(N/A) Grignard reagents react with carbon dioxide (dry ice) to form salts of carboxylic acids,which upon acidification with mineral acid yield the corresponding carboxylic acids.
General reaction:
$R-Mg-X + O=C=O \xrightarrow{\text{dry ether}} R-COO^-Mg^+X$
$R-COO^-Mg^+X \xrightarrow{H_3O^+} RCOOH + Mg(OH)X$
Example $(i)$: Preparation of ethanoic acid from methyl bromide:
$CH_3Br \xrightarrow{Mg, \text{dry ether}} CH_3MgBr$
$CH_3MgBr + CO_2 \xrightarrow{\text{dry ether}} CH_3COOMgBr$
$CH_3COOMgBr \xrightarrow{H_3O^+} CH_3COOH + Mg(OH)Br$
Example $(ii)$: Preparation of benzoic acid from bromobenzene:
$C_6H_5Br \xrightarrow{Mg, \text{dry ether}} C_6H_5MgBr$
$C_6H_5MgBr + CO_2 \xrightarrow{\text{dry ether}} C_6H_5COOMgBr$
$C_6H_5COOMgBr \xrightarrow{H_3O^+} C_6H_5COOH + Mg(OH)Br$