaker.Hence, the order of reactivities of methyl halide in the formation of Grignard reagent is, \(\mathrm{CH}_{3} \mathrm{I}>\mathrm{CH}_{3} \mathrm{Br}>\mathrm{CH}_{3} \mathrm{Cl}\).
