Which of the following chlorides is expected to hydrolyse in water mainly by anchimeric assistance rather than by an ordinary SN2 attack of solvent?

A $CF X_{3} - CH X_{2} - Cl$
B $C X_{6} H X_{5} - S - CH X_{2} CH X_{2} - Cl$
C $(CH X_{3}) X_{2} CHCH X_{2} - Cl$
D $CH X_{3} CH X_{2} CH X_{2} CH X_{2} - Cl$

Answer: B

Only

C X_{6} H X_{5} - S - CH X_{2} CH X_{2} - Cl

has a donor atom (sulfur) positioned beta to the leaving group so that a three-membered episulfonium ion can form. The other three chlorides lack any such group; they react only by slow unassisted SN2 attack.

Practice Organic Reaction Mechanisms III: Nucleophilic Substitution More on Neighbouring group participation

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