Grignard Reagents and Carbon Dioxide

Grignard reagents react with carbon dioxide to make carboxylic acids. This is a way of adding one carbon atom to a chain and forming a -COOH group at the same time.The reaction happens in two stages. First, the nucleophilic δ− carbon of the Grignard reagent attacks the δ+ carbon of CO₂. Carbon dioxide is usually added as crushed solid carbon dioxide (dry ice), or bubbled through the solution as a dry gas. An intermediate magnesium salt forms:CH₃CH₂MgBr + CO₂ → CH₃CH₂COOMgBrSecond, the salt is hydrolysed with dilute acid (for example dilute hydrochloric acid), which releases the carboxylic acid:CH₃CH₂COOMgBr + H⁺ → CH₃CH₂COOH + Mg²⁺ + Br⁻So ethylmagnesium bromide, with a two-carbon ethyl group, gives propanoic acid, with three carbons. The product always has one more carbon than the R group. In the same way, phenylmagnesium bromide gives benzoic acid, C₆H₅COOH.This gives a useful three-step route from a halogenoalkane to a carboxylic acid with a longer chain: halogenoalkane → Grignard reagent (Mg, dry ether) → carboxylic acid (CO₂, then dilute acid).Worked example: 0.0500 mol of ethylmagnesium bromide reacts completely with CO₂. The ratio is 1 : 1, so 0.0500 mol of propanoic acid (M_r = 74.0) forms, a mass of 0.0500 × 74.0 = 3.70 g.