Answer :
The order from fastest to slowest for the rates of the SN2 reactions of these alkyl bromides with CH3S-/DMSO is 2>3>1. Option C.
Primary alkyl halides undergo SN2 reactions faster than secondary and tertiary ones. Compound IV reacts faster in the SN1 reaction than in the SN2 reaction. This is due to the steric hindrance and difficult access of the nucleophile from the side opposite the leaving group.
The rate-limiting step in the reaction is the attack of the nucleophile on the substrate. Therefore, SN2 is easier to run against stronger nucleophiles. Since there are two molecules in the transition state, the reaction is a bimolecular reaction, indicated by the number 2 in the SN2 symbol. The kinetics of the SN2 reaction is first order within the substrate and first order within the nucleophile.
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