The key difference between Wittig and Wittig Horner reaction is that Wittig reaction uses phosphonium ylides, whereas Wittig Horner reaction uses phosphonate-stabilized carbanions.
Wittig reaction and Wittig Horner reactions are important synthesis reactions in organic chemistry, which produce alkenes from aldehydes or ketones. These reactions differ from each other depending on the reactants that are included in the reaction, along with aldehyde or ketone.
CONTENTS
1. Overview and Key Difference
2. What is Wittig Reaction
3. What is Wittig Horner Reaction
4. Side by Side Comparison – Wittig vs Wittig Horner Reaction in Tabular Form
5. Summary
What is Wittig Reaction?
Wittig reaction is a type of coupling reaction in which aldehydes or ketones react with phosphonium ylides to give an alkene. Further, this reaction is also named as Wittig olefination reaction because it forms an olefin as the final product. Also, this reaction was named after the scientist Georg Wittig. The phosphonium ylide is named as Wittig reagent because this reactant is specific for the Wittig reaction. Along with the alkene, this reaction gives another product, triphenylphosphine oxide. The general reaction is as follows:
Figure 01: Wittig Reaction
Wittig reaction is important in the production of alkenes in organic synthesis. It is a type of coupling reaction because it is involved in the coupling of aldehydes and ketones to triphenylphosphonium ylides. The nature of the produced alkene depends on the stability of the ylide. i.e. unstabilized ylides give Z-alkenes, and stabilized ylides give E-alkene. However, the formation of E-alkene is highly selective in this reaction.
What is Wittig Horner Reaction?
Wittig Horner reaction is a type of coupling reaction in which aldehydes or ketones couple with phosphonate-stabilized carbanions to give E-alkenes. This reaction was named after three scientists: Leopold Horner, William S. Wadsworth and William D. Emmons. And, it is a variation of the Wittig reaction. But, in contrast to Wittig reaction, this Wittig Horner reaction uses phosphonate-stabilized carbanions instead of phosphonium ylides. These carbanions are more nucleophilic and less basic. Moreover, this reaction favours the production of E-alkene. The general reaction is as follows:
Figure 02: Wittig Horner Reaction
We can observe a high selectivity for the E-alkene under conditions such as increasing the steric bulk of the aldehyde, high reaction temperature, using solvents such as DME, etc. Thus, this is named as the stereoselectivity of the Wittig Horner reaction.
What is the Difference Between Wittig and Wittig Horner Reaction?
Wittig reaction and Wittig Horner reaction are important synthesis reactions that produce alkenes from aldehydes or ketones. The key difference between Wittig and Wittig Horner reaction is that Wittig reaction uses phosphonium ylides, whereas Wittig Horner reaction uses phosphonate-stabilized carbanions. Therefore, the reactants of Wittig reaction are aldehyde or ketone and phosphonium ylides, while the reactants of Wittig Horner reaction are aldehydes or ketones with phosphonate-stabilized carbanions.
Wittig reaction was named after the scientist Georg Wittig while the Wittig Horner reaction was named after three scientists: Leopold Horner, William S. Wadsworth and William D. Emmons. Moreover, the Wittig reaction gives either E-alkene or Z-alkene depending on the nature of the ylide, i.e. unstabilized ylides give Z-alkene, and stabilized ylides give E-alkenes. However, Wittig Horner reaction gives only E-alkene. So, this is another difference between Wittig and Wittig Horner reaction.
Summary – Wittig vs Wittig Horner Reaction
Wittig reaction and Wittig Horner reaction are important synthesis reactions in organic chemistry and produce alkenes from aldehydes or ketones. The key difference between Wittig and Wittig Horner reaction is that Wittig reaction uses phosphonium ylides, whereas Wittig Horner reaction uses phosphonate-stabilized carbanions.
Reference:
1. “Wittig Reaction”. Wikipedia.Org, 2019, Available here.
2. “Wittig-Horner Reaction”. Organic-Chemistry.Org, 2019, Available here.
3. “Horner–Wadsworth–Emmons Reaction”. Wikipedia.Org, 2019, Available here.
Image Courtesy:
1. “Wittig Reaktion” By Roland Mattern – Roland1952 (Public Domain) via Commons Wikimedia
2. “Horner-Wadsworth-Emmons Reaction Example” By K assumed. Own work assumed (based on copyright claims) (Public Domain) via Commons Wikimedia