Which statement best describes an SN2 reaction?

Which statement best describes an SN2 reaction?

• A. It proceeds via a concerted mechanism with backside attack and inversion of configuration.
• B. It proceeds via a carbocation intermediate (SN1).
• C. It proceeds via a radical-chain mechanism.
• D. It is a purely elimination process.

Answer: (A)

SN2 reactions happen in one concerted step where the nucleophile attacks the carbon bearing the leaving group from the backside as the leaving group leaves. This simultaneous bond-making and bond-breaking causes inversion of configuration at a stereocenter because the approach is from the side opposite the leaving group. The rate depends on both the nucleophile and the substrate, so it’s second-order overall. Since there’s no discrete carbocation formed, SN2 is hindered by crowding at the carbon (methyl and primary substrates react fastest; tertiary are very slow). The other statements describe different pathways—SN1 goes through a carbocation, radical-chain mechanisms aren’t the SN2 pathway, and purely eliminative routes describe E2/E1 processes rather than substitution.

SN2 reactions happen in one concerted step where the nucleophile attacks the carbon bearing the leaving group from the backside as the leaving group leaves. This simultaneous bond-making and bond-breaking causes inversion of configuration at a stereocenter because the approach is from the side opposite the leaving group. The rate depends on both the nucleophile and the substrate, so it’s second-order overall. Since there’s no discrete carbocation formed, SN2 is hindered by crowding at the carbon (methyl and primary substrates react fastest; tertiary are very slow). The other statements describe different pathways—SN1 goes through a carbocation, radical-chain mechanisms aren’t the SN2 pathway, and purely eliminative routes describe E2/E1 processes rather than substitution.