Final answer:
When a carbonyl compound is treated with a Wittig reagent, a nucleophilic addition reaction occurs that forms a new carbon-carbon double bond and eliminates the oxygen atom from the carbonyl group.
Step-by-step explanation:
When a carbonyl compound is treated with a Wittig reagent, a nucleophilic addition reaction occurs. The Wittig reagent, which is an organophosphorus compound, reacts with the carbonyl compound to form a reactive intermediate called a ylide.
The ylide then undergoes a [2+2] cycloaddition with the carbonyl compound, resulting in the formation of a new carbon-carbon double bond and the elimination of the oxygen atom from the carbonyl group.