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In wittig reaction, the carbon oxygen double bond is converted to carbon carbon double bond in presence of phosphorus ylide and a base. Mixture o f cis trans isomer is often result when geometrical isomers possible. Phosphorus-stabilized carbanion an ylide bears no overall charge. But it has a negatively charged carbon atom bonded to a positively charged heteroatom. The phosphonium salt is treated with a strong base (usually butyllithium) to abstract a proton from the carbon atom bonded to phosphorus.

The phosphorus ylide has two resonance forms:

One with a double bond between carbon and phosphorus, and

Another with charges on carbon and phosphorus.

The doublebonded resonance form requires ten electrons in the valence shell of phosphorus, using a d orbital. The pi bond between carbon and phosphorus is weak, and the charged structure is the major contributor. The carbon atom actually bears a partial negative charge, balanced by a corresponding positive charge on phosphorus.