In which of the solvents below would the reaction shown take place at the fastest rate?

In which of the solvents below would the reaction shown take place at the fastest rate?

A. ethanol B. acetic acid C. dimethyl sulfoxide

D. water

Solution:

This is a substitution reaction since we replace Br with CN. The substrate is primary ( Br is attached to carbon which is attached to one carbon only). SN2 prefers a primary substrate. Therefore, this reaction is an SN2 reaction. SN2 reaction wants polar aprotic solvents such as: acetone, DMSO, DMF, and ethers. Polar aprotic solvents do not have acidic hydrogens. Dimethyl sulfide is the only polar aprotic solvent, choice C.