What is the Wittig Reaction Mechanism Reagents and Stepwise Example
Wittig reaction is an important name reaction of organic chemistry. It is used in organic synthesis for the preparation of alkenes. It is a coupling reaction which is also known as Wittig olefination. It is different from Wittig rearrangement. The reaction is carried out by Wittig reagent which is a triphenyl phosphonium ylide. It is prepared by phosphonium salt and phosphonium salt is prepared by the reaction of triphenylphosphine with an alkyl halide. Typically, tetrahydrofuran (THF) or diethyl ether are used as solvent in the reaction. The Wittig reaction is named after German Chemist Georg Wittig who discovered it in 1954. He got the Nobel Prize in Chemistry in 1979 which he shared with Herbert C. Brown.
What is Wittig Reaction?
Wittig reaction is a chemical reaction in which a carbonyl compound (aldehyde or ketone) reacts with a triphenyl phosphonium ylide to give an alkene. Thus, it is a useful reaction to convert aldehydes or ketones into alkenes. In this reaction Wittig reagent reacts with carbonyl compound and gives alkenes and triphenylphosphine oxide as side product. The reaction is given below (General form)
Thus, we can say two main components of Wittig reaction are as follows –
Carbonyl compound (aldehyde or ketone)
An Ylide
Carbonyl compounds are those compounds which have -C=O group in them and an ylide is a species which has opposite formal charges (positive and negative) on adjacent atoms. In Wittig reaction phosphonium ylide is used.
As you can see phosphonium ylide has a nucleophilic carbon. This carbon attacks on the carbon of the carbonyl group and initiates the reaction.
Mechanism of Wittig Reaction
Wittig reaction starts with the preparation of phosphonium ylide. Although ylides look like a difficult species, but their synthesis or preparation is quite easy and straightforward. Their preparation reactions simply follow a SN2 (bimolecular nucleophilic substitution) reaction mechanism.
Preparation of Wittig Reagent - Triphenyl phosphine reacts with alkyl halide and forms triphenyl phosphonium salt. Now this triphenyl phosphonium salt is made to react with a strong base (such as CH3-Li) to give triphenyl phosphonium ylide.
The Phosphate atom of triphenyl phosphine has a lone pair of electrons and act as an excellent nucleophile. This nucleophile attacks from the back on alkyl halide and displaces the leaving group (halide ion) which shows this reaction (preparation of ylide) follows SN2 reaction mechanism. Thus, formed phosphonium salt reacts with a strong base and goes through deprotonation and gives phosphonium ylide. Reaction mechanism of preparation of Wittig reagent is given below –
Step 1. Reaction of alkyl halide with triphenylphosphine -
Remember, in the above step, we use either primary or secondary alkyl halide. Tertiary halide cannot be used.
Step 2. Deprotonation
After preparation of ylide mechanism of the Wittig reaction takes place by following three steps –
Step 1. Attack of ylide carbon on carbonyl - Now the above prepared phosphonium ylide reacts with carbonyl compound (aldehyde or ketone). The Ylide carbon attacks on the carbonyl group due to the pi-electrons of the carbonyl group shifts towards the oxygen atom. Thus, betaine is formed.
Step 2. Attack of oxygen on phosphorus – Negatively charged oxygen atom attacks on positively charged phosphorus atom and forms oxaphosphetane.
Step 3. Reverse [2+2] cycloaddition – In this step, in oxaphosphetane [2+2] reverse cycloaddition takes place which give rise to the main product alkene and side product triphenylphosphine oxide.
In many cases, step 1 and step 2 takes place simultaneously.
Examples of Wittig Reaction
Ylide reaction with cyclohexanone –
Use of Wittig reaction to form ring compound using 1-bromo-6- heptanone-
Significance of Wittig Reaction
Importance of Wittig reaction can be understood by its popularity among the various methods of preparation of alkenes from aldehydes and ketones. Wittig reaction can be used for carbonyl compounds containing many functional groups. As Wittig reagent shows reactions with functional groups containing carbonyl compounds as well. It is a very effective method of preparation of alkenes. The geometry of the double bond can easily be predicted in the alkenes prepared by Wittig reaction, if the ylide’s nature is known. The components used in Wittig reaction are readily available or can be easily synthesized. It results in the formation of a new carbon – carbon double bond, which allows increase in carbon chain.
Limitations of Wittig Reaction
With many advantages, Wittig reaction has few limitations which are listed below –
The main limitation of Wittig reaction is that the reaction proceeds mainly through betaine intermediate, which leads to Z – alkene.
The ylides lacking electron withdrawing groups form both Z and E isomers. Although Z – isomer dominates.
When sterically hindered ketones are used in Wittig reaction, the rate of reaction decreases.
Some variations of Wittig reactions are also available such as Schlosser modification.
