In an epoxide ring opening under basic conditions, which carbon is attacked?

• A. The carbon adjacent to the oxygen.
• B. The less substituted carbon.
• C. The more substituted carbon.
• D. It does not depend on substitution.

Answer: (B)

Under basic conditions, epoxide opening proceeds by an SN2-like backside attack on the less substituted carbon of the ring. The nucleophile is more able to approach and push electrons onto the ring oxygen when there is less steric hindrance, so the attack happens at the less substituted carbon. As the bond to oxygen breaks, the oxygen ends up as an alkoxide on the more substituted carbon; upon protonation, that oxygen becomes an OH, giving the hydroxyl on the more substituted carbon and the nucleophile on the less substituted carbon. This contrast with acidic opening, where the more substituted carbon often bears the developing positive character and is the preferred site of attack. So, the best choice is the less substituted carbon.