Grignard reaction: practice and mechanism
The CH₃ group of the Grignard reagent forms a new C–C bond to the carbonyl carbon: the product has one more carbon and an OH group.
Example
The methyl group of the Grignard reagent attacks the carbonyl carbon as a carbanion and forms a new C–C bond. Aldehydes give secondary, ketones tertiary alcohols. Esters react twice (the ketone intermediate is more reactive) and also give tertiary alcohols.
Mechanism step by step
- 1
In the Grignard reagent the C–Mg bond is strongly polarised: the carbon behaves like a carbanion (CH₃⁻).
- 2
It attacks the carbonyl carbon, a new C–C bond forms, and C=O becomes C–O–MgBr.
- 3
Acidic work-up (H₃O⁺) gives the alcohol. Esters: first a ketone, then a second attack → tertiary alcohol.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
- Here the methyl group sits on the oxygen (ether). The Grignard reagent is a carbon nucleophile and attacks the carbonyl carbon, not the oxygen.
- It does not stop at the ketone: the ketone is more reactive than the ester and immediately reacts with a second CH₃MgBr to the tertiary alcohol.
Often confused with: Reduction with sodium borohydride · Wittig reaction
Frequently asked questions
Why must everything be completely dry?
The Grignard reagent is a very strong base and would be protonated by water to methane at once.
What do you get with CO₂ or formaldehyde?
With CO₂ a carboxylic acid (one more carbon), with formaldehyde a primary alcohol.