Carboxylic acid preparation

Methods of Preparation of Carboxylic Acids (Class 12 Chemistry)

Carboxylic acids are one of the most important functional groups in organic chemistry. They are represented by the –COOH group and are widely used in medicines, food industries, polymers, preservatives, and biochemical processes. Because of their wide applications, understanding the different methods of preparation of carboxylic acids is essential for Class 12 Board Exams, JEE, NEET, and other competitive exams.

This article explains all major preparation methods in a clear, exam-focused, and easy-to-remember way. The content is SEO optimized to help your blog rank on Google and Google Discover.

1. Oxidation of Primary Alcohols and Aldehydes

The most common laboratory method to prepare carboxylic acids is the oxidation of primary alcohols and aldehydes. When a primary alcohol is oxidized, it first forms an aldehyde and then gets converted into a carboxylic acid.

CH₃CH₂OH → CH₃CHO → CH₃COOH

Common oxidizing agents used:

• Potassium permanganate (KMnO₄)
• Potassium dichromate (K₂Cr₂O₇) + H₂SO₄
• Jones reagent (CrO₃)
• Oxygen in presence of catalysts

This method is widely used because alcohols are cheap and easily available. Aldehydes undergo oxidation even more easily.

2. Preparation of Carboxylic Acids from Nitriles (Cyanides)

This method is very useful because it increases the carbon chain by one carbon atom. Nitriles on hydrolysis give carboxylic acids.

R–CN + 2H₂O → R–COOH + NH₃

Types of hydrolysis:

• Acidic hydrolysis (HCl or H₂SO₄)
• Basic hydrolysis (NaOH or KOH)

This method is extremely important in organic synthesis because it helps in chain extension reactions.

3. Oxidation of Alkyl Benzenes

Alkyl benzenes, such as toluene or ethylbenzene, when oxidized strongly, give only one product: benzoic acid. The entire side chain gets oxidized irrespective of its length.

C₆H₅–CH₃ →(KMnO₄)→ C₆H₅COOH

Condition: At least one benzylic hydrogen must be present.

4. Hydrolysis of Acyl Halides

Acyl chlorides react vigorously with water to produce carboxylic acids. The reaction is fast and exothermic.

RCOCl + H₂O → RCOOH + HCl

This method is widely used in organic synthesis laboratories.

5. Hydrolysis of Acid Anhydrides

Acid anhydrides produce carboxylic acids on reaction with water.

(RCO)₂O + H₂O → 2RCOOH

Acetic anhydride, for example, gives two molecules of acetic acid.

6. Hydrolysis of Esters (Acidic and Basic)

Esters are one of the most common starting materials because they are stable and easily available.

Acidic Hydrolysis

Ester + Water → Carboxylic acid + Alcohol

Basic Hydrolysis (Saponification)

RCOOR' + NaOH → RCOONa + R'OH →(H⁺)→ RCOOH

This method is used in soap formation, fat hydrolysis, and industrial organic processes.

7. Carboxylation of Grignard Reagents

Grignard reagents are strong nucleophiles. When they react with dry ice (solid CO₂), they form magnesium salts of carboxylic acids, which on hydrolysis give carboxylic acids.

R–MgX + CO₂ → RCOOMgX →(H⁺)→ RCOOH

This method also increases the carbon chain by one carbon atom, making it highly valuable in organic synthesis.

8. Oxidative Cleavage of Alkenes

Alkenes undergo oxidative cleavage with ozonolysis or hot alkaline KMnO₄ to form aldehydes, ketones, and acids depending on their structure.

If the double-bonded carbon has a hydrogen, the product is a carboxylic acid.

Example:

CH₃–CH=CH₂ → CH₃COOH + CO₂

Summary of All Methods

• Oxidation of primary alcohols and aldehydes
• Hydrolysis of nitriles
• Oxidation of alkyl benzenes
• Hydrolysis of acyl halides
• Hydrolysis of acid anhydrides
• Hydrolysis of esters
• Carboxylation of Grignard reagents
• Oxidative cleavage of alkenes

These reactions form the backbone of Class 12 Organic Chemistry and frequently appear in board examinations, JEE Main, and NEET.

FAQs – Preparation of Carboxylic Acids

Q1. Which is the best method for chain extension?
Hydrolysis of nitriles and Grignard reagent carboxylation both increase the carbon chain by one carbon.

Q2. Which method is used industrially?
Oxidation of alcohols and hydrocarbons is commonly used industrially because of low cost.

Q3. Which reagents convert alcohols to carboxylic acids?
KMnO₄, K₂Cr₂O₇, and CrO₃ are most commonly used.

Q4. Does ozonolysis always give carboxylic acid?
Only if the double-bonded carbon contains a hydrogen atom.

Q5. Which method is the fastest?
Acyl chlorides react instantly with water to give carboxylic acids.