Describe the properties and reactions of carboxylic acids, and explain esterification and hydrolysis of esters

What this dot point is asking

A carboxylic acid contains the carboxyl group, -COOH. An ester contains the -COO- linkage and is formed from a carboxylic acid and an alcohol. The two families are closely related through esterification and hydrolysis.

Properties and reactions of carboxylic acids

Carboxylic acids are weak acids: they only partially ionise in water.

They show typical acid reactions:

• With bases: neutralisation to give a salt and water.
• With reactive metals: producing a salt and hydrogen gas.
• With carbonates and hydrogen carbonates: producing a salt, water and carbon dioxide (effervescence), a useful test for the -COOH group.

The -COOH group hydrogen bonds strongly, so carboxylic acids have high boiling points and the smaller ones are very soluble in water.

Esterification

An ester forms when a carboxylic acid reacts with an alcohol, with concentrated sulfuric acid as catalyst:

Because esterification is an equilibrium, the yield is incomplete. By Le Chatelier's principle, using an excess of one reactant or removing the water (or ester) as it forms shifts the equilibrium towards the ester and improves the yield. This is a direct link back to the equilibrium chemistry of Unit 3.

Hydrolysis of esters

Hydrolysis is the reverse of esterification, splitting the ester with water.

• Acid hydrolysis: with water and an acid catalyst, the ester reverts to the carboxylic acid and the alcohol (an equilibrium).
• Base hydrolysis (saponification): with hot aqueous sodium hydroxide, the ester is split into the sodium salt of the carboxylic acid (a carboxylate) and the alcohol. This goes to completion because the carboxylate ion does not re-esterify, and it is the basis of soap making.

Why carboxylic acids are weak yet detectable

The acidity of carboxylic acids comes from the stability of the carboxylate ion left after the proton leaves: the negative charge is spread (delocalised) over both oxygen atoms, which stabilises the ion and favours ionisation more than for an alcohol, whose $\text{O-H}$ does not ionise appreciably. Even so, carboxylic acids are only weak acids, so they partially ionise and have $\text{p}K_a$ values around 4 to 5. A reliable distinguishing test exploits their reaction with carbonates: only a compound acidic enough, such as a carboxylic acid, fizzes with sodium hydrogen carbonate to release carbon dioxide, $\text{CH}_3\text{COOH} + \text{NaHCO}_3 \rightarrow \text{CH}_3\text{COONa} + \text{H}_2\text{O} + \text{CO}_2$. Phenols and alcohols do not, so the effervescence test confirms the $\text{-COOH}$ group.

Uses and why this matters

Esters are responsible for many fruit flavours and floral scents and are used as solvents and in fragrances; larger esters form fats, oils and biodiesel. The esterification equilibrium ties organic synthesis to the equilibrium principles of Unit 3, and ester hydrolysis underlies soap manufacture and the digestion of fats.