Carboxylic acids and esters (A-level only)

What you'll learn:

• How carboxylic acids behave as weak acids.
• How to synthesise esters from alcohols and carboxylic acids, and how to name them.
• The critical differences between acid-catalysed and alkaline hydrolysis of esters.
• The structure of fats and oils, and how they are used to make soap and biodiesel.

Carboxylic acids as weak acids

Carboxylic acids contain the functional group $-COOH$. The oxygen atoms in the carboxyl group pull electron density away from the hydrogen atom, allowing the $O-H$ bond to break and release an $H^{+}$ ion.

However, carboxylic acids are weak acids. This means they only partially dissociate in water. For example, ethanoic acid sets up the following equilibrium:

$${CH}_{3}COOH(aq)\rightleftharpoons {CH}_3{COO}^{-}(aq)+H^{+}(aq)$$

Because they are acids, they will react with bases, but their hallmark reaction in A-Level Chemistry is their reaction with carbonates. Carboxylic acids are strong enough to liberate carbon dioxide gas from metal carbonates (like sodium carbonate, ${Na}_2{CO}_3$) or hydrogencarbonates (like ${NaHCO}_3$).

$$2{CH}_{3}COOH+{Na}_2{CO}_3\to 2{CH}_{3}COONa+H_{2}O+{CO}_2$$

Making esters: Esterification

When you heat a carboxylic acid with an alcohol in the presence of a strong acid catalyst (usually concentrated sulfuric acid, $H_2{SO}_4$), they react to form an ester and water.

The reaction is reversible and reaches a dynamic equilibrium.

$$Carboxylic\; Acid+Alcohol\rightleftharpoons Ester+Water$$

Below is the reaction between ethanoic acid and ethanol to form ethyl ethanoate. Notice that the $-OH$ group is lost from the carboxylic acid, and the $H$ atom is lost from the alcohol.

Naming esters

Esters contain the functional group $-COO-$. Their names are made of two parts:

1. The first part comes from the alcohol and ends in -yl (e.g., methanol $\to$ methyl).
2. The second part comes from the carboxylic acid and ends in -oate (e.g., propanoic acid $\to$ propanoate).

Uses of esters

Esters have distinctive physical properties that make them incredibly useful in industry:

• Perfumes and food flavourings: Short-chain esters are volatile liquids with sweet, fruity smells.
• Solvents: Esters are polar liquids but lack hydrogen bonding between their own molecules, giving them low boiling points. They make excellent volatile solvents for polar organic compounds (e.g., ethyl ethanoate is used in nail varnish remover and glues).
• Plasticisers: Esters are added to rigid polymers (like PVC) during manufacturing. The ester molecules slip between the polymer chains, pushing them apart and weakening the intermolecular forces. This makes the plastic flexible.

Hydrolysis of esters

Hydrolysis is the exact reverse of esterification. You are using water to break the ester bond ($-COO-$) apart. Because water alone is very slow, hydrolysis is done under either acidic or alkaline conditions. The choice of condition changes the final products.

1. Acid hydrolysis

Heating an ester under reflux with a dilute acid (like dilute $HCl$ or $H_2{SO}_4$) splits the ester back into its parent carboxylic acid and alcohol. Because the reaction is reversible, you never get a 100% yield; it reaches equilibrium.

$${CH}_3{COOCH}_2{CH}_3+H_{2}O\rightleftharpoons {CH}_{3}COOH+{CH}_3{CH}_{2}OH$$

2. Alkaline hydrolysis

Heating an ester under reflux with a dilute alkali (like aqueous $NaOH$) splits the ester into an alcohol and a carboxylate salt. Because the carboxylic acid that would be formed immediately reacts with the alkali to form a salt, the products cannot recombine. The reaction is irreversible and goes to completion, making it the preferred method in industry.

$${CH}_3{COOCH}_2{CH}_3+NaOH\to {CH}_{3}COONa+{CH}_3{CH}_{2}OH$$

Fats, oils, and soaps

Animal fats and vegetable oils are naturally occurring esters. Specifically, they are esters made from long-chain carboxylic acids (often called fatty acids) and a single alcohol that contains three $-OH$ groups.

This alcohol is propane-1,2,3-triol, commonly known as glycerol. Because glycerol has three $-OH$ groups, it can form three ester bonds with three fatty acid molecules. The resulting molecule is called a triglyceride.

Saponification (Making soap)

If you hydrolyse a fat or oil under alkaline conditions (heating with concentrated $NaOH$), the three ester linkages break.

This produces:

• One molecule of glycerol (propane-1,2,3-triol).
• Three molecules of the sodium salt of the long-chain carboxylic acid.

These sodium salts of long-chain fatty acids are exactly what soap is.

$$Triglyceride+3NaOH\to Glycerol+3Sodium\; Carboxylate\; Salts\; (Soap)$$

Biodiesel

Vegetable oils are renewable but too viscous to be used directly in standard diesel engines. However, they can be converted into biodiesel, which flows much more easily.

To make biodiesel, vegetable oils (triglycerides) are reacted with methanol in the presence of a strong alkaline catalyst (like potassium hydroxide). This reaction swaps the heavy glycerol backbone for three small methyl groups.

Because we are swapping one alcohol (glycerol) out of the ester for another alcohol (methanol), this specific type of reaction is called transesterification.

$$Triglyceride+3{CH}_{3}OH\underrightarrow{KOH\; catalyst}Glycerol+3Methyl\; Esters\; (Biodiesel)$$