Working Out Esters, And What They Are For

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 4.41 — Deducing the formula of an ester from its alcohol and carboxylic acid, and the reverse (bold C statement — Paper 2 only)
• 4.42 — Esters as volatile compounds with distinctive smells, used in flavourings and perfumes (bold C statement — Paper 2 only)

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

Every ester has exactly two parents: one alcohol and one carboxylic acid. This page is about moving in both directions — given the two parents, work out the ester; given the ester, work out which two made it. It is bookkeeping rather than new chemistry, and it is worth a lot of marks.

Forwards — from the two parents to the ester

1. Take the ALCOHOL and change its ending from -anol to -YL. Methanol gives methyl, ethanol gives ethyl, propan-1-ol gives propyl.
2. Take the ACID and change its ending from -anoic acid to -ANOATE. Methanoic acid gives methanoate, ethanoic acid gives ethanoate, propanoic acid gives propanoate.
3. Write the alcohol's part FIRST and the acid's part second. Propan-1-ol with ethanoic acid gives PROPYL ETHANOATE.
4. For the formula, reverse the order: write the acid fragment first, then the alcohol's alkyl group. CH₃COO– from ethanoic acid, then C₃H₇– from propan-1-ol.
5. So propyl ethanoate is CH₃COOC₃H₇.
6. And remember the water: the equation is CH₃COOH + C₃H₇OH ⇌ CH₃COOC₃H₇ + H₂O.

Backwards — from the ester to its two parents

1. Find the –COO– group in the formula, which is the join.
2. Everything on the LEFT of it, including that carbon, came from the ACID. Add an H to the free oxygen to rebuild the –COOH.
3. Everything on the RIGHT of it came from the ALCOHOL. Add an –OH to rebuild it.
4. For methyl propanoate, C₂H₅COOCH₃: the C₂H₅COO– part rebuilds as C₂H₅COOH, propanoic acid.
5. The CH₃ on the right rebuilds as CH₃OH, methanol.
6. Check against the name: methyl from methanol, propanoate from propanoic acid. It agrees.

Think of it like reading a hyphenated place name off a signpost

A road sign reading 'Stoke-on-Trent' can be taken apart without knowing anything about the place: there is a settlement called Stoke and a river called Trent, and the joining word tells you how they relate. Nobody has to memorise the pair. An ester name works identically. Find the join, read what is on each side, and put back what was removed to make the join — an H on one side, an OH on the other. The two parents fall out of the name every time.

Deducing an ester's molecular formula

Give the structural formula and the molecular formula of the ester made from methanol and ethanoic acid, and name it.

1. Methanol is CH₃OH, so its contribution is the CH₃ group and the name-part methyl.
2. Ethanoic acid is CH₃COOH, so its contribution is CH₃COO– and the name-part ethanoate.
3. The name is METHYL ETHANOATE — alcohol first.
4. The structural formula puts the acid fragment first: CH₃COOCH₃.
5. Counting atoms: three carbons, six hydrogens, two oxygens.
6. So the molecular formula is C₃H₆O₂, and the water lost in making it is not part of the ester.

Answer: Methyl ethanoate, CH₃COOCH₃, molecular formula C₃H₆O₂. Alcohol first in the name, acid first in the formula.

Something genuinely interesting falls out of this bookkeeping. Ethyl ethanoate has the molecular formula C₄H₈O₂ — and so does methyl propanoate, and so does propyl methanoate. Three different esters, three different smells, one molecular formula between them. They are isomers, and this is the clearest everyday demonstration on the whole course that a molecular formula does not identify a compound.

Statement 4.42 is about what esters are FOR. They are VOLATILE, meaning they evaporate readily at ordinary temperatures, and they have distinctive, usually pleasant smells. Those two properties together are exactly what a flavouring or a perfume needs: a smell only reaches the nose if the molecules leave the liquid and travel through the air.

So esters are used in food flavourings and in perfumes. Many of the esters used are the same compounds that occur naturally in fruit — the smell of a pear or a banana is largely a mixture of esters the plant made itself. A synthetic flavouring is not an imitation of a natural smell so much as the same molecules, prepared in a different place. That is worth stating plainly, because 'artificial flavouring' is often assumed to mean something chemically different from the natural one, and frequently it is not.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Deducing the ester from its two parents

1. Change the alcohol's -anol ending to -yl: this is the FIRST word of the ester's name.
2. Change the acid's -anoic acid ending to -anoate: this is the SECOND word.
3. For the formula, write the acid fragment first, then the alcohol's alkyl group.
4. Add water as the second product of the reaction.
5. Check the atom count: the ester has all the atoms of both parents, less one H₂O.

Deducing the two parents from the ester

1. Locate the –COO– group, which marks the join.
2. Take everything to the left of it, including that carbon, and add an H to the free oxygen to rebuild the carboxylic acid.
3. Take everything to the right of it and add an –OH to rebuild the alcohol.
4. Check your two answers against the ester's name, which should agree.

Four esters, made and named

The vocabulary of this page

Volatile

Evaporating readily at ordinary temperatures. Esters are volatile, which is why their smell reaches the nose.

-yl

The ending taken from the alcohol, written FIRST in an ester's name. Ethanol gives ethyl.

-anoate

The ending taken from the acid, written SECOND in an ester's name. Ethanoic acid gives ethanoate.

Flavouring

A substance added to food to give it a particular taste or smell. Many are esters.

