Organic Chemistry MCAT Practice Exam

The process of carbohydrate cyclization involves which of the following actions?

1. Formation of aldehydes

2. Attack of the anomeric carbon by the penultimate carbon

3. Mutarotation of the hydroxyl groups

4. Formation of polymers from monomers

The correct answer is: Attack of the anomeric carbon by the penultimate carbon

The process of carbohydrate cyclization involves the formation of a cyclic structure from a linear form of a carbohydrate. This typically occurs when a hydroxyl group on the penultimate carbon atom (the carbon next to the carbon that is the most functional, which is the one with the aldehyde or ketone group) attacks the carbonyl group (C=O) of the carbonyl carbon, leading to the formation of a ring structure. In this context, the anomeric carbon is the carbon that becomes a new stereocenter in the ring form; it was previously involved in the carbonyl group before the cyclization. Thus, the correct action described is indeed the attack of the anomeric carbon by the penultimate carbon, which is essential for forming the cyclic hemiacetal or hemiketal, depending on whether the original carbonyl was an aldehyde or a ketone. Other choices describe different processes or concepts associated with carbohydrates but do not directly explain the specific mechanism of cyclization. For instance, while the formation of aldehydes is related to the carbohydrate structure, it does not describe the cyclization process. Similarly, mutarotation refers to the change in optical rotation due to the interconversion between anomers, not the cyclization