What is the reaction of aldehydes and ketones with Grignard reagents?

General Reaction

Aldehydes and ketones undergo nucleophilic addition reactions with Grignard reagents (RMgX).

1. The carbanion-like carbon of the Grignard reagent (R–) acts as a nucleophile.
2. It attacks the electrophilic carbonyl carbon of the aldehyde/ketone.
3. This gives a tetrahedral alkoxide intermediate.
4. On acidic hydrolysis (H₃O⁺ or NH₄Cl(aq)), the alkoxide is protonated to give an alcohol.

Reactions by Substrate

1. Formaldehyde (H–CHO):
   • Reaction: H–CHO + RMgX → R–CH₂–O⁻MgX → (acid workup) primary alcohol (R–CH₂–OH).
2. Other Aldehydes (R–CHO):
   • Reaction: R–CHO + RMgX → R₂–CH–O⁻MgX → (acid workup) secondary alcohol.
3. Ketones (R₂–C=O):
   • Reaction: R₂C=O + RMgX → R₃C–O⁻MgX → (acid workup) tertiary alcohol.

Summary

• Formaldehyde → primary alcohol
• Aldehyde → secondary alcohol
• Ketone → tertiary alcohol

This reaction is widely used in alcohol synthesis and is a fundamental carbon–carbon bond-forming reaction in organic chemistry.