Phosphorus ylides are used in the Wittig reaction to convert aldehydes and ketones to alkenes. In the process a new carbon-carbon bond is formed. To prepare the triphenylphosphonium ylide reagent, an SN2 reaction is utilized between triphenylphosphine and an alkyl halide (followed by deprotonation). The ylide (an ylide is a dipolar substance with adjacent opposite charges) is then made to undergo a reaction with an aldehyde or ketone that is somewhat analogous to the reaction of aldehydes or ketones with Grignard reagents. The ylide carbanion electrons lead to bond formation with the positive carbon pole of the carbonyl group. A betaine is formed, which is unstable (a betaine is a dipolar substance with nonadjacent opposite charges.) Electron pair movement continues as oxygen departs its bond with carbon to bond with phosphorus. The two carbons of interest now possess a double bond between them. (Carbon-carbon bond forming reactions are of particular importance for organic synthesis. Other important reactions that form carbon-carbon bonds include the use of acetylide anion as an SN2 nucleophile, Grignard reagents, Gilman reagents, and the aldol & Claisen condensations.)
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