Carboxylic Acids (Higher Tier Overview)

Structure of Carboxylic Acids

Carboxylic acids contain the carboxyl functional group, which is written as -COOH. This group consists of a carbon atom double bonded to an oxygen atom and also bonded to a hydroxyl group (-OH).

The general molecular formula for carboxylic acids is C_nH_2nO₂. This means for every n carbon atoms, there are 2n hydrogen atoms and 2 oxygen atoms.

Common examples include:

• Methanoic acid (HCOOH), the simplest carboxylic acid, with one carbon atom.
• Ethanoic acid (CH₃COOH), also known as acetic acid, which is the main acid in vinegar.

The carboxyl group is responsible for the characteristic acidic properties of these compounds.

Properties of Carboxylic Acids

Carboxylic acids are weak acids. Unlike strong acids (e.g., hydrochloric acid), they only partially dissociate in water, releasing some hydrogen ions (H⁺) but not completely.

They have a sour taste and are corrosive, so care must be taken when handling concentrated solutions.

Carboxylic acids are soluble in water because they can form hydrogen bonds with water molecules. Their solutions are acidic due to the partial release of H⁺ ions.

Reactions of Carboxylic Acids

Carboxylic acids undergo several important reactions:

Reaction with Metals

Carboxylic acids react with reactive metals such as magnesium or zinc to produce a salt and hydrogen gas.

The general equation is:

$$\text{Carboxylic acid} + \text{Metal} \rightarrow \text{Salt} + \text{Hydrogen gas}$$

For example, ethanoic acid reacts with magnesium:

$$\mathrm{2CH_3COOH + Mg \rightarrow (CH_3COO)_2Mg + H_2}$$

The salt formed is magnesium ethanoate.

Reaction with Carbonates

Carboxylic acids react with metal carbonates (e.g., sodium carbonate) to produce a salt, carbon dioxide, and water. This reaction is often used to test for acids.

General equation:

$$\text{Carboxylic acid} + \text{Carbonate} \rightarrow \text{Salt} + \mathrm{CO_2} + \mathrm{H_2O}$$

Example with ethanoic acid and sodium carbonate:

$$\mathrm{2CH_3COOH + Na_2CO_3 \rightarrow 2CH_3COONa + CO_2 + H_2O}$$

The salt formed is sodium ethanoate.

Reaction with Alcohols (Esterification)

Carboxylic acids react with alcohols to form esters and water. This reaction is called esterification and requires an acid catalyst, usually concentrated sulfuric acid.

General equation:

$$\text{Carboxylic acid} + \text{Alcohol} \xrightarrow{\text{acid catalyst}} \text{Ester} + \mathrm{H_2O}$$

For example, ethanoic acid reacts with ethanol to form ethyl ethanoate and water:

$$\mathrm{CH_3COOH + C_2H_5OH \xrightarrow{H_2SO_4} CH_3COOC_2H_5 + H_2O}$$

Esters have distinctive fruity smells and are used in perfumes and flavourings.

For instance, if 1 mole of ethanoic acid reacts with 1 mole of ethanol, 1 mole of ethyl ethanoate and 1 mole of water are produced, showing the 1:1 mole ratio in esterification.

Uses of Carboxylic Acids

Carboxylic acids have many practical uses:

• Food preservation: Ethanoic acid is the main component of vinegar, used to preserve foods by creating an acidic environment that inhibits bacterial growth.
• Making esters: Esters formed from carboxylic acids and alcohols are used in perfumes, flavourings, and solvents because of their pleasant smells.
• Industrial uses: Carboxylic acids are used as starting materials in the manufacture of polymers, such as polyesters, which are important in textiles and plastics.