This page explains what esters are and looks at their simple physical properties such as solubility and boiling points. It includes an introduction to more complicated naturally-occurring esters like animal and vegetable fats and oils.
What are esters?
Esters are derived from carboxylic acids. A carboxylic acid contains the -COOH group, and in an ester the hydrogen in this group is replaced by a hydrocarbon group of some kind. This could be an alkyl group like methyl or ethyl, or one containing a benzene ring like phenyl.
A common ester - ethyl ethanoate
The most commonly discussed ester is ethyl ethanoate. In this case, the hydrogen in the -COOH group has been replaced by an ethyl group. The formula for ethyl ethanoate is:
Notice that the ester is named the opposite way around from the way the formula is written. The "ethanoate" bit comes from ethanoic acid. The "ethyl" bit comes from the ethyl group on the end.
Note: In my experience, students starting organic chemistry get more confused about writing names and formulae for esters than for almost anything else - particularly when it comes to less frequently met esters like the ones coming up next. Take time and care to make sure you understand!
A few more esters
In each case, be sure that you can see how the names and formulae relate to each other.
Notice that the acid is named by counting up the total number of carbon atoms in the chain - including the one in the -COOH group. So, for example, CH3CH2COOH is propanoic acid, and CH3CH2COO is the propanoate group.
Note: You can find more about naming acids and esters by following this link to a different part of this site.
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Fats and oils
Differences between fats and oils
Animal and vegetable fats and oils are just big complicated esters. The difference between a fat (like butter) and an oil (like sunflower oil) is simply in the melting points of the mixture of esters they contain.
If the melting points are below room temperature, it will be a liquid - an oil. If the melting points are above room temperature, it will be a solid - a fat.
The causes of the differences in melting points will be discussed further down the page under physical properties.
A simple introduction to their structures
Fats and oils as big esters
Esters can be made from carboxylic acids and alcohols. This is discussed in detail on another page, but in general terms, the two combine together losing a molecule of water in the process.
We'll start with a very, very simple ester like ethyl ethanoate - not something complicated like a fat or oil!
The diagram shows the relationship between the ethanoic acid, the ethanol and the ester.
This isn't intended to be a full equation. Water, of course, is also produced.
Note: The colour coding refers to the name of the ester and not strictly to the structure. When the ester is made, the water that is lost comes from the whole -OH group of the acid and just a single hydrogen from the alcohol. That means that as far as the structure is concerned the oxygen attached to the ethyl group actually ought to be coloured red. Don't worry about this at this level.
Now lets make the alcohol a bit more complicated by having more than one -OH group. The diagram below shows the structure of propane-1,2,3-triol (old name: glycerol).
Just as with the ethanol in the previous equation, I've drawn this back-to-front to make the next diagrams clearer. Normally, it is drawn with the -OH groups on the right-hand side.
If you make an ester of this with ethanoic acid, you could attach three ethanoate groups.
Now, make the acid chains much longer, and you finally have a fat.
Note: The colour coding is still there only to help you to see how the formulae are built up. In each case, if you want to be strictly accurate, the final oxygen in each row...