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.
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FAQs on Wittig Reaction
1. What is the Wittig reaction as per the CBSE Class 12 syllabus for the year 2025-26?
The Wittig reaction is a fundamental organic chemical reaction where a carbonyl compound, specifically an aldehyde or a ketone, reacts with a phosphorus ylide known as a Wittig reagent (triphenyl phosphonium ylide). The primary outcome of this reaction is the synthesis of an alkene, along with triphenylphosphine oxide as a by-product. It is a highly reliable method for forming a carbon-carbon double bond.
2. What are the two main components required for a Wittig reaction to occur?
The two essential components for a successful Wittig reaction are:
- A Carbonyl Compound: This is typically an aldehyde or a ketone, which contains the C=O group that will be transformed.
- A Phosphonium Ylide (Wittig Reagent): This is a special type of molecule with adjacent positive and negative charges. It acts as the nucleophile, with its negatively charged carbon atom attacking the carbonyl carbon.
3. How is the Wittig reagent (phosphonium ylide) prepared?
The preparation of a Wittig reagent is a two-step process:
- Step 1 (SN2 Reaction): Triphenylphosphine, an excellent nucleophile, attacks a primary or secondary alkyl halide. This forms a triphenylphosphonium salt.
- Step 2 (Deprotonation): The resulting phosphonium salt is then treated with a strong base, such as butyllithium (BuLi) or sodium hydride (NaH), which removes a proton from the carbon adjacent to the phosphorus atom, yielding the neutral phosphonium ylide.
4. What are the major applications and importance of the Wittig reaction?
The Wittig reaction is highly significant in organic synthesis for several reasons:
- It is a very versatile and effective method for creating alkenes from readily available aldehydes and ketones.
- The reaction allows for the specific placement of the double bond, which is often difficult to achieve with other methods.
- It can be used with a wide variety of functional groups present in the carbonyl compound without affecting them.
- It results in the formation of a new carbon-carbon bond, which is crucial for extending carbon chains in complex molecule synthesis.
5. Can you provide a common example of the Wittig reaction?
A classic example is the reaction of cyclohexanone with methylenetriphenylphosphorane (a Wittig reagent). The nucleophilic carbon of the ylide attacks the carbonyl carbon of cyclohexanone. Following the mechanism, the oxygen atom from the ketone is replaced, resulting in the formation of methylenecyclohexane (an alkene) and triphenylphosphine oxide.
6. Why is the Wittig reaction's mechanism crucial for understanding its outcome?
Understanding the mechanism is vital because it explains how the C=O bond is precisely replaced by a C=C bond. The process involves:
- Step 1: Nucleophilic attack by the ylide on the carbonyl carbon to form a betaine intermediate.
- Step 2: Ring formation where the negatively charged oxygen attacks the positively charged phosphorus, creating a four-membered ring intermediate called an oxaphosphetane.
- Step 3: The oxaphosphetane ring collapses in a reverse [2+2] cycloaddition, yielding the final alkene and the highly stable triphenylphosphine oxide, which drives the reaction forward.
7. Does the Wittig reaction typically produce E or Z isomers, and what influences this?
The stereochemical outcome depends on the type of ylide used. Generally, non-stabilised or simple ylides (e.g., from primary alkyl halides) react kinetically to predominantly form the Z-alkene (cis isomer). This is because the betaine intermediate forms and rapidly cyclises to the oxaphosphetane in a way that minimises steric hindrance in the transition state. In contrast, stabilised ylides often give the E-alkene (trans isomer) as the major product under thermodynamic control.
8. What are the main limitations of the Wittig reaction?
Despite its utility, the Wittig reaction has some limitations:
- Stereocontrol: While it can be selective, achieving a pure single isomer (either E or Z) can be challenging, often resulting in a mixture of both, with one dominating.
- Steric Hindrance: The reaction rate decreases significantly when using sterically hindered ketones, as the bulky groups prevent the ylide from easily attacking the carbonyl carbon.
- By-product Removal: The by-product, triphenylphosphine oxide, can sometimes be difficult to separate from the desired alkene product, complicating purification.
9. How can we use the Wittig reaction in retrosynthesis to plan the synthesis of a target alkene?
In retrosynthesis, you work backwards from the target molecule. To plan a synthesis using the Wittig reaction, you would mentally 'disconnect' the double bond of the target alkene. One carbon of the double bond comes from a carbonyl compound (aldehyde or ketone), and the other carbon comes from the Wittig reagent. This allows you to identify the two simpler starting materials needed to construct the alkene, making it a powerful strategic tool in planning complex syntheses.
