What is the mechanism of reaction of aldehydes and ketones?

• A. Nucleophilic addition
• B. Electrophilic substitution
• C. Radical substitution
• D. Electrophilic addition

Answer: (A)

Aldehydes and ketones react by nucleophilic addition to the carbonyl carbon. The C=O bond is highly polarized, with the carbon atom partially positive, so a nucleophile can attack it, forming a new bond to carbon while pushing the C=O electrons onto oxygen to give an alkoxide intermediate. After protonation (or subsequent reaction), you get the final product, such as an alcohol after reduction or a hemiacetal/acetal depending on the nucleophile.

This fits because the carbonyl carbon is the electrophilic site and the reaction proceeds through two-electron addition rather than substitution. It isn’t electrophilic substitution (which involves replacing a substituent on a saturated or aromatic system), nor radical substitution (which follows radical pathways), and while the outcome may involve addition of species like water or alcohols, the underlying mechanism is nucleophilic addition to the carbonyl carbon.