Which of the following is the double substitution product of the reaction below?
Solution: This is an intramolecular reaction. There are two leaving groups, bromines, on the substrate. The nucleophile is Na2S or S2- (Na+ is a spectator ion). First, sulfur will attack and replace one of the bromines in an SN2 reaction and then it will attack and replace the second bromine closing the ring.
There are 5 carbons going from the carbon with one bromine to the other. This eliminates choices C and D since these have 4 carbons attached to sulfur only.
SN2 reaction results in 1 product only with inverse stereochemistry. This eliminates choice B since this would be a product for an SN1 reaction (racemic mixture, two products). Choice A is the correct answer.
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