Question

As an SN2 solvent for reactions like the alkylation of acetaminophen, water would be a polar selection. Water is more polar that 2-butanone or DMSO. Why are polar, aprotic solvents better that polar, protic solvents (like water) for SN2 reactions

Answer 1

Answer: polar protic solvents solvate the nucleophile necessary for attack on the substrate in SN2 substitution.

Explanation:

Aprotic solvents are solvents that lack protons such as dimethyl sulphoxide (DMSO). DMSO has no exposed positive end. The positive end is buried inside the molecular structure. As a result of this, the nucleophile is not solvated. If the nucleophile is solvated, the rate of SN2 reaction will reduce drastically because the nucleophile becomes unavailable to attack the substrate. This solvation normally occur in polar protic solvents such as water because of the exposed positive end of the molecule which interacts with the nucleophile thereby reducing the rate of SN2 reaction.