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Esters

What you'll learn

  • How to recognise the ester functional group, −C(=O)−O−-\mathrm{C(=O)-O}-−C(=O)−O−.
  • How ethanol and ethanoic acid form ethyl ethanoate using an acid catalyst.
  • How to write and work backwards from structural and displayed formulae of esters.
  • Why esters are useful, and how to prepare a small sample safely in the lab.

Esters are a small Paper 2 only part of Edexcel IGCSE Chemistry, but the main pattern is very learnable once you see where each part of the molecule comes from.

Prerequisites: alcohols, carboxylic acids and functional groups

In organic chemistry, a functional group is the part of a molecule mainly responsible for its chemical reactions.

Alcohols contain the hydroxyl group, −OH-\mathrm{OH}−OH. For example, ethanol is CH3CH2OHCH_3CH_2OHCH3​CH2​OH.

Carboxylic acids contain the carboxyl group, −COOH-\mathrm{COOH}−COOH. For example, ethanoic acid is CH3COOHCH_3COOHCH3​COOH.

Definition

Functional group

A functional group is an atom or group of atoms in an organic molecule that gives the molecule its typical reactions.

The key idea for esters is that an alcohol and a carboxylic acid can join together, producing an ester and water.

The ester functional group

Esters contain the functional group:

−C(=O)−O−-\mathrm{C(=O)-O}-−C(=O)−O−

This is often written more compactly as −COO−-\mathrm{COO}-−COO−.

The C=OC=OC=O part is called a carbonyl group. In an ester, the carbonyl carbon is bonded to another oxygen atom, which then connects to a carbon chain.

Definition

Ester

An ester is an organic compound containing the −C(=O)−O−-\mathrm{C(=O)-O}-−C(=O)−O− functional group. Esters are commonly made by reacting a carboxylic acid with an alcohol.

Key Idea

The ester pattern

In an ester, the carboxylic acid provides the “alkanoate” part, and the alcohol provides the “alkyl” part of the name.

Making ethyl ethanoate

The ester you need to know by name is ethyl ethanoate.

It is made when ethanol reacts with ethanoic acid in the presence of an acid catalyst, usually concentrated sulfuric acid.

Definition

Catalyst

A catalyst increases the rate of a reaction but is not used up overall. In this reaction, concentrated sulfuric acid provides acidic conditions and speeds up ester formation.

The word equation is:

ethanoic acid + ethanol → ethyl ethanoate + water

The balanced symbol equation, with state symbols, is:

CH3COOH(l)+CH3CH2OH(l)→CH3COOCH2CH3(l)+H2O(l)CH_3COOH(l) + CH_3CH_2OH(l) \to CH_3COOCH_2CH_3(l) + H_2O(l)CH3​COOH(l)+CH3​CH2​OH(l)→CH3​COOCH2​CH3​(l)+H2​O(l)

This reaction is called esterification.

Formulae of ethyl ethanoate

A structural formula shows how atoms are connected, using groups such as CH3CH_3CH3​ and CH2CH_2CH2​.

A displayed formula shows every atom and every bond, including all the C–H bonds.

For ethyl ethanoate:

  • Structural formula: CH3COOCH2CH3CH_3COOCH_2CH_3CH3​COOCH2​CH3​
  • Condensed displayed form: CH3−C(=O)−O−CH2−CH3CH_3-C(=O)-O-CH_2-CH_3CH3​−C(=O)−O−CH2​−CH3​

The diagram shows both ways of representing the same ester.

Structural and displayed formulae of ethyl ethanoate

Common Mistake

Leaving out the linking oxygen

Ethyl ethanoate is CH3COOCH2CH3CH_3COOCH_2CH_3CH3​COOCH2​CH3​, not CH3COCH2CH3CH_3COCH_2CH_3CH3​COCH2​CH3​. If you leave out the single-bonded oxygen, you no longer have the ester functional group.

Naming esters from the reactants

The name of an ester has two words:

  1. The first word comes from the alcohol.
  2. The second word comes from the carboxylic acid, changing “-oic acid” to “-oate”.

For example:

  • methanol gives methyl
  • ethanol gives ethyl
  • propanol gives propyl

And:

  • methanoic acid gives methanoate
  • ethanoic acid gives ethanoate
  • propanoic acid gives propanoate

So:

ethanol + ethanoic acid → ethyl ethanoate

Definition

Alkyl group

An alkyl group is a hydrocarbon fragment from an alkane with one hydrogen removed, such as methyl, CH3−CH_3-CH3​−, or ethyl, CH3CH2−CH_3CH_2-CH3​CH2​−.

Example

Naming an ester from its reactants

  1. Identify the alcohol: ethanol gives the ethyl part of the ester name.

  2. Identify the carboxylic acid: propanoic acid gives the propanoate part of the ester name.

  3. Combine the name in the correct order: alcohol part first, acid part second, so the ester is ethyl propanoate.

  4. Write the structural formula by keeping the acid part as CH3CH2COO−CH_3CH_2COO-CH3​CH2​COO− and attaching the ethyl part, CH2CH3CH_2CH_3CH2​CH3​, to the oxygen: CH3CH2COOCH2CH3CH_3CH_2COOCH_2CH_3CH3​CH2​COOCH2​CH3​.

Tip

Name order versus formula order

The name usually starts with the alcohol part, but the structural formula is often written starting with the acid-derived part. Ethyl ethanoate is named alcohol-then-acid, but written as CH3COOCH2CH3CH_3COOCH_2CH_3CH3​COOCH2​CH3​.

Working backwards from an ester

You may also be given an ester and asked which alcohol and carboxylic acid made it.

