Organomagnesiums, 154. carbon dioxide

The reaction of organomagnesium compounds with carbon dioxide to give carboxylates, and thence carboxylic acids, was one of the first to be reported by Grignard [4], and Grignard also observed the important side-reactions leading to ketones and t-alcohols [5] ... 

The reaction of organomagnesium compounds with sulfur dioxide provides a good general method for the preparation of magnesium sulfinates, and thence to various derived compounds (cf. reactions with carbon dioxide see Section 6.4) ... 

This disconnection represents the retro-Grignard reaction. It leads almost automatically to organomagnesium or lithium compounds as equivalents of carbanions and aldehydes or ketones as equivalents to the carbocation in the synthesis of alcohols (Scheme 2.35), or carbon dioxide in the synthesis of carboxylic acids. 

The mechanism of Grignard carboxylation is similar to that of other Grignard reactions (Section 19.8). The organomagnesium halide adds to a C=0 bond of carbon dioxide in a typical nucleophilic addition reaction. Protonation of the carboxylate by addition of aqueous HCl in a separate step then gives the free carboxylic acid product. 

Ethereal solutions of organomagnesium compounds are sensitive to moisture, carbon dioxide, and air. Gilman and Hewlett179 have described an apparatus by means of which access of air can be prevented completely. 

Because the organomagnesium reagent reacts with water, carbon dioxide, and oxygen, it must be protected from air and moisture when it is used. The apparatus in which the reaction is to be conducted must be scrupulously dry, and the solvent must be completely anhydrous. In addition, diethyl ether is required as a solvent without the presence of an ether, the organomagnesium reagent will not form. 

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