Question 12.3: Predict whether each of the following substitution reactions...
Predict whether each of the following substitution reactions is likely to be S_{N}1 or S_{N}2:
Strategy
Look at the substrate, leaving group, nucleophile, and solvent. Then decide from the summaries at the ends of Sections 12-7 and 12-9 whether an S_{N}1 or an S_{N}2 reaction is favored. S_{N}1 reactions are favored by tertiary, allylic, or benzylic substrates, by good leaving groups, by nonbasic nucleophiles, and by protic solvents. S_{N}2 reactions are favored by primary substrates, by good leaving groups, by good nucleophiles, and by polar aprotic solvents.
(a) This is likely to be an S_{N}1 reaction because the substrate is secondary and benzylic, the nucleophile is weakly basic, and the solvent is protic.
(b) This is likely to be an S_{N}2 reaction because the substrate is primary, the nucleophile is a reasonably good one, and the solvent is polar aprotic.