The name order against the formula order

A. NAME: alcohol part first, acid part second. Ethanol plus propanoic acid gives ETHYL PROPANOATE.

B. FORMULA: acid fragment first, alcohol's alkyl group second. Ethyl propanoate is written C₂H₅COOC₂H₅.

Model answer [4 marks]

Deduce the alcohol and the carboxylic acid needed to make propyl methanoate, and give the ester's structural formula. [4]

The first part of the name, propyl, comes from the alcohol, so the alcohol is propan-1-ol, C₃H₇OH. The second part, methanoate, comes from the acid, so the acid is methanoic acid, HCOOH. In the formula the acid fragment is written first, giving HCOOC₃H₇. Water is also produced when the two react together with an acid catalyst, since esterification is a condensation reaction. The equation is HCOOH + C₃H₇OH ⇌ HCOOC₃H₇ + H₂O.

Model answer [3 marks]

Explain why esters are suitable for use in perfumes and food flavourings. [3]

Esters are volatile, meaning they evaporate readily at ordinary temperatures, so their molecules leave the liquid and travel through the air to reach the nose. They also have distinctive and usually pleasant, fruity smells. Both properties are needed together: a pleasant-smelling compound that did not evaporate would never be detected, and a volatile compound with an unpleasant smell would be useless. Many of the esters used are the same compounds that occur naturally in fruits.

Points that are frequently examined here

• Alcohol gives the -yl part, written FIRST in the name
• Acid gives the -anoate part, written SECOND in the name
• The formula reverses this: acid fragment first
• Water is always the second product, and is not part of the ester
• Esters are volatile AND pleasant-smelling — both are needed for the use
• Ethyl ethanoate, methyl propanoate and propyl methanoate are all C₄H₈O₂

Not this: An artificial flavouring must be chemically different from the natural smell it imitates.

This: Very often it is the identical compound. The ester giving a pear its smell is the same molecule whether the pear made it or a chemist did, and a molecule carries no record of where it was made.

Mark-losing trap. Do not forget the water. The ester contains every atom of both parents LESS one H₂O.

Mark-losing trap. Ethyl ethanoate and methyl propanoate share the formula C₄H₈O₂. Only the structure tells them apart.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · Name [1 mark] — Which ester is formed from methanol and ethanoic acid?
2. Grade 7 · Deduce [2 marks] — Ethyl propanoate is made from which alcohol?
3. Grade 8 · Explain [3 marks] — Explain why a compound must be volatile as well as pleasant-smelling in order to be useful as a perfume.
4. Grade 9 · Deduce [4 marks] — Select every statement that belongs in a full answer deducing the two parent compounds of the ester C₂H₅COOCH₃ and naming it.
5. 9+ · Determine [6 marks] — An ester made from ethanoic acid and an unknown alcohol has the molecular formula C₄H₈O₂. Determine the number of carbon atoms in the alcohol. (Aᵣ: H = 1, C = 12, O = 16)

The people behind this science

Two ways into the same idea — the one who worked out formulae by reasoning rather than by looking, and the one whose discovery this page turns into something you can smell. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Amedeo Avogadro — the one who worked out formulae by reasoning rather than by looking

This page asks a learner to determine a compound's formula purely from what it was made of — never having seen the molecule, and with no instrument involved. That is exactly the problem Avogadro faced and solved in 1811. His hypothesis, that equal volumes of gases contain equal numbers of particles, was what finally let chemists deduce molecular formulae from the proportions in which substances combine rather than guessing at them. He is the right figure to ask how much you can know about a molecule from arithmetic alone, and what such reasoning cannot tell you.

• “How much can you learn about a molecule from proportions alone?”
• “What can arithmetic never tell you about a compound?”
• “Why did chemists disagree about formulae for so long?”
• “How would you check a formula you had only deduced?”
• “What made you trust a conclusion nobody else accepted?”

Friedrich Wöhler — the one whose discovery this page turns into something you can smell

This page arrives at a striking fact: ethyl ethanoate, methyl propanoate and propyl methanoate all have the molecular formula C₄H₈O₂ and all smell different. That is isomerism, and Wöhler found it — but he found it in silver cyanate and silver fulminate, where the difference showed up as one compound detonating and the other not. Here the same principle shows up as a difference you can detect with your nose. He is the right person to ask why rearranging the same atoms should change a property as intimate as a smell.

• “Why should rearranging the same atoms change a smell?”
• “How did you convince chemists two identical analyses meant two substances?”
• “What property first showed you the two compounds were different?”
• “Is a smell as good a piece of evidence as a measurement?”
• “What would you want to measure before trusting your nose?”

Carry on through the course

• Previous lesson: Esters And How To Make One
• Next lesson: Addition Polymers
• What Organic Chemistry Is — Two facts about carbon generate several million compounds — and three words keep them in order.
• Naming, Drawing And Classifying — A name that tells you the structure, a method that finds every isomer, and three words that cover every organic reaction here.
• Crude Oil And Its Fractions — One mixture, separated by boiling point into six useful materials — and one causal chain that explains every trend down the column.
• Burning Fuels — Two ways a hydrocarbon can burn, and why one of them fills a room with a gas you cannot smell.
• Pollutants And Acid Rain — One pollutant comes out of the fuel and one comes out of the air — and telling them apart decides which fix works.
• All of Chemistry · Chemistry key terms · required practicals · equations · Incandio Science · The 4CH1 specification