Question

Which compound in each of the following pairs will react faster in S_N2 reaction with OH⁻?

(i) CH₃Br or CH₃I

(ii) (CH₃)₃CCl or CH₃Cl

Answer 1

(i) In the S_N2 mechanism, the reactivity of halides for the same alkyl group increases in the order. This happens because as the size increases, the halide ion becomes a better leaving group.

R−F << R−Cl < R−Br < R−I

Therefore, CH₃I will react faster than CH₃Br in S_N2 reactions with OH⁻.

(ii)

The S_N2 mechanism involves the attack of the nucleophile at the atom bearing the leaving group. But, in case of (CH₃)₃CCl, the attack of the nucleophile at the carbon atom is hindered because of the presence of bulky substituents on that carbon atom bearing the leaving group. On the other hand, there are no bulky substituents on the carbon atom bearing the leaving group in CH₃Cl. Hence, CH₃Cl reacts faster than (CH₃)₃CCl in S_N2 reaction with OH⁻.