Organic Chemistry MCAT Practice Exam

What reaction occurs at the anomeric carbon that affects the alpha/beta configuration?

1. SN1 reaction

2. SN2 reaction

3. Electrophilic addition reaction

4. Nucleophilic elimination reaction

The correct answer is: SN2 reaction

The reaction that occurs at the anomeric carbon, which affects the alpha/beta configuration in carbohydrate chemistry, involves the formation of either an alpha or beta anomer when a cyclic sugar is in equilibrium. This transformation typically involves the conversion of the open-chain aldehyde or ketone form to the cyclic hemiacetal or hemiketal form, where the anomeric carbon is the carbon that was the carbonyl carbon in the open chain. In this context, the SN2 reaction is relevant because it describes a type of nucleophilic substitution mechanism that can invert the configuration at the anomeric carbon. When a nucleophile attacks the anomeric carbon, it can lead to the formation of a new bond while displacing a leaving group, resulting in the conversion between the alpha (with the hydroxyl group on the opposite side of the CH2OH group) and beta (with the hydroxyl group on the same side as the CH2OH group) forms of the sugar. This mechanism is particularly fundamental in glycosylation reactions and other transformations in carbohydrate chemistry. The other types of reactions mentioned (SN1, electrophilic addition, and nucleophilic elimination) do not correctly represent the mechanism that directly affects the anomeric