Table of Contents
- 1 What makes a Wittig reagent stabilized?
- 2 What makes a ylide stabilized?
- 3 What is one of the limitations seen with the Wittig reaction?
- 4 What are stabilized and non stabilized ylides?
- 5 Why is the ylide a stable Nucleophile?
- 6 Why is the Wittig reaction important?
- 7 Which ylide is more stable?
- 8 Why are Wittig reactions exothermic?
- 9 Why do we use Wittig reagent with electron withdrawing groups?
- 10 What is the general mechanism of the Wittig reaction?
- 11 What is the difference between electron-withdrawing groups and electron-donating groups?
What makes a Wittig reagent stabilized?
Structure of the Wittig reagent It has been noted that dipolar phosphorus compounds are stabilized by p-d bonding. This bonding stabilization extends to carbanions adjacent to phosphonium centers. An ylide is defined as a compound with opposite charges on adjacent atoms, both of which have complete octets.
What makes a ylide stabilized?
What happens when we try to combine an aldehyde (or unsymmetrical ketone) with an unsymmetrical ylide? The ratio of the Z isomer decreases as electron-withdrawing groups are added to the ylide. These species are called, “stabilized ylides”, as they are less basic (and less reactive).
What are the conditions for a Wittig reaction?
| Wittig reaction | |
|---|---|
| Reaction type | Coupling reaction |
| Reaction | |
| aldehyde or ketone + triphenyl phosphonium ylide ↓ alkene + triphenylphosphine oxide | |
| Conditions |
What is one of the limitations seen with the Wittig reaction?
One limitation of the Wittig reaction is possibility of E and Z isomers of the alkene forming. With simple ylides, the product formed is usually primarily the Z-isomer, although a lesser amount of the E-isomer is often also formed.
What are stabilized and non stabilized ylides?
The ylides are of two types based on their relative stability. i) Non-stabilized ylides: The ylides with electron donating groups on negatively charged carbon are less stable and react faster. * The unstabilized ylides react faster and lead to (Z)-alkenes. * The stabilized ylides react slowly and lead to (E)-alkenes.
Why is triphenylphosphine oxide a major drawback to the Wittig reaction?
The formation of the alkene proceeds through the reaction of an aldehyde or ketone with a phosphonium ylide. The major drawback of the reaction is the generation of stoichiometric amounts of undesired triphenylphosphine oxide, a byproduct that frequently complicates the purification of the desired product.
Why is the ylide a stable Nucleophile?
It isn’t stable, it only forms transiently. Inductive effects and resonance stabilize the negative charge. The proximity of the positively charged phosphorous and negatively charged carbon stabilizes the charges. It is a stable Arrhenius base.
Why is the Wittig reaction important?
The Wittig reaction provides a path from aldehydes and ketones to alkenes, and consequently is a valuable tool in organic synthesis. For example, the Wittig reaction will convert an α,β-unsaturated ketone to a conjugated alkene.
Is Wittig a Stereoselectivity reaction?
The Z-stereoselective Wittig reaction consists of a stereoselective first step forming the syn oxaphosphetane. This is then followed by a stereospecific elimination of this intermediate to form the Z alkene.
Which ylide is more stable?
ii) Stabilized ylides: The ylides with electron withdrawing groups adjacent to the negatively charged carbon are more stable. These are usually stabilized by conjugation.
Why are Wittig reactions exothermic?
In the Wittig reaction, an ylide (typically triphenylphosphorus ylide), adds to an aldehyde or ketone to yield a four-membered heterocyclic intermediate called an oxaphosphetane. Cleavage of the oxaphosphetane to alkene and phosphine oxide products is exothermic and irreversible.
Which of the following is true about the Wittig reaction reaction of an aldehyde ketone with a phosphorus ylide )?
The Wittig reaction involves the reaction of an aldehyde or ketone with an phosphorus ylide (also called a Wittig reagent) to form an alkene. The phosphorus ylide acts as a nucleophile, attacking the aldehyde/keton. The answer is choice D, since the product of the Wittig reaction is an alkene, not an alkyne.
Why do we use Wittig reagent with electron withdrawing groups?
It is especially common to have a Wittig reagent with an electron-withdrawing group such as esters which stabilize the negative charge of the carbon connected to the phosphorous and also influence the stereochemistry of the reaction.
What is the general mechanism of the Wittig reaction?
Mechanism The general mechanism of the Wittig reaction is shown above. The phosphonium ion is deprotonated by base. The positively charged phosphorus atom is a strong electron-withdrawing group, which activates the neighboring carbon atom as a weak acid.
Why is Wittig reaction with non-stabilized ylides performed under inert atmosphere?
Hence the Wittig reaction with non-stabilized ylides is performed under inert atmosphere. ii) Stabilized ylides: The ylides with electron withdrawing groups adjacent to the negatively charged carbon are more stable. These are usually stabilized by conjugation.
What is the difference between electron-withdrawing groups and electron-donating groups?
Whereas electron-withdrawing groups tend to be meta-directors. Meaning that they direct subsequent EAS reactions to happen only at the meta positions. Here I have a picture of these two benzenes and an electron-donating group. We would expect to add the second EAS reagent in the ortho positions or in the para positions.
https://www.youtube.com/watch?v=hZ4y5WsjdDw