Organic Chemistry Relative Rate(s)

Question

Predict the relative rates of reaction for 1-chlorobutane and 1-bromobutane with sodium iodide.

Is 1-bromobutane faster then 1-chlorobutane or is 1-chlorobutane faster then 1-bromobutane?

Answer 1

This question is testing your understanding of relative rates in SN2 reactions. Remember, SN2 reactions involve the attack of a nucleophile which kicks off a leaving group on your electrophile. The faster rate will be for the reaction with the electrophile bearing the better leaving group - either bromine or chlorine.

One factor that contributes to leaving group stability is size. In this case, bromine is a bigger atom than chlorine. When the nucleophile (iodide in sodium iodide) kicks it off, bromine's large size helps it stabilize the negative charge it gets. So this means that bromine is more willing to jump off than chlorine, making the reaction with 1-bromobutane faster.