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Pramod Gupta

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 It should not be confused with the Wittig rearrangement.Wittig reactions are most commonly used to couple aldehydes and ketones to singly substituted phosphine ylides. With simple ylides this results in almost exclusively the Z-alkene productBecause of its reliability and wide applicability, the Wittig reaction has become a standard tool for synthetic organic chemists.[18]The most popular use of the Wittig reaction is for the introduction of a methylene group using methylenetriphenylphosphorane (Ph3P=CH2Structure of the ylideThe Wittig reagent may be written in the phosphorane form (the more familiar representation) or the ylide form:However the phosphorane resonance requires expansion of the octet on phosphorus. This hypervalency cannot (yet) be explained well in terms of standard bonding theory, and this resonance is rather less favoured than the more familiar p?p overlap seen in ?-bonded compounds as alkenes or imines. This means that the ylide form is a significant contributor, and the carbon is quite nucleophilic.