INTRODUCING ESTERS

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 In this case, the hydrogen in the -COOH group has been replaced by an ethyl group.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.

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.

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.

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.

The acid CH₃(CH₂)₁₆COOH is called octadecanoic acid, but the old name is still commonly used. This is stearic acid.

The full name for the ester of this with propane-1,2,3-triol is propane-1,2,3-triyl trioctadecanoate. But the truth is that almost everybody calls it (not surprisingly!) by its old name of glyceryl tristearate.

Saturated and unsaturated fats and oils

If the fat or oil is saturated, it means that the acid that it was derived from has no carbon-carbon double bonds in its chain. Stearic acid is a saturated acid, and so glyceryl tristearate is a saturated fat.

If the acid has just one carbon-carbon double bond somewhere in the chain, it is called mono-unsaturated. If it has more than one carbon-carbon double bond, it is polyunsaturated.

Those same terms will then apply to the esters that are formed.

All of these are saturated acids, and so will form saturated fats and oils:

Oleic acid is a typical mono-unsaturated acid:

. . . and linoleic and linolenic acids are typical polyunsaturated acids.

You might possibly have come across the terms "omega 6" and "omega 3" in the context of fats and oils.

Linoleic acid is an omega 6 acid. It just means that the first carbon-carbon double bond starts on the sixth carbon from the CH₃ end.

Linolenic acid is an omega 3 acid for the same reason.

Because of their relationship with fats and oils, all of the acids above are sometimes described as fatty acids.

Physical properties

Simple esters

I am thinking here about things like ethyl ethanoate.

Boiling points

The small esters have boiling points which are similar to those of aldehydes and ketones with the same number of carbon atoms.

Like aldehydes and ketones, they are polar molecules and so have dipole-dipole interactions as well as van der Waals dispersion forces. However, they don't form hydrogen bonds, and so their boiling points aren't anything like as high as an acid with the same number of carbon atoms.

For example:

Solubility in water

The small esters are fairly soluble in water but solubility falls with chain length.

For example:

The reason for the solubility is that although esters can't hydrogen bond with themselves, they can hydrogen bond with water molecules.

One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one of the lone pairs on one of the oxygen atoms in an ester for a hydrogen bond to be formed.

There will also, of course, be dispersion forces and dipole-dipole attractions between the ester and the water molecules.

Forming these attractions releases energy. This helps to supply the energy needed to separate water molecule from water molecule and ester molecule from ester molecule before they can mix together.

As chain lengths increase, the hydrocarbon parts of the ester molecules start to get in the way.

By forcing themselves between water molecules, they break the relatively strong hydrogen bonds between water molecules without replacing them by anything as good. This makes the process energetically less profitable, and so solubility decreases.

The physical properties of fats and oils

Solubility in water

None of these molecules are water soluble. The chain lengths are now so great that far too many hydrogen bonds between water molecules would have to be broken - so it isn't energetically profitable.

Melting points

The melting points determine whether the substance is a fat (a solid at room temperature) or an oil (a liquid at room temperature).

Fats normally contain saturated chains. These allow more effective van der Waals dispersion forces between the molecules. That means you need more energy to separate them, and so increases the melting points.

The greater the extent of the unsaturation in the molecules, the lower the melting points tend to be because the van der Waals dispersion forces are less effective.

Why should this be? We are talking about molecules of very similar sizes and so the potential for temporary dipoles should be much the same in all of them. What matters, though, is how close together the molecules can get.

van der Waals dispersion forces need the molecules to be able to pack closely together to be really effective. The presence of carbon-carbon double bonds in the chains gets in the way of tidy packing.

Here is a simplified diagram of a saturated fat:

The hydrocarbon chains are, of course, in constant motion in the liquid, but it is possible for them to lie tidily when the substance solidifies. If the chains in one molecule can lie tidily, that means that neighbouring molecules can get close.

That increases the attractions between one molecule and its neighbours and so increases the melting point.

Unsaturated fats and oils have at least one carbon-carbon double bond in at least one chain.

There isn't any rotation about a carbon-carbon double bond and so that locks a permanent kink into the chain. That makes packing molecules close together more difficult. If they don't pack so well, the van der Waals forces won't work as well.

This effect is much worse for molecules where the hydrocarbon chains either end of the double bond are arranged cis to each other - in other words, both of them on the same side of the double bond:

If they are on opposite sides of the double bond (the trans form) the effect isn't as marked. It is, however, rather more than the diagram below suggests because of the changes in bond angles around the double bond compared with the rest of the chain.

Trans fats and oils have higher melting points than cis ones because the packing isn't affected quite as much. Naturally occurring unsaturated fats and oils tend to be the cis form.

Where would you like to go now?

To the esters menu . . .

To the menu of other organic compounds . . .

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