This ends our coverage on Wittig Reaction. We hope you enjoyed learning and were able to grasp the concepts. We hope after reading this article you will be able to answer questions related to this topic. If you are looking for solutions of NCERT Textbook problems based on this topic, then log on to Vedantu website or download Vedantu Learning App. By doing so, you will be able to access free PDFs of NCERT Solutions as well as Revision notes, Mock Tests and much more.
FAQs on Wittig Reaction in Organic Chemistry for Alkene Synthesis
1. What is the Wittig reaction in organic chemistry?
The Wittig reaction is an organic reaction that converts an aldehyde or ketone into an alkene using a phosphonium ylide. It is widely used for carbon–carbon double bond formation in synthesis.
General reaction:
R1CHO + Ph3P=CHR2 → R1CH=CHR2 + Ph3P=O
Key points:
- The carbonyl oxygen is replaced by a C=C bond.
- A by-product, triphenylphosphine oxide (Ph3P=O), is formed.
- It is especially useful for synthesizing defined alkenes in organic chemistry.
2. What is a phosphonium ylide in the Wittig reaction?
A phosphonium ylide is a neutral molecule containing adjacent positively charged phosphorus and negatively charged carbon atoms, written as Ph3P+–CH−R. It is the key reagent in the Wittig reaction.
Structure features:
- Formed from a triphenylphosphonium salt.
- Has resonance forms: Ph3P+–CH−R ↔ Ph3P=CHR.
- The negatively charged carbon acts as a nucleophile toward carbonyl compounds.
3. How is a Wittig reagent prepared?
A Wittig reagent (phosphonium ylide) is prepared by reacting triphenylphosphine with an alkyl halide, followed by deprotonation with a strong base.
Step-by-step preparation:
- Step 1: Formation of phosphonium salt
Ph3P + R–Br → Ph3P+–R Br− - Step 2: Deprotonation
Ph3P+–CH2R + Base → Ph3P=CHR + HB
4. What is the mechanism of the Wittig reaction?
The Wittig reaction mechanism involves nucleophilic addition of a phosphonium ylide to a carbonyl compound, forming an intermediate that collapses to give an alkene and triphenylphosphine oxide.
Main steps:
- 1. Nucleophilic attack: The ylide carbon attacks the carbonyl carbon.
- 2. Betaine formation: A zwitterionic intermediate (betaine) is formed.
- 3. Oxaphosphetane intermediate: The betaine cyclizes to a four-membered ring.
- 4. Elimination: The ring breaks to form alkene + Ph3P=O.
5. What is the general equation of the Wittig reaction?
The general equation of the Wittig reaction is: R1R2C=O + Ph3P=CHR3 → R1R2C=CHR3 + Ph3P=O.
Where:
- R1 and R2 are alkyl or aryl groups from an aldehyde or ketone.
- Ph3P=CHR3 is the phosphonium ylide.
- The product is an alkene plus triphenylphosphine oxide.
6. What is the difference between stabilized and unstabilized ylides?
The difference between stabilized and unstabilized ylides lies in the presence of electron-withdrawing groups that affect their stability and alkene selectivity.
- Unstabilized ylides: No electron-withdrawing group; typically give Z-alkenes as major products.
- Stabilized ylides: Contain groups like –COOR or –CN; usually give E-alkenes as major products.
7. Why does the Wittig reaction form alkenes?
The Wittig reaction forms alkenes because the carbonyl C=O bond is replaced by a new carbon–carbon double bond during elimination of triphenylphosphine oxide.
Reason:
- The ylide carbon bonds to the carbonyl carbon.
- A cyclic intermediate forms and then breaks down.
- The strong P=O bond in Ph3P=O drives the reaction forward.
8. Can the Wittig reaction be used with both aldehydes and ketones?
Yes, the Wittig reaction works with both aldehydes and ketones to produce alkenes, although aldehydes are generally more reactive.
Comparison:
- Aldehydes: React faster and often give higher yields.
- Ketones: React more slowly due to steric hindrance.
9. What is an example of a Wittig reaction?
An example of a Wittig reaction is the conversion of benzaldehyde to styrene using a methylide ylide.
Example reaction:
C6H5CHO + Ph3P=CH2 → C6H5CH=CH2 + Ph3P=O
Here:
- Benzaldehyde reacts with methylenetriphenylphosphorane.
- The product is styrene (C6H5CH=CH2).
- Triphenylphosphine oxide is formed as a by-product.
10. What are the advantages of the Wittig reaction in organic synthesis?
The main advantage of the Wittig reaction is its ability to form carbon–carbon double bonds with predictable alkene products under relatively mild conditions.
Key advantages:
- Selective formation of alkenes from carbonyl compounds.
- Control over E/Z stereochemistry depending on ylide type.
- Mild reaction conditions compared to some elimination methods.
- Widely used in pharmaceuticals and complex molecule synthesis.
