Ylides

Definition

Ylides are a class of organic compounds containing a positively charged atom (usually phosphorus or sulfur) bonded to a negatively charged atom, typically carbon. They are important intermediates in various organic reactions, particularly in the Wittig reaction for the formation of alkenes.

5 Must Know Facts For Your Next Test

1. Ylides have a formal positive charge on the heteroatom (usually phosphorus or sulfur) and a formal negative charge on the adjacent carbon atom.
2. The carbon-heteroatom bond in an ylide is polarized, with the carbon bearing a partial negative charge and the heteroatom bearing a partial positive charge.
3. Ylides are highly reactive species and are often used as nucleophiles in organic reactions, such as the Wittig reaction for the synthesis of alkenes.
4. The Wittig reaction involves the reaction of an aldehyde or ketone with a phosphonium ylide to form an alkene and a phosphine oxide byproduct.
5. Sulfonium ylides are less common than phosphonium ylides but can also be used in organic synthesis, for example, in the synthesis of epoxides.

Review Questions

• Explain the formal charge distribution in an ylide and how it contributes to the reactivity of these species.
  • Ylides have a unique formal charge distribution, with a positively charged heteroatom (usually phosphorus or sulfur) and a negatively charged adjacent carbon atom. This polarized carbon-heteroatom bond gives the ylide a highly reactive nature, allowing it to act as a nucleophile in various organic reactions. The formal charges create a dipole moment that facilitates the ylide's ability to engage in nucleophilic addition or substitution reactions, such as the Wittig reaction for the synthesis of alkenes.
• Describe the role of ylides in the Wittig reaction and the significance of this transformation in organic synthesis.
  • The Wittig reaction is a powerful tool in organic synthesis that utilizes phosphonium ylides to convert aldehydes or ketones into alkenes. In this reaction, the ylide acts as a nucleophile, attacking the carbonyl carbon of the aldehyde or ketone. This forms a betaine intermediate, which then rearranges to eliminate a phosphine oxide byproduct and generate the desired alkene product. The Wittig reaction is particularly useful for the stereoselective synthesis of alkenes, as the reaction can be tuned to favor the formation of either the E or Z isomer. This makes the Wittig reaction an essential transformation in the toolbox of organic chemists.
• Analyze the differences between phosphonium ylides and sulfonium ylides, and discuss their respective applications in organic synthesis.
  • Phosphonium ylides and sulfonium ylides are both types of ylides, but they differ in the identity of the positively charged heteroatom. Phosphonium ylides, where the positively charge is on a phosphorus atom, are much more commonly used in organic synthesis, particularly in the Wittig reaction for alkene formation. Sulfonium ylides, on the other hand, have a positively charged sulfur atom and are less frequently employed. Despite their differences, both types of ylides can be used as nucleophiles in various organic transformations. Sulfonium ylides have found applications in the synthesis of epoxides and other heterocyclic compounds, but their reactivity and selectivity are generally less predictable than their phosphonium counterparts. The choice between phosphonium and sulfonium ylides in a particular synthesis will depend on the specific reaction conditions, the desired product, and the availability of the starting materials.

Related terms

Wittig Reaction: A chemical reaction that uses an ylide to convert an aldehyde or ketone into an alkene.

Phosphonium Ylide: An ylide where the positively charged atom is phosphorus, commonly used in the Wittig reaction.

Sulfonium Ylide: An ylide where the positively charged atom is sulfur, also used in organic synthesis.