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Organic Chemistry 2 · Reaction Mechanism

The Wittig Reaction

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On this page 7 sections
  1. In 30 seconds
  2. Why this matters
  3. The college version
  4. Eli explains
  5. Worked example
  6. Key takeaway
  7. Study tools

In 30 seconds

The converts an aldehyde or ketone into an alkene by treating it with a (Ph3P=CR2). The ylide's carbon attacks the carbonyl carbon to form a four-membered , which collapses to the alkene and (Ph3P=O). The reaction installs a carbon–carbon double bond exactly where the C=O was, and the alkene geometry depends on whether the ylide is stabilized (E-selective) or unstabilized (Z-selective).

Why this matters

The Wittig reaction is a workhorse in the synthesis of pharmaceuticals, vitamins, and natural products because it builds carbon–carbon double bonds at defined positions and with predictable geometry. It has been used in industrial syntheses of β-carotene (a vitamin A precursor) and in the construction of retinoids and polyene antibiotics, where controlling the geometry of each double bond is essential for biological activity.

The college version

1. Phosphonium Salts and Ylide Formation

A is made by treating triphenylphosphine (Ph3P, a good nucleophile) with a primary or secondary alkyl halide in an Sₙ2 reaction: Ph3P + RCH2X → [Ph3P-CH2R]+ X−. Deprotonation of the acidic α-carbon of this salt with a strong base (e.g., butyllithium) generates the ylide, Ph3P=CHR. The ylide is best described by two resonance forms: the neutral ylene Ph3P=CHR and the charge-separated ylide Ph3P+-CH−R. The negative carbon of the second form is the nucleophilic center.

2. Carbonyl-to-Alkene Conversion

The ylide carbon attacks the carbonyl carbon, and after ring closure and collapse, the C=O becomes C=C. The product alkene is fully predictable: each former carbonyl carbon now bears the two substituents it carried, plus the two groups originally on the ylide carbon. The reaction works for both aldehydes and ketones and tolerates many functional groups, making it a cornerstone of alkene synthesis.

3. Stabilized vs. Unstabilized Ylides and E/Z Selectivity

Unstabilized ylides carry only alkyl or hydrogen groups on the ylide carbon. They are highly reactive and react under kinetic control to favor the Z (cis) alkene. Stabilized ylides carry an electron-withdrawing group (ester, ketone, aldehyde, or nitrile) conjugated with the ylide carbon; they are less reactive and react under thermodynamic control to favor the E (trans) alkene. This distinction is the key to predicting alkene geometry.

How it works

  1. Triphenylphosphine attacks an alkyl halide to make a phosphonium salt.
  2. A strong base deprotonates the α-carbon to form the ylide.
  3. The ylide carbon attacks the carbonyl carbon (π electrons go to oxygen).
  4. The oxyanion closes onto phosphorus to form the four-membered oxaphosphetane.
  5. The ring collapses, forming C=C and a strong P=O bond.
  6. The alkene and triphenylphosphine oxide are the products.
  7. Unstabilized ylides give Z (cis) alkenes; stabilized ylides give E (trans) alkenes.

Common confusions

Do not confuseWithDifference
YlideEnolateYlide is P=C−; enolate is C=C-O− — different heteroatoms
OxaphosphetaneEpoxideOxaphosphetane has P and C in a 4-ring; epoxide is a 3-ring with two C and one O
TriphenylphosphineTriphenylphosphine oxidePh3P is the reagent; Ph3P=O is the oxidized byproduct
Z-selective ylideE-selective ylideUnstabilized → Z; stabilized (EWG) → E
Wittig alkeneAlkene from eliminationWittig fixes the double-bond position; elimination gives mixtures of isomers

Memory aids

"Wittig Welds the double bond Where the oxygen Was." Remember the driving force with "P=O is the POwer that pulls the reaction forward," and the geometry with "Z for Zesty (unstabilized, fast) and E for Electron-poor (stabilized)."

Quick review

Topic Recap

The Wittig reaction turns a carbonyl into an alkene through a phosphorus ylide, passing through a four-membered oxaphosphetane that collapses with loss of triphenylphosphine oxide. The strong P=O bond drives the reaction, and the alkene geometry is set by the ylide type: unstabilized ylides give Z alkenes, stabilized ylides give E alkenes. Its predictable double-bond placement makes it a cornerstone of alkene retrosynthesis.

Knowledge Check

  1. Starting from CH3CH2Br, outline how to make the ylide Ph3P=CHCH3.
  2. Predict the alkene product from cyclohexanone and Ph3P=CH2.
  3. What is the byproduct of every Wittig reaction, and why does it form?
  4. An reacting with an aldehyde gives predominantly which alkene geometry?
  5. How would you retrosynthetically make CH3CH=CHCH3 using a Wittig reaction?

