Chapter  19: Carboxylic Acids

Carbonation of Grignard Reagents, RMgX, by CO₂

Reaction usually in Et₂O or THF followed by H₃O⁺ work-up

Summary

• Grignard reagents react with dry ice (solid CO₂)  followed by aqueous acid work-up to give carboxylic acids.
• CO₂ can be thought of as a being a dicarbonyl compound :  O=C=O
• Note that the carboxylic acid contains one extra C atom compared to the original halide from which the Grignard reagent was prepared.

• Reaction of RLi and RMgX with esters
• Hydride Reductions of Aldehydes and Ketones
• Hydride Reductions of Esters

NUCLEOPHILIC ADDITION OF RMgX TO CARBON DIOXIDE
Step 1: The nucleophilic C in the Grignard reagent adds to the electrophilic C in the polar carbonyl group, electrons from the C=O move to the electronegative O creating an intermediate magnesium carboxylate complex.
Step 2: This is the  work-up step, a simple acid/base reaction. Protonation of the carboxylate oxygen creates the carboxylic acid product from the intermediate complex.

© Dr. Ian Hunt, Department of Chemistry