17-12 Addition of Phosphorous Ylides:The Wittig teao5oCaag2cpnooaststabtady Cn2etYegnia8yfeatawatsbocauethentntegroup H ofphosphonium salts gives oewg2eVieaeataraavecetee tnthe prenc of ethere 11 11 Cyanohydrins are useful intermediates because the nitrile group can be modified by further reaction. Addition of Phosphorous Ylides: The Wittig Reaction 17-12 A phosphorus ylide contains a carbanion that is stabilized by an adjacent, positively charged phosphorous group. A ylide is capable of attacking aldehydes and ketones in a reaction called the Wittig reaction, which allows the synthesis of alkenes from aldehydes and ketones. Deprotonation of phosphonium salts gives phosphorous ylides. Ylides can be prepared from haloalkanes by a two-step sequence: The phosphonium salt can be deprotonated by bases such as alkoxides, sodium hydride or butyl lithium, giving the ylide: The Wittig reaction forms carbon-carbon double bonds. In the Wittig reaction, an ylide reacts with an aldehyde or ketone by coupling the ylide carbon to that of the aldehyde or ketone: In the Wittig reaction, the position of the newly formed double bond is unambiguous: The mechanism of the Wittig reaction involves the formation of a phosphorous betaine, a dipolar species of a type known as a zwitterion. Wittig reactions can be carried out in the presence of ether, ester, halogen, alkene and alkyne functions. Wittig reactions are only sometimes stereoselective; cis-trans mixtures of alkenes may be produced