Methods of Preparation of Carboxylic Acids
Quick Notes
- Oxidation of primary alcohols and aldehydes gives carboxylic acids.
- Alkylbenzenes undergo side-chain oxidation to give aromatic acids.
- Nitriles and amides hydrolyse under acidic or basic conditions to give acids.
- Grignard reagents form acids upon reaction with CO2 and hydrolysis.
- Acyl halides, anhydrides, and esters convert to acids through hydrolysis.
Full Notes
Carboxylic acids can be synthesised from a wide range of starting materials, including alcohols, aldehydes, hydrocarbons, nitriles, Grignard reagents, and derivatives of acids. Each method targets the formation of the carboxyl group by oxidation, hydrolysis, or nucleophilic substitution.
From Primary Alcohols and Aldehydes
Primary alcohols are easily oxidised to carboxylic acids using strong oxidising agents. Common oxidising agents include:
Potassium permanganate (KMnO4) or Potassium dichromate (K2Cr2O7)

- Works in neutral, acidic, or alkaline media.
Chromium trioxide (CrO3) in acidic medium (Jones reagent).

Aldehydes are also oxidised to carboxylic acids using mild oxidising agents.
From Alkylbenzenes
Aromatic carboxylic acids are prepared by oxidising alkylbenzenes with chromic acid or acidic/alkaline KMnO4.

- Primary and secondary alkyl groups are oxidised.
- Tertiary alkyl groups do not undergo oxidation under these conditions.
- Substituted alkenes can also be oxidised similarly to form carboxylic acids.
From Nitriles and Amides
Hydrolysis of nitriles (RCN) and amides (RCONH2) leads to the forming of carboxylic acids.
The nitrogen-containing groups are replaced by the hydroxyl of the acid.

- From Nitriles:
- R–CN + 2H2O + H+ → R–COOH + NH4+ (acidic hydrolysis)
- R–CN + H2O + OH− → R–COO− + NH3 (basic hydrolysis)
- From Amides:
- R–CONH2 + H2O + H+ → R–COOH + NH4+
From Grignard Reagents
Grignard reagents (RMgX) react with carbon dioxide to form the magnesium salt of a carboxylic acid, which yields the acid upon hydrolysis.

Steps:
- RMgX + CO2 → RCOOMgX
- RCOOMgX + H3O+ → RCOOH + Mg2+ + X−
For Example: CH3MgBr + CO2 → CH3COOH (after hydrolysis)
From Acyl Halides and Acid Anhydrides
Both acyl halides and acid anhydrides undergo hydrolysis to form carboxylic acids.
- From Acyl Halides (RCOCl): RCOCl + H2O → RCOOH + HCl
- This reaction can be carried out in alkaline conditions, forming a carboxylate salt which can then be protonated using a dilute acid to obtain the carboxylic acid.
- From Acid Anhydrides: (RCO)2O + H2O → 2RCOOH

From Esters
Esters undergo hydrolysis in acidic or basic conditions to form the corresponding carboxylic acid and an alcohol.
- Acidic Hydrolysis: RCOOR′ + H2O (H+/heat) → RCOOH + R′OH

- Basic Hydrolysis (Saponification): RCOOR′ + OH− → RCOO− + R′OH, then acidify to get RCOOH

Summary
- Primary alcohols and aldehydes oxidise to carboxylic acids.
- Alkylbenzenes oxidise at the side chain to benzoic acids.
- Nitriles and amides hydrolyse to give acids.
- RMgX + CO2 followed by hydrolysis gives acids.
- Acyl halides, anhydrides, and esters hydrolyse to acids.