Wittig Reaction
Definition and meaning of Wittig Reaction in chemistry.
The Wittig reaction converts an aldehyde or ketone into an alkene by reacting it with a phosphorus ylide. It forms a new carbon-carbon double bond in a predictable location and produces triphenylphosphine oxide as a byproduct.
In more detail
The reaction is a reliable way to build alkenes from carbonyl compounds. A phosphorus ylide, sometimes called a Wittig reagent, is a molecule that has a carbon atom carrying a negative charge next to a positively charged phosphorus atom. Chemists make the ylide by treating a triphenylphosphonium salt with a strong base, which removes a proton from the carbon next to phosphorus.
When the ylide meets an aldehyde or ketone, its negatively charged carbon attacks the carbonyl carbon. The two molecules join and pass through a four-membered ring called an oxaphosphetane. This ring then breaks apart, sending the oxygen to phosphorus and forming the new C=C double bond.
The strong phosphorus-oxygen bond in triphenylphosphine oxide is a major driving force for the reaction. A useful feature of the Wittig reaction is that the double bond always appears exactly where the old carbonyl carbon was. This makes the product easy to predict, unlike some elimination reactions that can place the double bond in more than one spot.
The carbonyl oxygen is replaced by the carbon that came from the ylide. The stability of the ylide often controls the shape of the product. Unstabilized ylides, which lack electron-withdrawing groups, tend to favor the cis (Z) alkene.
Stabilized ylides, which carry groups such as an ester next to the charged carbon, usually favor the trans (E) alkene. Because it is dependable and works on many substrates, the Wittig reaction is widely used to make everything from simple lab products to complex molecules like vitamin A.
Key facts
| Field | Organic Chemistry |
|---|---|
| Reaction type | carbonyl to alkene |
| Reagent | phosphorus ylide |
| Byproduct | triphenylphosphine oxide |
| Key intermediate | oxaphosphetane |
| Driving force | strong P=O bond |
| Stereochemistry | ylide stability controls E or Z |
| Named for | Georg Wittig |
Treating benzaldehyde with the ylide made from methyltriphenylphosphonium bromide and a strong base gives styrene (phenylethene). The carbonyl oxygen leaves as triphenylphosphine oxide, and a new C=C double bond forms where the aldehyde group used to be.
Frequently asked questions
What is a phosphorus ylide?
It is a molecule with a negatively charged carbon directly bonded to a positively charged phosphorus atom. The carbon acts as a nucleophile and attacks the carbonyl carbon of an aldehyde or ketone.
Where does the new double bond form?
Exactly at the carbon that used to be the carbonyl carbon. This predictable placement is a main reason the Wittig reaction is so useful in synthesis.
Does the Wittig reaction work on carboxylic acids or esters?
No, it is designed for aldehydes and ketones. Esters and acids are much less reactive toward ordinary Wittig reagents.
How is the E or Z product controlled?
Unstabilized ylides tend to give the cis (Z) alkene, while stabilized ylides bearing electron-withdrawing groups tend to give the trans (E) alkene.