Which of these undergo SN2: chlorobenzene, methylbenzene, iodopentane, or chloropentane?

Answer 1

For SN2 reactions, you want an unhindered substrate. The reactivity of substrates for SN2 is CH₃ > 1° > 2° > 3°.

Due to the resonance stabilization of benzene, substituted benzenes do not undergo SN2 reactions, therefore, chlorobenzene and methylbenzene can be eliminated as the answers.

Between iodopentane and chloropentane, I am assuming they are 1-iodopentane and 1-chloropentane, respectively, making them both 1° alkyl halides. For alkyl halides, iodide is a better leaving group than chloride, therefore, 1-iodopentane is the best substrate for an SN2 reaction.