Rank the following alkyl bromides in order of decreasing reactivity (from fastest to slowest) as a substrate in an S_(N)2 reaction.

Rank the following alkyl bromides in order of decreasing reactivity (f

1.	Rank the following alkyl bromides in order of decreasing reactivity (from fastest to slowest) as a substrate in an S_(N)2 reaction.
Answer» Solution :We examine the CARBON bearing the leaving group in each instance to ASSESS the steric hindrance to an `S_(n)2`reaction at that carbon. In C it is `3^(@)`, therefore, three groups would HINDER the approach of a nucleophile, so this alkyl bromide would react most slowly. In the carbon bearing the leaving group is `2^(@)` (two groups hinder the approach of the nucleophile), while in both A and B it is `1^(@)` (one group hin ders the nucleophile.s approach). Therefore, D would react faster than C, but slower than either A or B. But, what about A and B? They are both `1^(@)`alkyl bromides, but B has a methyl group on the carbon adjacent to the one bearing the bromine, which would provide steric hindrance to the approaching nucleophile that would not be present in A. The order of reactivity, therefore, is `A gt B gt Dgt gt C`.