18.1 Carboxylic Acids

2026 Syllabus Objectives

By the end of these notes, you should be able to:

  1. Recall the reactions by which carboxylic acids can be produced:

    • (a) Oxidation of primary alcohols and aldehydes with acidified K₂Cr₂O₇ or acidified KMnO₄ and refluxing
    • (b) Hydrolysis of nitriles with dilute acid or dilute alkali followed by acidification
    • (c) Hydrolysis of esters with dilute acid or dilute alkali and heat followed by acidification

  2. Describe:

    • (a) The redox reaction with reactive metals to produce a salt and H₂(g)
    • (b) The neutralisation reaction with alkalis to produce a salt and H₂O(l)
    • (c) The acid–base reaction with carbonates to produce a salt and H₂O(l) and CO₂(g)
    • (d) Esterification with alcohols with concentrated H₂SO₄ as catalyst
    • (e) Reduction by LiAlH₄ to form a primary alcohol

Main Content

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What are Carboxylic Acids?

Carboxylic acids are organic compounds that contain a special functional group called the carboxyl group (-COOH). This group is made up of a carbon atom double-bonded to one oxygen atom (C=O) and single-bonded to a hydroxyl group (-OH).

The general structure is: R-COOH (where R represents the rest of the carbon chain)

Examples:

  • Methanoic acid (HCOOH) - 1 carbon
  • Ethanoic acid (CH₃COOH) - 2 carbons (found in vinegar)
  • Propanoic acid (CH₃CH₂COOH) - 3 carbons

Carboxylic acids are weak acids. This means they only partially split apart (dissociate) in water to release hydrogen ions (H⁺).

For example: CH₃COOH + H₂O ⇌ CH₃COO⁻ + H₃O⁺

Notice the reversible arrow (⇌). This shows that most of the acid molecules stay intact, with only a small amount releasing H⁺ ions. This is why carboxylic acids have a pH less than 7, but not as low as strong acids like hydrochloric acid.

How Carboxylic Acids are Produced

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1(a). Oxidation of Primary Alcohols and Aldehydes

Carboxylic acids can be made by oxidation - a chemical process where oxygen is added or hydrogen is removed from a molecule.

What can be oxidized to make carboxylic acids?

  • Primary alcohols (alcohols where the -OH group is attached to a carbon with only one other carbon attached)
  • Aldehydes (compounds with a -CHO group)

What oxidizing agents are used?

Two common oxidizing agents are used:

  1. Acidified potassium dichromate(VI) - written as K₂Cr₂O₇/H⁺ or acidified K₂Cr₂O₇

    • Color change: orange to green
    • The orange dichromate ions (Cr₂O₇²⁻) get reduced to green chromium(III) ions (Cr³⁺)

  2. Acidified potassium manganate(VII) - written as KMnO₄/H⁺ or acidified KMnO₄

    • Color change: purple to colorless
    • The purple manganate ions (MnO₄⁻) get reduced to colorless manganese(II) ions (Mn²⁺)

What conditions are needed?

Reflux - This is a special technique where the reaction mixture is continuously boiled and the vapors are condensed back into the flask. This ensures:

  • Complete reaction without losing any volatile substances
  • The reaction goes to completion to form the carboxylic acid

Why reflux? If you don't use reflux, volatile products like aldehydes might evaporate away before they can be oxidized further.

Oxidation pathway for primary alcohols:

Step 1: Primary alcohol → Aldehyde (partial oxidation) Step 2: Aldehyde → Carboxylic acid (complete oxidation)

Example:

Ethanol → Ethanal → Ethanoic acid

CH₃CH₂OH + [O] → CH₃CHO + H₂O

CH₃CHO + [O] → CH₃COOH

Overall: CH₃CH₂OH + 2[O] → CH₃COOH + H₂O

(The symbol [O] represents the oxidizing agent)

Oxidation of aldehydes:

Aldehydes are easily oxidized to carboxylic acids using the same reagents and reflux conditions.

Example:

Propanal → Propanoic acid

CH₃CH₂CHO + [O] → CH₃CH₂COOH

Key Point: You need to heat under reflux with an excess of the oxidizing agent to ensure the primary alcohol or aldehyde is completely converted to the carboxylic acid.

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