Wittig reaction: practice and mechanism
In the Wittig reaction the oxygen is swapped for CH₂: C=O becomes C=CH₂ and Ph₃P=O leaves.
Example
The phosphorus ylide attacks the carbonyl carbon; via an oxaphosphetane an alkene and triphenylphosphine oxide form. The oxygen is replaced by the CH₂ group: C=O becomes C=CH₂.
Mechanism step by step
- 1
The ylide Ph₃P⁺–CH₂⁻ attacks the carbonyl carbon with its carbon.
- 2
A four-membered ring with P and O forms (oxaphosphetane).
- 3
The ring breaks into the alkene and triphenylphosphine oxide; the very strong P=O bond drives the reaction.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- That would be a Grignard product (alcohol). The ylide does not transfer a methyl group but forms a C=C double bond via the oxaphosphetane.
Often confused with: Grignard reaction · Reduction with sodium borohydride
Frequently asked questions
How is the ylide made?
Triphenylphosphine and a haloalkane (SN2) give a phosphonium salt; a strong base (e.g. BuLi) deprotonates it.
What is the advantage over an elimination?
The double bond forms exactly where the C=O was; there are no regioisomers.