Organic Chemistry MCAT Practice Exam

What is the role of alcohol in the formation of hemiacetals and acetals?

1. Acts as a nucleophile

2. Acts as an electrophile

3. Acts as a reducing agent

4. Acts as a leaving group

The correct answer is: Acts as a nucleophile

In the formation of hemiacetals and acetals, alcohol primarily acts as a nucleophile. The reaction involves the nucleophilic attack of the alcohol on the electrophilic carbon atom of an aldehyde or ketone. This carbonyl carbon is electron-deficient due to the polarization of the carbon-oxygen double bond, making it susceptible to nucleophilic attack. When the alcohol (the nucleophile) approaches the carbonyl carbon, it donates a pair of electrons to form a bond, resulting in the formation of a hemiacetal when one equivalent of alcohol reacts with an aldehyde or ketone. If the reaction continues with the addition of an additional equivalent of alcohol, a further reaction occurs, leading to the formation of an acetal. This transformation is an equilibrium process, and the role of alcohol as a nucleophile is critical throughout. In contrast, other designated roles such as acting as an electrophile, reducing agent, or leaving group do not accurately describe the function of alcohol in this context.