NSWChemistrySyllabus dot point

Inquiry Question 5: How are esters formed, what are their properties, and how are they used?

Investigate the structural formulae, properties, applications, formation by esterification, and hydrolysis (including saponification) of esters

Q: 2019 HSC HSC: Outline the difference between acid hydrolysis and base hydrolysis (saponification) of an ester, using methyl ethanoate as the example. Write equations for both.

**Acid hydrolysis** is the reverse of esterification. Dilute $H_2SO_4$ catalyst, reflux: $$CH_3COOCH_3 + H_2O \underset{}{\overset{H_2SO_4}{\rightleftharpoons}} CH_3COOH + CH_3OH$$ The reaction is **reversible** and reaches equilibrium. Products are the carboxylic acid and the alcohol. **Base hydrolysis (saponification)** uses aqueous NaOH, reflux: $$CH_3COOCH_3 + NaOH \rightarrow CH_3COONa + CH_3OH$$ The reaction is **irreversible** because the carboxylate salt $CH_3COO^-$ cannot react back with the alcohol. Products are the sodium salt of the acid (a soap if the acid is a long fatty acid) and the alcohol. **Key differences.** Acid hydrolysis is an equilibrium that gives the free acid; base hydrolysis goes to completion and gives the carboxylate salt. The name saponification reflects soap-making from fats and oils (long-chain esters) with NaOH. Markers reward (1) both balanced equations with correct reversibility arrows, (2) acid hydrolysis as equilibrium, (3) saponification as irreversible because of the salt.

A focused answer to the HSC Chemistry Module 7 dot point on esters. Naming as alkyl alkanoates, the equilibrium esterification with concentrated H2SO4 catalyst, acid and base hydrolysis (saponification), applications as flavours, fragrances and biodiesel, and worked HSC past exam questions.

Generated by Claude OpusReviewed by Better Tuition Academy9 min answerUpdated 2026-05-18

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What this dot point is asking

NESA wants you to name esters as alkyl alkanoate, write the esterification equation between an alcohol and a carboxylic acid with concentrated $H_2SO_4$ catalyst, describe the reflux procedure, list common applications of esters, and write both acid and base hydrolysis equations including saponification of long-chain esters to make soap.

The answer

Structure and naming

An ester contains the linkage $-COO-$ . The general formula is $R-COO-R'$ , where $R$ comes from the carboxylic acid and $R'$ comes from the alcohol.

Esters are named in two words: alkyl (from the alcohol) alkanoate (from the acid, including the carbonyl carbon).

Ester	Acid + alcohol	Smell/use
methyl ethanoate IMATH_9	ethanoic acid + methanol	solvent
ethyl ethanoate IMATH_10	ethanoic acid + ethanol	nail polish remover, pear
methyl butanoate IMATH_11	butanoic acid + methanol	apple
pentyl ethanoate IMATH_12	ethanoic acid + pentan-1-ol	banana
octyl ethanoate IMATH_13	ethanoic acid + octan-1-ol	orange

To work backwards from an ester to its parent acid and alcohol: split at the single-bonded $C-O$ , add $H$ to the acid oxygen and add $OH$ to the alcohol carbon.

Esterification (Fischer esterification)

An alcohol reacts with a carboxylic acid in the presence of a concentrated $H_2SO_4$ catalyst, heated under reflux, to give an ester and water:

R-COOH + R'-OH \underset{}{\overset{H_2SO_4, \text{reflux}}{\rightleftharpoons}} R-COO-R' + H_2O

The reaction is reversible and reaches equilibrium, typically with 60 to 70% conversion. Concentrated $H_2SO_4$ has two roles:

Catalyst: protonates the carbonyl oxygen of the acid, making the carbonyl carbon more electrophilic and easier for the alcohol to attack.
Dehydrating agent: absorbs the water byproduct, shifting equilibrium right by Le Chatelier's principle.

Reflux is essential because the reactants are volatile (boiling points 65 to 120 degrees C). Reflux returns evaporated reactants to the flask, so the mixture stays at temperature for long enough to reach equilibrium without losing material.

Purification. Pour the cooled mixture into saturated sodium hydrogencarbonate to neutralise unreacted acid (effervescence stops when complete). Separate the organic layer, dry over anhydrous $MgSO_4$ , then distil at the boiling point of the ester.

Physical properties of esters

Esters have a polar $C=O$ but no $O-H$ , so they cannot hydrogen-bond to each other. Boiling points are lower than the parent acid and alcohol of similar molar mass, comparable to ketones. Small esters are volatile liquids with characteristic fruity smells.

Solubility in water decreases sharply with chain length: methyl ethanoate is somewhat soluble, ethyl ethanoate has limited solubility (about 8% w/w), and esters above C6 are essentially insoluble. Esters are good solvents for non-polar organic compounds (paints, varnishes, glues).

Applications

Flavours and fragrances: short-chain esters give fruits their characteristic smells. Synthetic esters are added to lollies, drinks, perfumes and air fresheners.
Solvents: ethyl ethanoate is the active solvent in nail polish remover; butyl ethanoate is used in lacquers.
Biodiesel: methyl esters of long-chain fatty acids ( $C_{16}$ to $C_{18}$ ) are biodiesel, made by transesterification of vegetable oil with methanol and a NaOH catalyst.
Plasticisers: dialkyl phthalate esters soften PVC.
Fats and oils: triesters of glycerol with fatty acids (triglycerides) are biological lipids.

