Wittig Reaction

Animated mechanism of the Wittig reaction between acetaldehyde and a phosphonium ylide.

1. (Not shown in animation) The phosphonium ylide is formed from the SN2 reaction of triphenylphosphine and bromoethane, followed by deprotonation by strong base. The ylide contains a nucleophilic carbanion stabilized by the adjacent phosphorus.
2. The ylide and aldehyde undergo a concerted [2+2] cycloaddition, forming a four-membered oxaphosphetane ring intermediate.
3. The oxaphosphetane decomposes to yield the alkene product (in this case, 2-butene), driven by the formation of the strong P=O bond in triphenylphosphine oxide.