Wittig reaction: alkene formation

Wittig Reaction Definition:

The Wittig reaction is a useful way of forming alkenes from phosphorus ylides (Wittig reagents) and aldehydes or ketones.

Wittig Reaction Explanation:

The chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide yielding alkene, and triphenylphosphine oxide.

phosphorus ylide or Wittig reagent is a suitable nucleophile because new carbon-carbon bond can be made in this way. Phosphorus ylides are prepared from an alkyl halide and triphenylphosphine such that triphenylphosphine performs a nucleophilic attack on the alkyl halide via the SN2 reaction followed by deprotonation with a strong base.

The mechanism of this reaction consists of two steps. In the first step, the negatively polarized carbon in the ylide functions as a nucleophile and attacks the electrophilic carbon of carbonyl group. The oxygen from carbonyl group immediately attacks phosphorus from Wittig reagent which is positively charged. This simultaneously double addition is called [2+2] cycloaddition, and the product called an oxaphosphetane or oxaphosphacyclobutane. This intermediate then in the second step undergoes fragmentation to give the alkene product.

Close Menu


Are you ready for your next Ochem Exam?