The anions $Br - (CH X_{2}) X_{n} - COO X^{-}$ cyclise to lactones by intramolecular SN2 attack of the carboxylate oxygen on the C-Br carbon. Under identical conditions in water, which anion cyclises fastest?

A $n = 5$ (6-bromohexanoate, forming a seven-membered lactone)
B $n = 3$ (4-bromobutanoate, forming a five-membered lactone)
C $n = 2$ (3-bromopropanoate, forming a four-membered lactone)
D $n = 1$ (bromoethanoate, forming a three-membered lactone)

Answer: B

Ring-closure rates depend on ring strain and on the entropy cost of freezing rotors. Five-membered ring formation combines negligible strain with a small entropy penalty, so

n = 3

is fastest. Three- and four-membered rings are strained, and the seven-membered ring suffers from larger entropy loss and transannular strain, so those are slower.

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

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The anions Br−(CHX2​)Xn​−COOX− cyclise to lactones by intramolecular SN2 attack of the carboxylate oxygen on the C-Br carbon. Under identical conditions in water, which anion cyclises fastest?

Answer: B

More questions on Neighbouring group participation

The anions $Br - (CH X_{2}) X_{n} - COO X^{-}$ cyclise to lactones by intramolecular SN2 attack of the carboxylate oxygen on the C-Br carbon. Under identical conditions in water, which anion cyclises fastest?