(i) Between CH3Br and CH3I, CH3I will react faster via the SN2 mechanism. In SN2 mechanism, C – X bond breaks and the faster it breaks faster is the reaction.
I– is a better leaving group. Owing to its large size, the C – I bond breaks faster than the C – Br bond and reaction proceeds further at a greater rate.
(ii) The order of reactivity in an SN2 reaction depends on minimal steric hindrance around the carbon involved in the C – X bond. Lesser the steric hindrance felt by the incoming nucleophile,more reactive will be the alkyl halide towards SN2 reaction.
Based on this, CH3Cl will react faster than (CH3)3CCl.