Edexcel International A Level (IAL) Chemistry (YCH11) - Unit 4 - 15.11 Preparation methods-Study Notes - New Syllabus

15.11 Preparation of Carboxylic Acids

Carboxylic acids can be prepared by several important organic reactions, including:

• Oxidation of primary alcohols
• Oxidation of aldehydes
• Hydrolysis of nitriles

1. Preparation from Primary Alcohols

Primary alcohols can be oxidised first to aldehydes and then further oxidised to carboxylic acids.

Oxidising Agent

Acidified potassium dichromate(VI), \( \mathrm{K_2Cr_2O_7/H^+} \)

Conditions

• Heat under reflux to ensure complete oxidation.
• Excess oxidising agent used.

General Equation

\( \mathrm{RCH_2OH + 2[O] \rightarrow RCOOH + H_2O} \)

Example

\( \mathrm{CH_3CH_2OH + 2[O] \rightarrow CH_3COOH + H_2O} \)

Observation

• Dichromate(VI) solution changes:

Orange → Green

2. Preparation from Aldehydes

Aldehydes are readily oxidised to carboxylic acids.

Oxidising Agents

• Acidified dichromate(VI)
• Tollens’ reagent
• Fehling’s/Benedict’s solution

General Equation

\( \mathrm{RCHO + [O] \rightarrow RCOOH} \)

Example

\( \mathrm{CH_3CHO + [O] \rightarrow CH_3COOH} \)

Reason Aldehydes Oxidise Easily

• Carbonyl carbon is attached to a hydrogen atom.
• Oxidation does not require breaking C–C bonds.

3. Preparation from Nitriles

Nitriles can be hydrolysed to produce carboxylic acids.

Reagents and Conditions

• Dilute acid
• Heat under reflux

General Equation

\( \mathrm{RCN + 2H_2O + H^+ \rightarrow RCOOH + NH_4^+} \)

Example

\( \mathrm{CH_3CN + 2H_2O + H^+ \rightarrow CH_3COOH + NH_4^+} \)

Importance of Nitrile Hydrolysis

• Increases carbon chain length by one carbon atom.
• Useful in organic synthesis.

Summary Table

Key Features

• Primary alcohols oxidise to carboxylic acids under reflux.
• Aldehydes oxidise readily to acids.
• Nitrile hydrolysis forms carboxylic acids and increases chain length.
• Acidified dichromate(VI) is a common oxidising agent.