Ylide Stability

Definition

Ylides are neutral, dipolar compounds containing a positively charged atom (typically phosphorus or sulfur) bonded to a negatively charged carbon atom. The stability of ylides is a crucial factor in determining their reactivity, particularly in the context of the Wittig reaction, a powerful tool for the formation of carbon-carbon double bonds.

5 Must Know Facts For Your Next Test

1. The stability of a ylide is determined by the nature of the substituents attached to the positively charged atom and the negatively charged carbon atom.
2. Ylides with electron-withdrawing substituents on the positively charged atom and electron-donating substituents on the negatively charged carbon are generally more stable.
3. Increased stability of the ylide leads to a slower rate of reaction with the carbonyl compound in the Wittig reaction, resulting in a higher degree of stereoselectivity.
4. The choice of substituents on the ylide can be used to control the stereochemistry of the alkene product formed in the Wittig reaction.
5. The stability of the ylide also affects the equilibrium between the ylide and the betaine intermediate in the Wittig reaction mechanism.

Review Questions

• Explain how the stability of a ylide affects the outcome of the Wittig reaction.
  • The stability of the ylide is a key factor in the Wittig reaction, as it determines the rate of the reaction and the stereochemistry of the alkene product. Stable ylides, with electron-withdrawing substituents on the positively charged atom and electron-donating substituents on the negatively charged carbon, react more slowly with the carbonyl compound. This slower rate of reaction leads to a higher degree of stereoselectivity in the formation of the alkene. The stability of the ylide also affects the equilibrium between the ylide and the betaine intermediate in the reaction mechanism, which can further influence the stereochemical outcome.
• Analyze the relationship between ylide stability and the choice of substituents on the ylide.
  • The stability of a ylide is directly related to the nature of the substituents attached to the positively charged atom and the negatively charged carbon. Ylides with electron-withdrawing substituents on the positively charged atom (typically phosphorus) and electron-donating substituents on the negatively charged carbon are generally more stable. This increased stability is due to the ability of the electron-withdrawing substituents to stabilize the positive charge on the phosphorus atom, while the electron-donating substituents can stabilize the negative charge on the carbon atom. The choice of these substituents can be used to control the reactivity and stereochemistry of the ylide in the Wittig reaction, as more stable ylides tend to be less reactive but more stereoselective.
• Evaluate the role of ylide stability in the mechanism of the Wittig reaction and its implications for the formation of alkene products.
  • The stability of the ylide is a crucial factor in the mechanism of the Wittig reaction, as it directly impacts the equilibrium between the ylide and the betaine intermediate. More stable ylides, with electron-withdrawing substituents on the positively charged atom and electron-donating substituents on the negatively charged carbon, favor the formation of the betaine intermediate. This intermediate then undergoes a cycloreversion to form the alkene product and the phosphine oxide byproduct. The stability of the ylide also affects the rate of the reaction, with more stable ylides reacting more slowly with the carbonyl compound. This slower rate of reaction leads to a higher degree of stereoselectivity in the formation of the alkene, as there is more time for the betaine intermediate to adopt the preferred conformation before the cycloreversion step. Therefore, the stability of the ylide is a key consideration in the Wittig reaction, as it allows for the controlled synthesis of alkenes with desired stereochemistry.

Related terms

Wittig Reaction:

A organic reaction in which a phosphorus ylide (a phosphonium ylide) reacts with a carbonyl compound (an aldehyde or a ketone) to form an alkene and a phosphine oxide.

Phosphonium Ylide:

A type of ylide where the positively charged atom is a phosphorus atom, which is bonded to a negatively charged carbon atom.

Nucleophilic Addition:

A type of reaction where a nucleophile (an electron-rich species) adds to an electrophilic (electron-deficient) carbon center, such as a carbonyl carbon.