The Wittig reaction, which converts aldehydes or ketones to alkenes using phosphorus ylides, proceeds through a nucleophilic addition‒elimination process.

The reaction begins with the nucleophilic addition between a phosphorus ylide and the carbonyl compound. Due to its carbanionic character, phosphorus ylide acts as a strong nucleophile and attacks the electrophilic carbonyl group. This generates a charge-separated dipolar intermediate called betaine. The negatively charged oxygen atom and the positively charged phosphorus atom in betaine undergo a ring-closure reaction to generate the four-membered oxaphosphetane ring.

Figure1

In some cases, a concerted [2 + 2] cycloaddition between the Wittig reagent and the carbonyl compound results in the oxaphosphetane intermediate.

Figure2

The unstable oxaphosphetane ring undergoes fragmentation to yield the desired alkene molecule along with a stable triphenylphosphine oxide as the by-product. The driving force for the Wittig reaction is the formation of a strong P=O bond in the phosphine oxide molecule.

Figure3

Tags
Wittig ReactionAldehydesKetonesAlkenesPhosphorus YlidesNucleophilic AdditionCarbanionic CharacterCharge separated Dipolar IntermediateBetaineOxaphosphetane Ring2 2 CycloadditionFragmentationTriphenylphosphine OxideP O Bond

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12.18 : Aldehydes and Ketones to Alkenes: Wittig Reaction Mechanism

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12.1 : Structures of Aldehydes and Ketones

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12.2 : IUPAC Nomenclature of Aldehydes

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12.3 : IUPAC Nomenclature of Ketones

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12.4 : Common Names of Aldehydes and Ketones

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12.5 : IR and UV–Vis Spectroscopy of Aldehydes and Ketones

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12.6 : NMR Spectroscopy and Mass Spectrometry of Aldehydes and Ketones

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12.7 : Preparation of Aldehydes and Ketones from Alcohols, Alkenes, and Alkynes

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12.8 : Preparation of Aldehydes and Ketones from Nitriles and Carboxylic Acids

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12.9 : Preparation of Aldehydes and Ketones from Carboxylic Acid Derivatives

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12.10 : Nucleophilic Addition to the Carbonyl Group: General Mechanism

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12.11 : Aldehydes and Ketones with Water: Hydrate Formation

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12.12 : Aldehydes and Ketones with Alcohols: Hemiacetal Formation

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12.13 : Protecting Groups for Aldehydes and Ketones: Introduction

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