Organic Chemistry MCAT Practice Exam

In which type of solvent does SN2 reactions typically occur best?

1. Polar protic solvents

2. Apolar solvents

3. Polar aprotic solvents

4. Non-polar solvents

The correct answer is: Polar aprotic solvents

In SN2 reactions, the best solvent type is polar aprotic solvents. This is because SN2 mechanisms involve a nucleophile attacking an electrophile to displace another group, often in a single concerted step. Polar aprotic solvents support this process as they can solvate cations well while leaving anions (like nucleophiles) relatively free and less hindered. In polar aprotic solvents, the solvent molecules do not form strong hydrogen bonds with the nucleophile due to the lack of hydrogen atoms attached to electronegative atoms. As a result, the nucleophile remains highly reactive, facilitating a more effective attack on the electrophile. This combination of enhanced nucleophilic strength with an efficient transition state leads to faster reaction rates. On the other hand, polar protic solvents stabilize both the nucleophile and the leaving group through strong hydrogen bonding, which can hinder the nucleophile's reactivity. Apolar solvents do not provide polarity necessary for solvation of ionic species, leading to poor reaction kinetics, while non-polar solvents lack the capability to stabilize charged species that are central to SN2 mechanisms. Consequently, the unique properties of polar aprotic solvents make them ideal for SN2 reactions.