Bondline / Reactions

Wittig reaction

A phosphorus ylide adds to an aldehyde or ketone; a four-membered oxaphosphetane forms and falls apart into an alkene and a very strong P=O bond. Drawn here with Me₃P instead of Ph₃P to keep it small.

Carbonyl addition · 3 steps

Conditions

Ph₃P=CHR (made from a phosphonium salt and a strong base such as BuLi), THF.

Mechanism in 3 steps

Red curly arrows show where each pair of electrons moves.

P+OP+O−+
Step 1The ylide carbon attacks the carbonyl carbon.
P+O−PO
Step 2The oxygen anion bonds to phosphorus, closing a four-membered ring (oxaphosphetane).
POPO+
Step 3The ring breaks apart: a C=C and a P=O form. The strong P=O bond drives the reaction.

About carbonyl addition reactions

The carbon of a C=O is δ+, so nucleophiles attack it and the π electrons move onto oxygen. Weak nucleophiles often need acid to activate the carbonyl first.

Practise this mechanismDraw the curly arrows yourself and Bondline checks each one. Or add it to your notes and watch it in 3D.

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