The Wittig reaction uses phosphorus ylides [2] to transform aldehydes and ketones [1] into alkenes [3] . Triphenylphosphine oxide is obtained as a by-product [4].

In the mechanism of the reaction, the ylide and the carbonyl combine to form an oxaphosphetane that breaks, leaving the final alkene free.

Stage 1. Ethanal and ylide combine to form phosphethane.


Stage 2. The phosphethane breaks down to form the alkene and triphenylphosphine oxide.


{nota}Example - Obtain 2-methylbut-2-ene using Wittig.

The alkene is broken at the double bond and the group circled in green is added to each carbon.{/nota}

Phosphorus ylides are prepared by reaction of haloalkanes and triphenylphosphine, followed by deprotonation of carbon with strong base (lithium organometallics).