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The Wittig reaction converts a carbonyl to an alkene using a phosphorus ylide (R₃P=CR₂). The ylide is generated by deprotonation of a phosphonium salt. Reaction with an aldehyde or ketone proceeds through a betaine intermediate to form a four-membered cyclic intermediate, ultimately yielding the alkene and triphenylphosphine oxide. Stabilized ylides (with electron-withdrawing groups) are more selective and stereospecific.
Draw the ylide formation and the cycloaddition/retrocycloaddition mechanism. Compare the reactivity and E/Z selectivity of stabilized versus non-stabilized ylides.
From your work with Grignard reagents and carbonyl chemistry, you know that carbon nucleophiles can attack the electrophilic carbonyl carbon. The Wittig reaction uses the same logic — a carbon nucleophile attacks a C=O — but the outcome is fundamentally different: instead of producing an alcohol, it replaces the C=O entirely with a C=C double bond. This makes the Wittig reaction one of the most powerful and predictable methods for alkene synthesis in organic chemistry.
The reactive species is a phosphorus ylide, a molecule of the form R₃P=CR'₂ where phosphorus bears a formal positive charge and the adjacent carbon a formal negative charge. You generate this ylide in two steps. First, a phosphine (typically triphenylphosphine, PPh₃) attacks an alkyl halide in an SN2 reaction to form a phosphonium salt. Then a strong base (often n-BuLi) removes a proton from the carbon adjacent to phosphorus, producing the ylide. The carbanion character of this carbon is what makes it nucleophilic enough to attack a carbonyl.
When the ylide encounters an aldehyde or ketone, the nucleophilic ylide carbon attacks the electrophilic carbonyl carbon to form a betaine — a zwitterionic intermediate with both positive (on phosphorus) and negative (on oxygen) charges. This betaine collapses into a four-membered ring called an oxaphosphetane, containing C–C, C–O, O–P, and P–C bonds. The oxaphosphetane then fragments in a retro-[2+2] cycloaddition: the strong P=O bond in triphenylphosphine oxide (Ph₃P=O) forms, and the alkene is released. The thermodynamic driving force is the exceptional strength of the P=O bond (~540 kJ/mol), which makes the overall reaction essentially irreversible.
The practical power of the Wittig reaction lies in its regiochemical predictability: the new C=C bond forms exactly where the C=O was, with no ambiguity about where the double bond ends up. This is a significant advantage over elimination reactions, which can give mixtures of regioisomers. Stereoselectivity depends on the ylide type. Non-stabilized ylides (no electron-withdrawing groups on the ylide carbon) tend to give the Z-alkene (cis) through kinetic control. Stabilized ylides (with an adjacent ester, nitrile, or other electron-withdrawing group) favor the E-alkene (trans) through thermodynamic control. This tunability — choosing your ylide to select the desired geometric isomer — makes the Wittig reaction a cornerstone of retrosynthetic planning for target molecules containing specific alkene geometries.