Answers and Rationales

  1. Treat CH3CH2Br with Ph3P to form the phosphonium salt [Ph3P-CH2CH3]+ Br−, then deprotonate with a strong base (e.g., BuLi) to give Ph3P=CHCH3.
  2. Methylenecyclohexane. The =CH2 group replaces the carbonyl oxygen of cyclohexanone.
  3. Triphenylphosphine oxide (Ph3P=O). It forms because the very strong P=O bond makes the reaction thermodynamically favorable and irreversible.
  4. The Z (cis) alkene, because unstabilized ylides react under kinetic control.
  5. Disconnect the central C=C to give acetaldehyde (CH3CHO) and the ylide Ph3P=CHCH3 (from ethyl bromide), or the reverse pairing; either aldehyde + ylide combination retrosynthetically delivers the alkene.
Eli, the EliExplains learning guide

Eli explains

The same idea, in plain words

Explain it like I’m 10

Imagine you want to replace the oxygen of a carbonyl with a brand-new carbon partner, turning C=O into C=C. The Wittig reaction does this with a special "carbon with a handle": a phosphorus atom (carrying three bulky phenyl groups) is attached to a carbon that also carries a negative charge. That charged carbon is exactly the kind of electron-rich group that will latch onto the hungry carbonyl carbon.

A useful comparison: think of the ylide as a delivery truck carrying a new carbon atom. The truck (triphenylphosphine) backs up to the carbonyl, drops off its carbon cargo to form a temporary four-sided "package" (the oxaphosphetane), and then drives away — but it drives away carrying the oxygen with it, leaving behind the new carbon–carbon double bond.

This comparison stops being exact because the phosphorus "truck" does not physically scoop up the oxygen and leave. Electrons rearrange in a concerted ring-closing and ring-opening: a new P–O bond forms while the C–O and C–P bonds break, and the driving force is the formation of a very strong phosphorus–oxygen double bond, not any literal carrying away.

Simple Example

Treating acetone (CH3COCH3) with the ylide Ph3P=CH2 gives 2-methylpropene (CH2=C(CH3)2) plus triphenylphosphine oxide. The =CH2 group simply takes the place of the original oxygen.

Worked example

The mechanism of the Wittig reaction (electron movement is described before the products):

  1. Nucleophilic attack. The ylide carbon's lone pair attacks the electrophilic carbonyl carbon. A double-headed arrow runs from that lone pair to carbon while the C=O π electrons move onto oxygen. This gives a betaine-like intermediate, R2C(O−)-CHR-PPh3+.
  2. Ring closure to the oxaphosphetane. The negatively charged oxygen donates its lone pair to the positively charged phosphorus (arrow from O to P), forming a four-membered ring (the oxaphosphetane) containing C–O and P–C bonds.
  3. Cycloreversion. The four-membered ring opens in a concerted step: the C–O and P–C bonds break while the new C=C π bond and the new P=O bond form. Two arrows are drawn — one from the C–O bond to the C–C bond (forming the alkene) and one from the P–C bond to oxygen (forming the P=O). The products are the alkene and triphenylphosphine oxide.
  4. Driving force. The formation of the very strong P=O bond (roughly 140 kcal/mol) makes the reaction irreversible and pulls the equilibrium toward the alkene.

For alkene prediction, simply delete the C=O oxygen and connect the two carbons with a double bond, transferring the ylide carbon's substituents onto the former carbonyl carbon.

Key takeaways

  • High yield: The Wittig reaction converts C=O to C=C at a precisely chosen position.
  • High yield: Byproduct is triphenylphosphine oxide; the strong P=O bond is the thermodynamic driving force.
  • High yield: Unstabilized ylide → Z alkene; stabilized ylide → E alkene.
  • Phosphonium salt = Ph3P + RCH2X (Sₙ2); ylide = deprotonation with a strong base.
  • The ylide carbon (with its substituents) becomes one end of the new double bond.
  • The oxaphosphetane is the four-membered intermediate; it is not isolated but is the last species before the alkene forms.
  • Retrosynthesis: any alkene can be disconnected to a carbonyl and a ylide; choose the disconnection that gives an accessible alkyl halide for the ylide.

Keep learning

Ready to build on this? Continue to the next lesson.

Study tools & related lessonsYou’ll learn to · Key vocabulary · Related

You’ll learn to

  • Describe how a phosphorus ylide is made from a phosphonium salt and how it converts a carbonyl into an alkene.
  • Write the mechanism of the Wittig reaction, including the oxaphosphetane intermediate and the triphenylphosphine oxide byproduct.
  • Predict alkene products and their E/Z geometry from a given ylide and carbonyl compound.
  • Contrast stabilized and unstabilized ylides and apply the Wittig reaction retrosynthetically.

Key vocabulary

Wittig reaction
Ylide + aldehyde/ketone → alkene + Ph3P=O
Phosphorus ylide
Ph3P=CR2, a carbon nucleophile
Phosphonium salt
[Ph3P-CH2R]+ X−
Ylide formation
Strong-base deprotonation of the phosphonium salt
Carbonyl-to-alkene conversion
C=O becomes C=C
Oxaphosphetane
Four-membered C–O–P–C ring intermediate
Triphenylphosphine oxide
Ph3P=O byproduct
Alkene product prediction
Replace O with =CR2
E/Z selectivity
Geometry of the product double bond
Stabilized ylide
Ylide with an EWG on the carbon
Unstabilized ylide
Ylide with only H/alkyl groups
Retrosynthetic use
Disconnect an alkene into carbonyl + ylide

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