Question

Explain the preparation of carboxylic acids from Grignard reagent. Give an equation.

Answer 1

Hint: We know that it is a nucleophilic reagent that acts like a very strong base in chemical reactions. Its unique properties are due to the carbon-metal bond present in it and because carbon is bonded to metal, it can form carbanion in chemical reactions but it is important to keep it away from the moisture.

Complete answer:
The important observation is the nucleophilic addition of Grignard reagent over the substrate as alkyl carbanion is a very strong base and powerful nucleophile, that’s with compounds containing carbonyl carbons it readily forms nucleophilic addition product and common thing over here is the acidic work up to form the final product, because the addition compound formed after the nucleophilic addition of Grignard reagent over substrate will easily get hydrolyzed in acidic medium.
Grignard reagent reacts with solid \[C{{O}_{2}}\] (dry ice) to form magnesium salt of carboxylic acid. This is followed by acidification to obtain carboxylic acid. For example, the reaction between methyl magnesium bromide and carbon dioxide followed by acid hydrolysis will give acetic acid.
\[\underset{Grignard~~\operatorname{Re}agent}{\mathop{R-Mg-X}}\,+\underset{\begin{smallmatrix}
 Carbon \\
 dioxide
\end{smallmatrix}}{\mathop{C{{O}_{2}}}}\,\xrightarrow{Dry~Ether}\underset{Salts~~of~~Carboxlyic~~acid}{\mathop{R-C(=O)-{{O}^{-}}Mgx}}\,\xrightarrow{{{H}_{3}}{{O}^{+}}}\underset{Carboxylic~~acid}{\mathop{R-COOH}}\,\]

Additional Information:
French chemist Victor Grignard discovered this reagent and won the \[1912\] Nobel Prize for that. From the scientist's name it is known as Grignard reagent. Grignard is a very important reagent in chemistry as it can be used to form a variety of compounds. On reacting with alkyl halide it forms higher alkanes.

Note:
Remember that while preparing Grignard reagent it is important to have anhydrous condition because Grignard reagents react with water and form alkane. In fact with compounds containing active hydrogen like carboxylic acid, alcohol etc.