Grignard reagent + CO₂ → carboxylic acid template

Phenylmagnesium bromide attacks the carbon of CO₂ (dry ice) to give the magnesium carboxylate, which acid work-up turns into benzoic acid.

Mechanism step by step

Carboxylation with dry ice adds one carbon; acid work-up frees the acid

1. Addition to CO₂ – The C–Mg bond attacks the CO₂ carbon; a C=O becomes C–O⁻
2. Acid work-up – Aqueous acid protonates the carboxylate

Key points

• Adds one carbon: R–X → R–MgX → R–COOH. It works for aryl, vinyl and tertiary halides, where the nitrile route (SN2 with CN⁻) fails.
• The carboxylate salt is negatively charged and unreactive, so a second equivalent does not add (unlike an ester).
• The Grignard solution is poured onto excess crushed dry ice, protected from moist air; any water would give benzene instead.
• ¹³CO₂ or ¹⁴CO₂ puts an isotope label on the carboxyl carbon only.

Drawn with 63 atoms, 7 arrows, 8 shapes – every one editable in MolDraw.

Sources

• Clayden, Greeves & Warren, Organic Chemistry, 2nd ed., OUP 2012, ch. 9 Textbook
• Kürti & Czakó, Strategic Applications of Named Reactions in Organic Synthesis, Elsevier 2005 Textbook