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

(i) Since I^- ion is a better leaving group than Br^- ion, therefore, CH₃I reacts faster CH₃Br in S_N2 reaction with OH^- ion.

Note: Better the leaving group, faster is the S_N2 reaction.

(ii) On steric grounds, 1^◦ alkyl halides are more reactive than tert-alkyl halides in S_N2 reactions. Therefore, CH₃Cl will react at a faster rate than (CH₃)₃CCl in a S_N2 reaction with OH- ion.(bulkier the g8roup, slower is the S_N2 reaction.)