What is the Difference Between Wittig and Wittig Horner Reaction

Wittig and Wittig Horner reactions are two methods used in chemistry to create double bonds (alkenes) from carbonyl compounds. Even though they give similar results, they work in different ways and are used for different purposes.

What is the difference between Wittig and Wittig Horner reaction? In the Wittig reaction, the starting material is phosphonium ylide whereas, in the Wittig Horner reaction, the starting material is phosphonate ester anion.

Key Areas Covered

1. What is Wittig Reaction 
     – Definition, Facts, Features
2. What is Wittig Horner Reaction 
     – Definition, Facts, Features
3. Similarities Between Wittig and Wittig Horner Reaction
    – Outline of Common Features
4. Difference Between Wittig and Wittig Horner Reaction 
     – Comparison of Key Differences
5. FAQ – Wittig and Wittig Horner Reaction
     – Answers to Frequently Asked Questions

Key Terms

Wittig Reaction, Wittig Horner Reaction, Horner-Wadsworth-Emmons (HWE) Reaction

What is Wittig Reaction

Wittig reaction is a way to introduce a desired carbon chain between two carbonyl groups (aldehydes or ketones). The starting material is phosphonium ylide, a molecule with a positively charged phosphorus atom bonded to a carbon holding a negative charge. The negatively charged carbon makes it highly reactive. A strong base like sodium hydride (NaH) is used to deprotonate a phosphonium salt, generating the ylide.

The ylide then reacts with the aldehyde or ketone. The negatively charged carbon on the ylide forms a new bond with the positively charged carbon of the carbonyl group, while the oxygen from the carbonyl grabs hydrogen from the ylide, forming a water molecule. This process creates the desired alkene and a byproduct called triphenylphosphine oxide (Ph3P=O).

Advantages of Wittig reaction include having control over the location of the double bond, being able to work with various aldehydes and ketones and having high productive yields.

What is Wittig Horner Reaction

The Wittig Horner reaction, also known as the Horner-Wadsworth-Emmons (HWE) reaction, allows the formation of carbon-carbon double bonds (alkenes). Instead of a phosphonium ylide, the HWE reaction utilizes a stabilized phosphonate ester anion. This anion has a negative charge on a carbon atom bonded to a phosphorus atom, similar to the ylide.

The main difference is the presence of an electron-withdrawing group (EWG) attached to the same carbon. This EWG stabilizes the negative charge, making the anion more nucleophilic (electron-donating) but less basic compared to a ylide. The HWE reaction produces a water-soluble phosphate ester as a byproduct, making product purification much easier compared to the troublesome triphenylphosphine oxide (Ph3P=O) in the Wittig reaction.

Similarities Between Wittig and Wittig Horner Reaction

1. Both reactions are mainly used to form carbon-carbon double bonds, resulting in the synthesis of alkenes.
2. These reactions involve phosphorus ylides as key intermediates.

Difference Between Wittig and Wittig Horner Reaction

Definition

• Wittig reaction is a chemical reaction that forms alkenes through the reaction of a phosphonium ylide with a carbonyl compound. On the other hand, Wittig-Horner reaction (Horner-Wadsworth-Emmons reaction) forms alkenes by reacting phosphonate esters with carbonyl compounds in the presence of a base.

Starting Materials

• Wittig reaction uses a phosphonium ylide (PPh3=CHR) as the reactive component. These are made by reacting a phosphonium salt with a strong base. Horner-Wadsworth-Emmons reaction, on the other hand, uses a phosphonate ester (P(O)(OR)(OR)CH2-CHR) as the reactant.

Byproducts

• Wittig reaction produces triphenylphosphine oxide (Ph3P=O) as a byproduct, which can be difficult to separate from the desired alkene product. Meanwhile Wittig Horner reaction generates a water-soluble phosphate ester byproduct, which makes it easier to remove during purification through water washes.

Selectivity

• Wittig reaction often favors the formation of the E-isomer of the alkene, while Wittig Horner reaction yields a mixture of E and Z isomers.

Sensitivity 

• Wittig reaction can be sensitive to acidic or basic functional groups on the starting materials due to the reactivity of the ylide, while Wittig Horner reaction is more tolerant of various functional groups because the phosphonate ester is less basic than the ylide.

Conclusion

Wittig and Wittig Horner reactions are two methods used in chemistry to create double bonds (alkenes) from carbonyl compounds. The main difference between Witting and Witting Horner reaction is that in Wittig reaction starting material is phosphonium ylide where as in Wittig Horner reaction the starting material is phosphonate ester anion.

FAQ: Wittig and Wittig Horner Reaction

1. What is the difference between Wittig reaction and Wittig rearrangement?

The Wittig reaction turns aldehydes/ketones into alkenes, while the Wittig rearrangement transforms ethers into alcohols. This is the basic difference between Wittig reaction and Wittig rearrangement.

2. What is the application of Wittig Horner reaction?

Wittig Horner reaction is particularly useful for making compounds with E-stereochemistry, which is needed for many drugs and materials.

3. What is the mechanism of the Horner Wittig Emmons reaction?

Horner Wittig Emmons reaction involves deprotonating a phosphonate, nucleophilic attack on a carbonyl, then cyclization and elimination to form the alkene and a phosphate.

4. What is the phase transfer catalyst in the Horner Emmons experiment?

The Horner-Wadsworth-Emmons reaction often benefits from phase transfer catalysts. Common examples include quaternary ammonium salts like tetrabutylammonium bromide (TBAB).

5. What are the advantages of Wittig reaction?

Advantages of Wittig reaction include precise control of double bond placement, unlike elimination reactions, and compatibility with various functional groups on the carbonyl compound. It also allows the prediction of double bond geometry based on the Wittig reagent.

Reference:

1. “Wittig Reaction – An Overview.” Science Direct.
2. “Horner–Wadsworth–Emmons reaction.” Wikipedia. Wikipedia Foundation.

Image Courtesy:

1. “Wittig Reaktion”  By Roland Mattern – Roland1952 (Public Domain) via Commons Wikimedia
2. “Horner-Wadsworth-Emmons-Mechanism“V1” By Jü – Own work (CC BY-SA 4.0) via Commons Wikimedia