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.

S_N2 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.