Describe the acidity of carboxylic acids and the esterification and hydrolysis of esters.

What this dot point is asking

TASC expects you to describe the acidity and reactions of carboxylic acids, and the formation and hydrolysis of esters, with correct equations.

Carboxylic acids as weak acids

The carboxyl group $-\text{COOH}$ donates a proton to give a carboxylate ion, but only partly, so carboxylic acids are weak acids:

They are weak because the equilibrium lies well to the left, although the carboxylate ion is stabilised by delocalisation of its negative charge over both oxygen atoms, which makes carboxylic acids more acidic than alcohols. They show the usual acid reactions:

• with a base: salt and water, for example $\text{CH}_3\text{COOH} + \text{NaOH} \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O}$;
• with a carbonate or hydrogencarbonate: salt, water and carbon dioxide (effervescence);
• with a reactive metal: salt and hydrogen gas.

The effervescence test with sodium carbonate distinguishes a carboxylic acid (reacts) from a phenol or alcohol (no reaction).

Esterification

When a carboxylic acid is heated with an alcohol and a concentrated sulfuric acid catalyst, an ester forms and water is eliminated. This is a condensation reaction.

The reaction is reversible and reaches equilibrium, so it does not go to completion. Esters often have sweet, fruity smells and are used as flavourings, fragrances and solvents.

Hydrolysis of esters

Esters can be split back into their components by hydrolysis:

• Acid hydrolysis uses water with an acid catalyst and simply reverses esterification, giving the carboxylic acid and the alcohol. Because it is the reverse of an equilibrium reaction, it does not go to completion.
• Base hydrolysis (saponification) uses sodium hydroxide and gives the carboxylate salt and the alcohol. It goes to completion because the carboxylate salt cannot react back to the ester. Saponification of fats and oils produces soap.

Structure and properties

A carboxylic acid molecule can hydrogen bond strongly (it has both $\text{O}-\text{H}$ and $\text{C}=\text{O}$), so carboxylic acids have high boiling points and the smaller ones are very soluble in water. Esters cannot donate a hydrogen bond (they have no $\text{O}-\text{H}$), so for similar size they boil lower than the parent acid and are less soluble in water, which is why esters separate as an oily layer.

In the exam, show esterification as a reversible reaction with water as a product and a sulfuric acid catalyst, name the ester alcohol-part first, and distinguish acid hydrolysis (reversible) from base hydrolysis (complete).