Ylide

An ylide (/ˈɪlaɪd/)[1] or ylid (/ˈɪlɪd/) is a neutral dipolar molecule containing a formally negatively charged atom (usually a carbanion) directly attached to a heteroatom with a formal positive charge (usually nitrogen, phosphorus or sulfur), and in which both atoms have full octets of electrons.

The result can be viewed as a structure in which two adjacent atoms are connected by both a covalent and an ionic bond; normally written X+–Y−.

Many ylides may be depicted by a multiply bonded form in a resonance structure, known as the ylene form, while the actual structure lies in between both forms:[citation needed] The actual bonding picture of these types of ylides is strictly zwitterionic (the structure on the right) with the strong Coulombic attraction between the "onium" atom and the adjacent carbon accounting for the reduced bond length.

[citation needed] Phosphonium ylides are used in the Wittig reaction, a method used to convert ketones and especially aldehydes to alkenes.

The positive charge in these Wittig reagents is carried by a phosphorus atom with three phenyl substituents and a bond to a carbanion.

Phosphorus ylides are important reagents in organic chemistry, especially in the synthesis of naturally occurring products with biological and pharmacological activities.

These ylides can be generated by condensation of an α-amino acid and an aldehyde or by thermal ring opening reaction of certain N-substituted aziridines.

Stable carbenes also have a ylidic resonance contributor, e.g.: Halonium ylides can be prepared from allyl halides and metal carbenoids.