Illustrated Glossary of Organic Chemistry

Wittig reaction: A reaction in which a phosphonium ylide reacts with an aldehyde or ketone to produce an alkene.

Shown below is a typical Wittig reaction sequence, with mechanism. Some atoms have been colored to show their pathway from reactants to products.

SN2 reaction of triphenylphosphine (Ph3P) with iodomethane (CH3I) produces methyl triphenylphosphonium iodide (Ph3PCH3+  I-), a phosphonium salt.

Phosphonium salt deprotonation by butyllithium (CH3CH2CH2CH2Li) forms methyl triphenylphosphonium ylide (Ph3P=CH2) and butane (CH3CH2CH2CH3).

The phosphonium ylide adds to benzaldehyde to produce styrene (PhCH=CH2), along with triphenylphosphine oxide (Ph3P=O) as a by-product. The four-membered ring intermediate is an oxaphosphetane.