To reverse the process, split the ester at the single bond after the ester oxygen:

acid part−C(=O)−O ∣ alcohol part\mathrm{acid\ part{-}C(=O)-O\ |\ alcohol\ part}acid part−C(=O)−O ∣ alcohol part

The part containing C=OC=OC=O came from the carboxylic acid. The carbon chain attached after the single-bonded oxygen came from the alcohol.

Example

Finding the reactants from an ester formula

  1. Start with the ester CH3COOCH3CH_3COOCH_3CH3​COOCH3​ and locate the ester group: CH3C(=O)OCH3CH_3C(=O)OCH_3CH3​C(=O)OCH3​.

  2. Split after the single-bonded oxygen: CH3COO−CH_3COO-CH3​COO− is the acid-derived part, and CH3−CH_3-CH3​− is the alcohol-derived part.

  3. Turn the acid-derived part back into a carboxylic acid by adding H to the final O: CH3COOHCH_3COOHCH3​COOH, which is ethanoic acid.

  4. Turn the alcohol-derived part back into an alcohol by adding OHOHOH: CH3OHCH_3OHCH3​OH, which is methanol.

  5. Therefore, CH3COOCH3CH_3COOCH_3CH3​COOCH3​ is methyl ethanoate, made from methanol and ethanoic acid.

Common Mistake

Splitting the ester in the wrong place

When finding the original acid and alcohol, split the single bond between the ester oxygen and the alcohol-derived carbon. Do not break the C=OC=OC=O double bond.

Properties and uses of esters

Esters are often volatile and have distinctive smells.

Definition

Volatile

A volatile substance evaporates easily, so its particles can enter the air at room temperature or with gentle warming.

Many small esters have sweet, fruity smells. Because they evaporate easily, their vapour reaches your nose quickly, even when only a small amount is present.

Esters are used in:

  • food flavourings, such as fruit-flavoured sweets and drinks
  • perfumes, because they often have pleasant smells and evaporate into the air

In the lab, you should never sniff chemicals directly. You should waft the smell gently towards your nose.

Practical: preparing ethyl ethanoate

To prepare a small sample of ethyl ethanoate, you react ethanol with ethanoic acid using a few drops of concentrated sulfuric acid.

Typical apparatus includes:

  • boiling tube
  • beaker of hot water as a water bath
  • clamp or test-tube holder
  • dropping pipettes
  • sodium carbonate solution or water for separating the ester after heating

Method

  1. Add small measured volumes of ethanol and ethanoic acid to a boiling tube.

  2. Carefully add a few drops of concentrated sulfuric acid.

  3. Warm the boiling tube in a hot water bath for a few minutes. Do not heat the mixture directly with a flame.

  4. Pour the warm reaction mixture into sodium carbonate solution or water.

  5. Look for oily ester droplets or a thin layer floating near the surface, then carefully waft the smell towards your nose.

Preparing ethyl ethanoate using a hot water bath and sodium carbonate solution

The sodium carbonate solution helps neutralise leftover acid. With ethanoic acid, this produces carbon dioxide gas, so you may see fizzing:

2CH3COOH(aq)+Na2CO3(aq)→2CH3COONa(aq)+H2O(l)+CO2(g)2CH_3COOH(aq) + Na_2CO_3(aq) \to 2CH_3COONa(aq) + H_2O(l) + CO_2(g)2CH3​COOH(aq)+Na2​CO3​(aq)→2CH3​COONa(aq)+H2​O(l)+CO2​(g)

Variables and expected results

The main reactants chosen determine which ester forms. If you are comparing preparations, keep the volumes of alcohol and acid, number of catalyst drops, water bath temperature and heating time the same.

Expected observations for ethyl ethanoate include:

  • sweet or fruity smell
  • oily droplets or a layer because the ester is only slightly soluble in water
  • fizzing if sodium carbonate neutralises excess acid
Common Mistake

Safety in the ester practical

Ethanol is flammable, so use a hot water bath rather than direct heating. Concentrated sulfuric acid is corrosive, so wear eye protection and handle it carefully. Always waft smells; do not sniff directly.

Common Mistake

Overheating the mixture

If you heat too strongly, volatile reactants or ester can evaporate away, and ethanol vapour can catch fire. Gentle warming is enough.

Exam technique

In the exam

  1. Look for the ester functional group −C(=O)−O−-\mathrm{C(=O)-O}-−C(=O)−O−, not just any oxygen-containing group.

  2. For naming, use alcohol first, acid second: ethanol + ethanoic acid makes ethyl ethanoate.

  3. For the practical, mention the acid catalyst, hot water bath, sodium carbonate or water separation step, fruity smell, and wafting for safety.

Self review

Check yourself

  • What ester forms from methanol and propanoic acid?
  • Which alcohol and carboxylic acid could form CH3CH2COOCH3CH_3CH_2COOCH_3CH3​CH2​COOCH3​?
  • Why is a hot water bath used instead of direct heating when preparing an ester?
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Reaction diagram showing ethanol plus ethanoic acid forming ethyl ethanoate and water, with the ester functional group and the acid-derived and alcohol-derived parts labelled

Esters are organic compounds that contain the functional group −C(=O)−O−-\mathrm{C(=O)-O}-−C(=O)−O−, often shortened to −COO−-\mathrm{COO}-−COO−. They form when a carboxylic acid reacts with an alcohol.

This reaction is called esterification, and concentrated sulfuric acid is used as an acid catalyst. A key example is the production of ethyl ethanoate from ethanoic acid and ethanol.

CH3COOH+CH3CH2OH→CH3COOCH2CH3+H2O CH_3COOH + CH_3CH_2OH \to CH_3COOCH_2CH_3 + H_2O CH3​COOH+CH3​CH2​OH→CH3​COOCH2​CH3​+H2​O

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What are the two components of an ester functional group?

Esters Revision Guide

  1. IGCSE
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