EstersIB MYP Chemistry: Revision notes
Section 1
What is an ester?
Esters are organic compounds with the functional group -COO-. A carbon atom is double bonded to one oxygen atom and single bonded to another oxygen atom, which is joined to a second carbon chain.
The general shape is R-COO-R', where R and R' are carbon chains. For example, ethyl ethanoate is CH₃COOC₂H₅.
Do not mix up the groups: the carboxylic acid has -COOH (it ends in H), but the ester has -COO- with a carbon chain on both sides.
Section 2
Making an ester: esterification
An ester is made by esterification: a carboxylic acid reacts with an alcohol. The mixture is warmed with a few drops of concentrated sulfuric acid, which acts as a catalyst. Water is the other product.
carboxylic acid + alcohol ⇌ ester + water
CH₃COOH + C₂H₅OH ⇌ CH₃COOC₂H₅ + H₂O
The reaction is reversible, so the ester is never made in 100% yield. Notice that the atoms balance: both sides have C₄H₁₀O₃ in total.
Concentrated sulfuric acid is not a reactant. It is the catalyst, so it is not written in the equation as a reactant.
Section 3
Naming esters
An ester name has two words. The first word comes from the alcohol (the alkyl group): methyl, ethyl, propyl, butyl. The second word comes from the carboxylic acid and ends in -oate: methanoate, ethanoate, propanoate, butanoate.
- ethanol + butanoic acid gives ethyl butanoate
- propan-1-ol + ethanoic acid gives propyl ethanoate
- methanol + ethanoic acid gives methyl ethanoate
To name an ester from its formula, find the -COO- group. The chain on the C=O side is the acid part (-oate). The chain on the O side is the alcohol part (-yl).
Students often write the acid name first. Remember: the alcohol gives the first word, so ethanol plus butanoic acid is ethyl butanoate, not butyl ethanoate.
Section 4
Smell and properties
Many esters are volatile, which means they evaporate easily at room temperature, and they have sweet, fruity smells. For example, ethyl butanoate smells of pineapple. Small esters are usually colourless liquids that do not mix well with water.
This is why a fruity smell is a quick sign that an ester has formed when you pour the reaction mixture into cold water.
Section 5
Uses of esters
The properties of esters decide how they are used:
- Flavourings: artificial fruit flavours in sweets, drinks and ice cream
- Perfumes: the pleasant smell and volatility mean the scent reaches the nose
- Solvents: esters such as ethyl ethanoate dissolve many substances and evaporate quickly, so they are used in nail varnish remover, glues and paints
Synthetic esters are cheaper and more reliable than flavours extracted from fruit, but some people prefer natural flavours.
That's the notes covered.
Carry on to the next subtopic.
Exam questions on Esters
- A flavour company makes an artificial pineapple flavouring for sweets. The flavouring is an ester, made in the laboratory by heating a carboxylic acid with an alcohol in the presence of a catalyst.Outline how the ester is made in the laboratory, naming the catalyst.2 marks
- A student warms ethanoic acid with propan-1-ol and a few drops of concentrated sulfuric acid in a test tube. She pours the mixture into a beaker of cold water and notices a sweet, fruity smell.State the role of the concentrated sulfuric acid and give one safety precaution the student should take.2 marks
- Ethyl ethanoate is a colourless liquid used as a solvent in nail varnish remover and in some glues. A factory makes it from ethanoic acid and ethanol.Write a balanced symbol equation, using molecular or condensed formulae, for the formation of ethyl ethanoate from ethanoic acid and ethanol.3 marks
Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).