Arrange the following alkyl bromides in decreasing order of their reactivity towards nucleophilic substitution with alcoholic AgCN : (I) CH ₃- CH ₂- Br (II) (III)

Step-by-step solution

The reaction of alkyl halides with alcoholic AgCN predominantly follows the S_N1 mechanism. This is because the Ag ^+ ion acts as a Lewis acid, coordinating with the leaving halogen atom and facilitating the cleavage of the C-Br bond to form a carbocation intermediate (along with the precipitation of AgBr ). The reactivity of an alkyl halide in an S_N1 reaction is directly proportional to the stability of the carbocation formed. The carbocations generated from the given alkyl bromides are: (I) CH₃-CH₂-Br CH₃-CH₂^+