Hydrolysis: the reverse reactions

Acid hydrolysis. Reflux the ester with dilute sulfuric acid; the reverse of esterification:

R-COOR' + H_2O \underset{}{\overset{H_2SO_4}{\rightleftharpoons}} R-COOH + R'-OH

Reversible, reaches equilibrium. Useful to identify an unknown ester by isolating its acid and alcohol fragments.

Base hydrolysis (saponification). Reflux the ester with aqueous NaOH:

R-COOR' + NaOH \rightarrow R-COONa + R'-OH

Irreversible because the carboxylate anion $R-COO^-$ is unreactive and cannot recombine with the alcohol. The reaction goes to completion, giving the sodium salt of the carboxylic acid and the alcohol.

When the ester is a triglyceride (the natural form of fats and oils), saponification with NaOH gives glycerol plus three sodium fatty-acid salts, which are soap:

\text{Triglyceride} + 3 NaOH \rightarrow 3 R-COONa + \text{glycerol}

This is the chemistry of soap-making: hot fat or oil is stirred with sodium hydroxide solution, the soap is salted out with brine, washed, and pressed into bars. KOH gives softer potassium soaps (liquid soaps).

Mechanism in brief

Fischer esterification is acid-catalysed nucleophilic acyl substitution. The acid is protonated on the carbonyl O, the alcohol O attacks the carbonyl C, proton transfers occur, water leaves, and a proton is lost from the protonated ester. You do not need the full curly-arrow mechanism for HSC, but you do need to know what each reactant contributes.

Common traps

Naming esters in the wrong order. Always alkyl (from alcohol) first, then alkanoate (from acid). Do not write "ethanoate ethyl".

Forgetting the equilibrium arrows. Acid hydrolysis and esterification are reversible. Saponification is not. Get the arrows right.

Using dilute $H_2SO_4$ for esterification. You need concentrated $H_2SO_4$ , both as catalyst and dehydrator. Dilute acid does not drive the equilibrium.

Skipping the bicarbonate wash. Without it, the product is contaminated with unreacted carboxylic acid and traces of $H_2SO_4$ .

Confusing transesterification with esterification. Biodiesel is made by transesterification (swapping the alcohol on an existing ester), not by direct esterification of fatty acid plus methanol.

In one sentence

An ester $R-COO-R'$ forms when a carboxylic acid and an alcohol are heated under reflux with concentrated $H_2SO_4$ catalyst (Fischer esterification, reversible), and the ester can be hydrolysed back to its components with dilute acid (reversible) or with NaOH (irreversible, saponification, the basis of soap making).

Past exam questions, worked

Real questions from past NESA papers on this dot point, with our answer explainer.

2022 HSC5 marksDescribe an experimental procedure to prepare ethyl ethanoate in the laboratory. Include a balanced equation, the catalyst, the reflux setup, and how the product is isolated and identified.

Show worked answer →

A 5 mark answer needs the equation, the role of the catalyst, the apparatus, and the purification and identification steps.

Equation:

CH_3COOH_{(l)} + C_2H_5OH_{(l)} \underset{}{\overset{H_2SO_4}{\rightleftharpoons}} CH_3COOC_2H_{5(l)} + H_2O_{(l)}

Procedure.

In a round-bottomed flask, mix about 10 mL ethanoic acid and 10 mL ethanol. Add 5 mL concentrated $H_2SO_4$ slowly while swirling and cooling. Add anti-bumping granules.
Attach a reflux condenser and heat with a water bath or heating mantle at about 70 degrees C for 20 minutes. Reflux returns volatile reactants to the flask while the reaction reaches equilibrium.
After cooling, transfer to a separating funnel and add saturated $NaHCO_3$ solution to neutralise excess acid (until no more $CO_2$ effervescence).
Discard the aqueous layer. Dry the organic layer over anhydrous $MgSO_4$ .
Distil at 77 degrees C (the boiling point of ethyl ethanoate) to isolate the pure product.

Identification. Sweet, fruity, pear-drop smell. Confirm by boiling point and refractive index.

Role of catalyst. Concentrated $H_2SO_4$ catalyses the reaction by protonating the carbonyl and also absorbs the water byproduct, shifting equilibrium right (Le Chatelier).

Markers reward (1) the balanced equation with reversible arrows, (2) reflux setup with diagram or description, (3) the catalyst role (both proton donor and dehydrator), (4) purification with bicarbonate wash, (5) identification by smell and boiling point.

2019 HSC3 marksOutline the difference between acid hydrolysis and base hydrolysis (saponification) of an ester, using methyl ethanoate as the example. Write equations for both.

Show worked answer →

Acid hydrolysis is the reverse of esterification. Dilute $H_2SO_4$ catalyst, reflux:

CH_3COOCH_3 + H_2O \underset{}{\overset{H_2SO_4}{\rightleftharpoons}} CH_3COOH + CH_3OH

The reaction is reversible and reaches equilibrium. Products are the carboxylic acid and the alcohol.

Base hydrolysis (saponification) uses aqueous NaOH, reflux:

CH_3COOCH_3 + NaOH \rightarrow CH_3COONa + CH_3OH

The reaction is irreversible because the carboxylate salt $CH_3COO^-$ cannot react back with the alcohol. Products are the sodium salt of the acid (a soap if the acid is a long fatty acid) and the alcohol.

Key differences. Acid hydrolysis is an equilibrium that gives the free acid; base hydrolysis goes to completion and gives the carboxylate salt. The name saponification reflects soap-making from fats and oils (long-chain esters) with NaOH.

Markers reward (1) both balanced equations with correct reversibility arrows, (2) acid hydrolysis as equilibrium, (3) saponification as irreversible because of the salt.