What is the Difference Between Wittig and Wittig Horner Reaction?
🆚 Go to Comparative Table 🆚The Wittig and Wittig-Horner reactions are both important synthesis reactions in organic chemistry that produce alkenes from aldehydes or ketones. The key difference between the two reactions lies in the reactants they use:
- Wittig Reaction: This reaction uses phosphonium ylides as reactants. It has two main steps: the first step is the deprotonation reaction of a phosphonium salt to obtain a phosphorous ylide, which then reacts with a compound containing a carbonyl group (an aldehyde or a ketone) to give the corresponding alkene and phosphine oxide. The Wittig reaction can produce either E-alkene or Z-alkene, depending on the nature of the ylide.
- Wittig-Horner Reaction: Also known as the Horner-Wadsworth-Emmons (HWE) reaction, this reaction uses phosphonate-stabilized carbanions as reactants. In contrast to phosphonium ylides used in the Wittig reaction, phosphonate-stabilized carbanions are more nucleophilic but less basic. The Wittig-Horner reaction produces only E-alkene. The reaction mechanism begins with the deprotonation of the phosphonate to give the carbanion, which then undergoes nucleophilic addition onto the aldehyde or ketone.
In summary, the main difference between the Wittig and Wittig-Horner reactions is the type of reactants they use: phosphonium ylides in the Wittig reaction and phosphonate-stabilized carbanions in the Wittig-Horner reaction. This difference in reactants results in the Wittig reaction producing either E-alkene or Z-alkene, while the Wittig-Horner reaction produces only E-alkene.
Comparative Table: Wittig vs Wittig Horner Reaction
The Wittig and Wittig-Horner reactions are both important synthesis reactions that produce alkenes. Here is a table summarizing the differences between the two reactions:
| Feature | Wittig Reaction | Wittig-Horner Reaction |
|---|---|---|
| Reactants | Phosphonium ylides with aldehydes or ketones | Phosphonate-stabilized carbanions with aldehydes or ketones |
| Mechanism | Two-step process: deprotonation of a phosphonium salt followed by reaction with a compound containing a carbonyl group | Modified version of the Wittig reaction, using more nucleophilic and less basic carbanions |
| Stereoselectivity | Favors the production of Z-alkenes | Favors the production of E-alkenes under certain conditions, such as increasing the steric reaction temperature or using solvents like DME |
| Advantages | Versatile and widely used for alkene synthesis | Provides higher E-alkene selectivity and uses more nucleophilic and less basic carbanions |
| Disadvantages | Waste product is phosphine oxide | May require more specific conditions to achieve high E-alkene selectivity |
In summary, the key difference between the Wittig and Wittig-Horner reactions is the type of reactant used (phosphonium ylides vs. phosphonate-stabilized carbanions) and the resulting stereoselectivity of the product (Z-alkenes vs. E-alkenes).
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