Wittig Reaction: Mechanism & Stereochemistry
The Wittig Reaction is one of the most reliable methods for synthesizing Alkenes from Aldehydes or Ketones using a special organophosphorus reagent called a Phosphonium Ylide (or Wittig Reagent). Its greatest advantage is the complete control over the position of the double bond.
1. General Reaction
An aldehyde or ketone reacts with a triphenylphosphonium ylide to form an alkene and triphenylphosphine oxide.
Driving Force: The formation of the extremely strong Phosphorus-Oxygen double bond ($P=O$) in triphenylphosphine oxide is the thermodynamic driving force of this reaction.
2. Preparation of Wittig Reagent (Ylide)
The ylide is prepared in two steps via an $S_N2$ reaction followed by an acid-base reaction.
Step 1: Phosphonium Salt Formation
Triphenylphosphine ($PPh_3$) acts as a nucleophile and attacks a primary or secondary alkyl halide.
Step 2: Ylide Formation
A strong base (like Butyl Lithium, $BuLi$, or $NaH$) removes an acidic proton from the carbon adjacent to phosphorus.
The product is a resonance hybrid called an Ylide (a species with adjacent opposite charges).
3. Detailed Mechanism
The reaction proceeds through a concerted cycloaddition pathway.
Step 1: Nucleophilic Addition
The nucleophilic carbon of the ylide attacks the carbonyl carbon, while the carbonyl oxygen attacks the phosphorus atom. This forms a four-membered cyclic intermediate called Oxaphosphetane.
Step 2: Fragmentation
The oxaphosphetane collapses. The strong $P=O$ bond forms, breaking the C-P and C-O bonds, releasing the alkene.
4. Stereochemistry (Z vs E)
Key Rule
- Unstabilized Ylides: (R is Alkyl) typically yield Z-Alkenes (Cis) as the major product.
- Stabilized Ylides: (R is EWG like $-COOR, -CN$) typically yield E-Alkenes (Trans) as the major product.
5. Examples
A. Synthesis of Methylenecyclohexane
Reaction of Cyclohexanone with Methylenetriphenylphosphorane.
$$ \text{Cyclohexanone} + Ph_3P=CH_2 \rightarrow \text{Methylenecyclohexane} + Ph_3P=O $$B. Synthesis of Styrene
Reaction of Benzaldehyde with Methylenetriphenylphosphorane.
$$ Ph-CHO + Ph_3P=CH_2 \rightarrow Ph-CH=CH_2 \text{ (Styrene)} + Ph_3P=O $$6. Comparison with Elimination
Unlike elimination reactions (E1/E2) which usually follow Zaitsev's rule (forming the most substituted alkene) and can give mixtures, the Wittig reaction forms the double bond exactly where the carbonyl group was, providing superior Regiocontrol.
Wittig Reaction Quiz
Test your concepts on Ylides and Alkene synthesis. 10 MCQs with explanations.
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