Grignard Reaction
Grignard Reaction: The polarized carbanion-like carbon of the Grignard reagent attacks the electrophilic carbonyl carbon of the aldehyde; the C=O π bond…
What happens
The polarized carbanion-like carbon of the Grignard reagent attacks the electrophilic carbonyl carbon of the aldehyde; the C=O π bond breaks and a new C–C bond forms, giving a magnesium alkoxide that is protonated on aqueous workup to the neutral alcohol.
Reaction summary
PhCHO + PhMgBr → Ph₂CH–OMgBr → (H₃O⁺) → Ph₂CH–OH
Reagents and conditions
- Starting material: Benzaldehyde
- Reagents: Phenylmagnesium bromide (PhMgBr)
- Conditions: Dry ether, 0°C → RT; then aqueous H₃O⁺ workup
- Products: Diphenylmethanol (benzhydrol)
Mechanism
- Nucleophilic addition: PhMgBr's carbanion-like carbon attacks the carbonyl carbon of benzaldehyde; the C=O π electrons shift onto oxygen.
- Magnesium alkoxide intermediate: A tetrahedral magnesium alkoxide forms: the new C–C bond and a C–O–MgBr linkage are both in place.
- Aqueous acidic workup: Dilute aqueous acid protonates the alkoxide oxygen, releasing the neutral secondary alcohol.
Selectivity
- Chemoselectivity: Grignards also add to ketones, esters (2 equiv → tertiary alcohol), CO₂, and epoxides, aldehydes are the most reactive of the common carbonyl electrophiles.
Common mistakes
- Forgetting Grignard reagents are destroyed by any acidic proton (water, alcohols, terminal alkynes), must run under strictly anhydrous conditions.
- Adding excess Grignard to an ester and stopping at the ketone stage, esters react twice, giving a tertiary alcohol, not a ketone.
Memory trick
"Grignard flips the polarity": think of the reagent's carbon as carbanion-like (R–MgBr ≈ R⁻), always attacking a carbon that's electron-poor (C=O), never the reverse.