Carboxylic acid from Grignard and CO₂: practice and mechanism
Mg turns the haloalkane into a Grignard reagent. It attacks CO₂, and after acidification a carboxylic acid with one more carbon forms.
Example
Magnesium turns the haloalkane (or aryl bromide) into the Grignard reagent. It attacks CO₂ at the carbon, and acidic work-up gives the carboxylic acid with one carbon more than the starting material.
Mechanism step by step
- 1
Magnesium inserts into the C–halogen bond in ether: R–MgX forms.
- 2
The nucleophilic carbon attacks the carbon of CO₂ (e.g. poured onto dry ice): a carboxylate forms.
- 3
Acidification protonates the carboxylate to the carboxylic acid with one more carbon.
Practice tasks
Which product forms, which reagent is needed? After each answer you see the explanation.
Which main product forms?
Typical mistakes
Often confused with: SN2 with cyanide (chain extension) · Malonic ester synthesis
Frequently asked questions
How does this differ from the nitrile route?
Both add one carbon. The nitrile route (NaCN then hydrolysis) needs SN2 and fails with tertiary or aryl halides; the Grignard route works with them.
Why does the carboxylate not react further?
The negatively charged carboxylate is hardly electrophilic, so a second Grignard attack practically does not happen.