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. 

   

Carboxylic Acids (3)

We have seen in a previous blog that carboxylic acids react with alcohols to produce esters and water.

The question I want to deal with in this blog is this: does the alcohol –OH group end up in the water molecule or the ester?

In other words which of these two reactions takes place on esterification:

In this reaction the oxygen of the alcohol –OH group is found in the water molecule.

But in the second reaction the oxygen of the ethanol –OH group is found in the ester.

Which is it and how can we find out which it is?

In a famous experiment in 1938 two American chemists Irving Roberts and Harold Urey found out the destination of the alcoholic oxygen using isotopic labelling of the oxygen atom in the alcohol

In the reaction below, they used isotopically labelled oxygen ¹⁸O in the methanol to see which of the two reactions below actually took place. 

C₆H₅CO | OH     +    CH₃¹⁸OH     ⇋   C₆H₅CO¹⁸OCH₃    +  H₂O ……..A

Or

C₆H₅COO | H    +    CH₃¹⁸OH     ⇋    C₆H₅COOCH₃    +  H₂¹⁸O ……..B

They used a mass spectrometer to determine the masses of the products and found that the labelled oxygen appeared in the ester and not in the water molecule.

This meant that equation A was correct not equation B.

The oxygen of the alcohol appears in the ester.

The mechanism involves the loss of an –OH group from the acid. 

This means that the mechanism begins with the alcohol making a nucleophilic attack on the acid as the mechanism shows below.

Step 1 involves the protonation of the carbonyl group, the strong acid catalyst facilitates this.

Step 2 involves the nucleophilic addition of methanol to the protonated carboxylic acid. 

Methanol is a nucleophile because of the lone pair of electrons on the oxygen atom.

Step 3 involves the elimination of a water molecule due to proximity of the -H and -OH groups.

At this point, the oxygen of the acids -OH group ends up in the water molecule not the ester. 

Step 4 is the final step in which the species on the right above deprotonates to form the ester.

The product in this example is methyl benzoate.

Examination of the reactants and products IR spectra reveal the change in major functional groups.

Here is benzoic acid’s IR spectrum:

Note the large absorption around 1700cm^-1 due to the carbonyl group

Here next is methanol’s IR spectrum

This spectrum is characterised by the large absorption at around 3300cm^-1 due to the H bonded OH group.

Now contrast those two spectra with that of methyl benzoate:

The strong absorption at 3300cm^-1 is no longer present because the ester contains no H bonded OH group. 

The strong carbonyl absorption is still present at about 1700 cm^-1 because this is central to the ester functional group

We have seen then that the mechanism for the esterification of a carboxylic acid involves an addition elimination reaction.

In this reaction substitution of the –OH group of the acid by the CH₃O- group of the methanol occurs.

This reaction mechanism was first confirmed when Irving Roberts and Harold Urey used isotopic labelling to follow the reaction.   

Carboxylic Acids (2)

In this blog we are going to look at some of the reactions of the carboxylic acids, both those reactions that are typical of acids generally and those specific to this class of organic acids.

1. with reactive metals: 

This is a redox reaction.

Note the increase in oxidation number of the magnesium from 0 to +2.

It is not always easy to identify the organic atom or species that is reduced however.

The redox reactions are typical of minerals acids generally and because organic acids have replaceable hydrogens in their carboxyl groups then they too can take part in these reactions.

2. with bases 

This is not a redox reaction but a neutralisation reaction.

There is no change to the oxidation number of the magnesium ion.

Water forms as a result of the oxide ion abstracting two protons from two ethanoic acid molecules.

The resultant magnesium ethanoate is water soluble

3. with alkalis 

As we can see here with alkalis or soluble bases the hydroxide ion abstracts a proton and forms water.

Again the reaction is a neutralisation not a redox reaction.

The reaction is the typical weak acid–strong base titration example often given to illustrate the formation of an acid buffer. (which see here )

4. with PCl5

First, please note for simplicity I have drawn PCl₅ as a molecule whereas we are well aware that the white solid is an ionic structure formed from two ions PCl₄⁺ and PCl₆^-.

The cation is tetrahedral in shape and the anion is octahedral in shape.  

The product is ethanoyl chloride, one of a number of carboxylic acid derivatives which we shall consider in another blog.  

5. with alcohols: esterification

  

Much is made of the esterification reaction in college and Advanced level chemistry partly because it is known to be challenging on several levels not least in the naming of esters.  

I have colour coded the names in an attempt to bring home the fact that the ester derives its name from both the acid and the alcohol.

The reaction does not go to completion but enters an equilibrium state.

Water is the only other product

Esters are insoluble in water.

They are another type of carboxylic acid derivative.

The reaction requires a catalytic amount of a strong acid such as H₂SO₄

6. with SOCl₂ thionyl chloride

Thionyl chloride acts as a powerful nucleophile and attacks the carbon of the carbonyl group.

The product is the highly volatile and easily hydrolysed ethanol chloride.  

This reaction occurs at room temperature and is as equally dangerous as the PCl₅ reaction given the toxicity of the products SO₂ and HCl.  

7. with LiAlH4

LiAlH4 is Lithium tetrahydridoaluminate (III) and it is a very powerful reducing agent.  

In dry ether and in the absence of moisture is acts as a nucleophile attacking the carboxylic acid with H-ions.  

The acid is reduced down in two stages to its original alcohol.  

  

The reactions described above are typical of the lower molecular mass carboxylic acids that are soluble in water.  

Those acids of higher molecular mass that are waxy solids do not enter into these reactions since they do not dissolve in the ionic media necessary. 

There is more discussion of the carboxylic acid derivatives such as  esters, acid chloride etc. in a later blog. 

Chemical Test

Finally a chemical test exists for the carboxyl group –COOH in water.

Simply add a spatula measure of sodium carbonate powder (Na₂CO₃)and watch the aqueous solution of carboxylic acid fizz as CO₂ is evolved.   

This is your opportunity to construct the balanced symbol equation for this reaction taking place in this chemical test.

As the pKa of carboxylic acids are greater than those of phenols this test easily distinguishes the acids from phenolic compounds.  

Phenolic compounds do not fizz with sodium carbonate powder or solution.