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Preparation of Carboxylic Acids

Synthesis via Oxidation, Hydrolysis, and Grignard Reactions.

By chemca Team • Updated Jan 2026

Carboxylic acids ($R-COOH$) are prepared by oxidizing alcohols/aldehydes, hydrolyzing nitriles/esters, or through the carboxylation of Grignard reagents.

1. From Primary Alcohols and Aldehydes

Strong Oxidation

Primary alcohols are oxidized to carboxylic acids using common oxidizing agents like acidified $KMnO_4$, acidic $K_2Cr_2O_7$, or Jones reagent ($CrO_3$ in $H_2SO_4$).

$$ R-CH_2OH \xrightarrow{[O]} R-CHO \xrightarrow{[O]} R-COOH $$

Example: Ethanol to Ethanoic Acid.

$$ CH_3CH_2OH \xrightarrow{Jones \text{ Reagent}} CH_3COOH $$

2. From Alkyl Benzenes

Side-Chain Oxidation

Aromatic carboxylic acids are prepared by vigorous oxidation of alkyl benzenes with alkaline $KMnO_4$.

$$ \underset{\text{Toluene}}{C_6H_5-CH_3} \xrightarrow{1. KMnO_4, KOH, \Delta} C_6H_5-COOK \xrightarrow{2. H_3O^+} \underset{\text{Benzoic Acid}}{C_6H_5-COOH} $$

Important Rule: The entire alkyl side chain is oxidized to the carboxyl group regardless of length, provided there is at least one benzylic hydrogen. Tertiary alkyl groups are not oxidized.

3. From Nitriles and Amides

Hydrolysis

Nitriles ($R-CN$) are hydrolyzed first to Amides and then to Carboxylic acids in the presence of $H^+$ or $OH^-$ as catalyst.

$$ R-CN \xrightarrow{H_2O/H^+} R-CONH_2 \xrightarrow{H_2O/H^+} R-COOH + NH_3 $$

Mild conditions can stop the reaction at the Amide stage.

4. From Grignard Reagents

Carboxylation (Dry Ice)

Grignard reagents react with solid Carbon Dioxide (Dry Ice) to form salts of carboxylic acids, which yield the acid upon acidification.

$$ R-MgX + O=C=O \xrightarrow{\text{Dry Ether}} R-COO^-MgX^+ \xrightarrow{H_3O^+} R-COOH $$

This method increases the carbon chain length by one.

5. From Acyl Halides and Esters

Hydrolysis of Derivatives

A. Acyl Chlorides: Hydrolyze readily with water.

$$ R-COCl + H_2O \rightarrow R-COOH + HCl $$

B. Esters: Acidic hydrolysis gives acid and alcohol.

$$ \underset{\text{Ethyl Benzoate}}{C_6H_5COOC_2H_5} \xrightarrow{H_3O^+} \underset{\text{Benzoic Acid}}{C_6H_5COOH} + C_2H_5OH $$

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