Esters

Esters are amazing molecules because you find them in all sorts of applications very often where you would least expect them to be.

On this chart there is the skeletal formulae, the name and the source of all the esters mentioned.

Having the name and skeletal formula allows you to match the name to each part of the ester.

So for example:

So as you can see esters are ubiquitous: they are found in all sorts of places.

And here is another fascinating fact about esters they are in space in a giant dust cloud in the constellation Sagittarius on which see here

So let’s look at the properties of esters.

The German chemist Leopold Gmelin was the first to give the name Ester to molecules with the –COO functional group. 

Here is a typical ester display formula: ethyl ethanoate

You can see the key structure is the carbonyl group attached to a single oxygen atom i.e. the  –COO group.

Solubility in water

As you might have already guessed the larger the alkyl groups on esters the less likely they are to be water-soluble.

Large esters especially fats and oils are insoluble in water.

You will see that they do possess the –COO functional groups that can form strong hydrogen bonds with water like this:

But the energy released when esters form these bonds is less and less able to break the van der Waals forces between the alkyl groups as these groups increase in size.

Small esters like ethyl methanoate and ethyl ethanoate are sparingly soluble in water.

Structure

Their names are based on the carboxylic acid from which they are derived.

We also remember that the name is the reverse of the way we write the structural formula:

Ethyl ethanoate

CH₃COOCH₂CH₃

The colour coding above matches the parts of the formula of the ester to its name so you can see how the name is the reverse of the formula.

The above was an example but it is always best to test yourself to see if you can work out the name from the formula and the formula from the name.

So here are two lists in which you need to work out the name from the formula and the formula from the name.

Formulae: what are the names of these esters?

CH₃COOCH₃

CH₃CH₂CH₂COOCH₂CH₃

HCOOCH₂CH₂CH₂CH₃

Names: what are the formulae of these esters?

Methyl propanoate

Hexyl septanoate

Phenyl butanoate

Synthesis

Esters are best synthesised from the corresponding acid chloride and alcohol.

Here is an example of this nucleophilic addition elimination reaction:

An alternative reaction is to use the alcohol and carboxylic acid but this is a slower reaction with a lower yield because the reaction is in equilibrium and does not go to completion.

Big esters

Esters can form polymers and lipids.

So for example glycerol and fatty acids produce lipid molecules in a reaction like this below:

This is tristearin, a form of lipid that is a triglyceride:

The fat will have a glycerol head and fatty acid tail.

These lipid molecules can be saturated or unsaturated:

Saturated:

Unsaturated:

The unsaturated fats can have cis and trans geometric isomers

Cis and trans fats:

Trans fats are quite toxic and difficult to breakdown in the digestive system.

Some places and states in the US ban the use of trans fats in baking and other products. 

Polymeric esters

Terylene is the original ICI trade name for the fibre spun from this polyester:

Biodegradable esters: biopol

Bipol is the biodegradable polymer polyhydroxybutyrate.

This is its structure:

It is environmentally friendly green polymer because is degrades relatively quickly in landfill sites.

It is produced from the fermentation of plant sugars and glucose usually derived from starch type plants such as sweet potato.

It has become a substitute plastic for plastic bags.

Such bags degrade if left for a few weeks become weak and unusable.