Ylide

An ylide is a neutral molecule with positive and negative charges on adjacent atoms.

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The most common ylides are phosphonium ylides, used in the Wittig reaction for double bond synthesis from carbonyl groups (C=O). The positive charge is carried by a phosphorus atom with three phenyl substituents and one bond to a carbon bearing a negative charge and two substituents, commonly alkyl groups. Ylides can be 'stabilised' or 'non-stabilised'. Non-stabilised ylides react readily with both aldehydes and ketones whereas stabilised will only react with aldehydes.

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Other common ylides include sulfonium and sulfoxonium ylides used in the preparation of epoxides, as well as certain nitrogen-based ylides.

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For more information see Wittig reaction#Wittig reagents.

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Wittig reaction: The Wittig reaction is a chemical reaction of an aldehyde or ketone with a triphenyl phosphonium ylide (often called a Wittig reagent) to give an alkene and triphenylphosphine oxide....