From Grignard reagent

A Grignard reagent, dissolved in dry ether, is added to solid carbon dioxide (dry ice); this addition gives a magnesium-carboxylate complex, R-COOMgX, which on subsequent acid hydrolysis gives the corresponding carboxylic acid: R-Mg-X + O=C=O (dry ice), in dry ether, gives R-COOMgX, and this complex with water/dilute HCl gives R-COOH + Mg(X)OH. A distinctive feature of this particular preparation, worth remembering: the dry ice serves TWO simultaneous roles in the reaction -- it is both the actual carbon-dioxide REAGENT being added across the Grignard's C-Mg bond, and, because it is used in its solid, very col …