4gEsters

Separate Chemistry only.

The functional group

The ester linkage, −COO−.

Compare it with the carboxyl group of an acid, −COOH. They look almost identical, and the difference is the whole story: in an ester the acidic hydrogen has been replaced by a carbon chain.

No hydrogen on the oxygen means no H⁺ to release — which is why esters are not acidic, even though they contain the same carbon and two oxygens arranged in nearly the same way.

Making an ester

alcohol + carboxylic acid ⇌ ester + water

CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O

Ethanol and ethanoic acid give ethyl ethanoate. The reaction needs a concentrated sulfuric acid catalyst.

Water is always the second product. Forgetting it is the most common omission in esterification questions — and it is where the alcohol's −OH and the acid's −OH went.

The structure of ethyl ethanoate

Structural formula: CH₃COOC₂H₅

Molecular formula: C₄H₈O₂ — four carbons in all, two from the acid and two from the alcohol.

The ester linkage sits in the middle: the acid section on one side, the alcohol section on the other, joined through the oxygen.

Properties and uses

Esters are volatile — they evaporate easily — and have distinctive, often pleasant smells.

Those two properties together are why they are used in food flavourings and perfumes.

Volatility is not incidental. A smell can only be detected if molecules reach the nose, so a pleasant-smelling compound that never evaporated would be useless in a perfume.

Separate Chemistry only.

The naming rule

An ester's name has two words, and each comes from one reactant:

alcohol first, acid second

So ethanol + ethanoic acid → ethyl ethanoate, and propanol + methanoic acid → propyl methanoate.

Identify which reactant is the alcohol first. Its stem becomes the first word, and getting the order wrong names a completely different ester.

Working backwards

Given methyl propanoate, read it in reverse:

• methyl → made from methanol
• propanoate → made from propanoic acid

Preparing ethyl ethanoate

1. Mix ethanol with ethanoic acid in a test tube or flask.
2. Add a few drops of concentrated sulfuric acid as catalyst.
3. Warm gently in a water bath, or heat under reflux.
4. Pour the mixture into sodium carbonate solution.
5. Identify the ester by its sweet, fruity smell.

Why a water bath

Ethanol and the ester are both flammable. A naked flame is a fire risk, so the heating is done indirectly.

Why sodium carbonate, not water

This is the step that carries the reasoning marks.

The mixture still contains unreacted ethanoic acid, whose sharp vinegar-like smell would mask the ester's. The sodium carbonate neutralises it — along with the sulfuric acid catalyst — so the sweet smell can be detected.

Pouring into plain water would only dilute the acid, not remove it. It is the neutralisation that matters.

(The sulfuric acid is odourless, so it is not what masks the smell; it is neutralised so the sample can be handled safely.)

Separate Chemistry only.

ethanol + ethanoic acid ⇌ ethyl ethanoate + water

Apparatus

Pear-shaped flask, reflux condenser, anti-bumping granules, water bath or electric heater, distillation apparatus, separating funnel.

Making it

1. Mix ethanol and ethanoic acid in the flask.
2. Add a few drops of concentrated sulfuric acid as catalyst, and anti-bumping granules.
3. Heat under reflux for about 20 minutes.
4. Rearrange for distillation and distil off the ester, which has the lowest boiling point of the mixture.

Ethyl ethanoate boils at 77 °C and ethanol at 78 °C — barely one degree apart, so simple distillation cannot separate them cleanly. That is exactly why the distillate is crude and has to be purified.

Purifying it

The crude distillate still contains unreacted acid and alcohol.

1. Shake with sodium carbonate solution in a separating funnel.
2. Release the pressure often — carbon dioxide is produced and the funnel builds up pressure.
3. Run off the lower aqueous layer; the ester is the upper layer.
4. Dry with an anhydrous salt, then redistil, collecting the fraction at the ester's boiling point.

Why each step

Reflux. The mixture is volatile and flammable. The condenser returns the vapour to the flask, so the mixture can be heated for a long time without boiling away — you heat without losing anything.

Water bath, not a naked flame. Ethanol and the ester are highly flammable. Vapour reaching a flame would ignite.

Sodium carbonate, not water. The unreacted ethanoic acid smells sharp and would mask the ester's smell. Carbonate neutralises it — and the sulfuric acid catalyst — and removes it into the water layer. Water would only dilute the acid, not remove it.

Anti-bumping granules. They give small, even bubbles instead of sudden violent boiling.

Identifying it

A sweet, fruity smell — the property that puts esters into perfumes and flavourings.

Why the yield is never 100%

Esterification is reversible and reaches equilibrium, so some acid and alcohol always remain. Material is lost at each transfer and in the separating funnel as well.

Safety

Where marks are lost

• Heating with a Bunsen directly.
• Washing with water instead of sodium carbonate.
• Forgetting that water is the second product of the reaction.