Which compound in the given pair would undergo S~2 reaction at a…

Which compound in the given pair would undergo S~2 reaction at a faster rate and why?
CH3 – CH2 – I and CH3 – CH2 – Br

Ans.  In the S² (bimolecular nucleophilic substitution) reaction, the rate is largely dependent on the stability of the transition state, which in turn is influenced by the strength of the carbon-halogen bond being broken.

The rate of an S² reaction is generally faster when the carbon-halogen bond is weaker. The carbon-halogen bond strength decreases in the order of C-F > C-Cl > C-Br > C-I due to the decrease in electronegativity of the halogen atom.

Comparing CH₃–CH₂–I and CH₃–CH₂–Br, the carbon-iodine bond in CH₃–CH₂–I is weaker than the carbon-bromine bond in CH₃–CH₂–Br. This is because iodine is larger and less electronegative than bromine, leading to a weaker bond.

Therefore, CH₃–CH₂–I would undergo an S² reaction at a faster rate compared to CH₃–CH₂–Br.