The rate of this reaction is markedly increased if a small amount of sodium iodide is added to the reaction mixture. The sodium iodide is not consumed by the reaction and is therefore considered to be a catalyst. Explain how the presence of iodide can speed up the rate of the reaction.

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Explanation:

SN2 reaction is a synchronous reaction in which the leaving group departs as the nucleophile is being attached to the substrate in a single step. This makes it imperative to use a good leaving group when carrying out an SN2 reaction such as the one shown in the question.

The addition of a small amount of NaI acts as a sort of external nucleophile which assists the departure of the leaving group thereby enabling the reaction to proceed faster. This occurs because the iodide ion is very good leaving group.

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