The Wittig reaction converts aldehydes or ketones to alkenes using phosphorus ylides. The reaction occurs via a nucleophilic addition‒elimination process generating a new C=C bond in the product. Nucleophilic addition begins with the reaction between phosphorus ylide and the carbonyl compound. Phosphorus ylide is a strong nucleophile that attacks the electrophilic carbonyl group to generate a charge-separated dipolar intermediate — a betaine. In the intermediate, the negatively charged oxygen atom and the positively charged phosphorus atom undergo a ring-closure reaction to give the four-membered oxaphosphetane ring. In some instances, the oxaphosphetane intermediate is formed via a concerted [2 + 2] cycloaddition between the Wittig reagent and the carbonyl compound. The unstable oxaphosphetane ring undergoes fragmentation to yield the desired alkene along with the stable by-product triphenylphosphine oxide. The formation of the strong P=O bond in the by-product is the driving force for the Wittig reaction.