Esterification and hydrolysis (SACE Stage 2 Chemistry)

What this dot point is asking

SACE expects you to write equations, name esters and their products, explain the equilibrium and how to drive the yield, and distinguish acid from base hydrolysis.

Lead worked calculation

Esterification

The concentrated sulfuric acid plays two roles: it catalyses the reaction and it acts as a dehydrating agent, removing water to shift the equilibrium toward the ester.

Naming esters

An ester name has two words. The first is the alkyl group from the alcohol (methyl, ethyl, propyl, ...). The second is the acid stem with -oate (methanoate, ethanoate, propanoate, ...). So ethanol plus ethanoic acid gives ethyl ethanoate. Reading an ester structure $\text{R-COO-R'}$: the $\text{R'}$ on the single-bonded oxygen came from the alcohol (alkyl, first word), and the $\text{R-COO}$ part came from the acid (-oate, second word).

Driving the equilibrium

Because esterification is an equilibrium, the yield can be raised using Le Chatelier's principle:

Hydrolysis: the reverse

Hydrolysis splits an ester apart using water. There are two routes:

• Acid hydrolysis: water with a dilute acid catalyst reverses esterification, giving the carboxylic acid and the alcohol. It is an equilibrium, so it does not go to completion. $\text{R-COO-R'} + \text{H}_2\text{O} \rightleftharpoons \text{R-COOH} + \text{R'-OH}$.
• Base hydrolysis (saponification): warming with aqueous $\text{NaOH}$ gives the carboxylate salt and the alcohol, and goes essentially to completion (irreversible) because the carboxylate ion does not react back. $\text{R-COO-R'} + \text{NaOH} \rightarrow \text{R-COO}^-\text{Na}^+ + \text{R'-OH}$.

Saponification is the industrial process for making soap from fats (esters of long-chain acids).

Why it matters

Esters give fruits and flowers their scents and are used as flavourings, solvents and in biodiesel. The forward reaction (condensation) and reverse (hydrolysis) are the same chemistry that builds and breaks down the lipids and proteins studied next, and saponification underpins soap manufacture.