Ylide
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An ylide is a neutral molecule with positive and negative charges on adjacent atoms.
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.
Other common ylides include sulfonium and sulfoxonium ylides used in the preparation of epoxides, as well as certain nitrogen-based ylides.
For more information see Wittig reaction#Wittig reagents.