Final answer:
An enolate ion adds to a carbonyl group through a nucleophilic addition reaction. This entails deprotonation of an aldehyde, ketone, or ester to form the enolate, which attacks the electrophilic carbonyl carbon and upon protonation, results in the creation of aldol or a ß-keto ester.
Step-by-step explanation:
When an enolate ion adds to a carbonyl group, it undergoes a nucleophilic addition reaction. This process begins with the formation of the enolate ion, typically from an aldehyde, ketone, or ester, by deprotonation with a base. The resulting enolate ion, being a strong nucleophile, can then attack an electrophilic carbonyl carbon in another molecule. Upon this nucleophilic attack, the electrons in the double bond between the oxygen and the carbon shift towards the oxygen, creating a negatively charged oxygen anion. This anion can then be neutralized by protonation, resulting in a new alcohol group and the formation of an aldol (in the case of aldehydes and ketones) or a ß-keto ester (in the case of esters). The newly formed aldol can undergo dehydration to yield an α,ß-unsaturated carbonyl compound, while the ß-keto ester typically remains stable.
The overall nucleophilic addition is influenced by reaction conditions. For example, in acidic conditions, the carbonyl oxygen is protonated, which makes the carbonyl carbon more electrophilic, facilitating the nucleophilic attack. However, in basic conditions, the nucleophile adds to the carbonyl carbon directly without the need for prior protonation.