MCQ Detailed View

A Grignard reagent is an organometallic compound of the general formula RMgX, where R is an alkyl or aryl group and X is a halogen (usually Cl, Br, or I). These reagents are strong nucleophiles and bases, widely used in organic synthesis for the formation of carbon–carbon bonds.

One of the most important reactions of Grignard reagents is with carbon dioxide (CO₂). When a Grignard reagent is treated with dry CO₂, the nucleophilic carbon atom of RMgX attacks the electrophilic carbon of CO₂. This forms a magnesium salt of the corresponding carboxylate anion. Upon hydrolysis with dilute acid (H₃O⁺), this salt is converted into the corresponding carboxylic acid (R–COOH).

For example:

RMgX + CO₂ → RCOOMgX →(H₃O⁺)→ RCOOH

This reaction is an efficient method for the synthesis of carboxylic acids directly from Grignard reagents. If R = CH₃, the product will be acetic acid. If R is phenyl, the product will be benzoic acid. Thus, by varying the alkyl or aryl group of the Grignard reagent, a wide range of carboxylic acids can be synthesized.

It is important to note that this specific reaction occurs only with CO₂. With other compounds such as CH₃CHO (acetaldehyde), the reaction would produce a secondary alcohol; with COCl₂ (phosgene), the reaction leads to more complex derivatives; and with HCN, addition leads to cyanohydrin formation.

Therefore, the unique reaction that yields a carboxylic acid is the one with carbon dioxide. This reaction highlights the versatility and synthetic importance of Grignard reagents in organic chemistry, making them a cornerstone of laboratory and industrial organic synthesis.