Esters are a derivative of carboxylic acids and formed from the reaction between a carboxylic acid or an acyl chloride and an alcohol – this process is known as esterification.
Making esters
1. with an alcohol
If we are making an ester with an alcohol and a carboxylic acid the reactants are refluxed in the presence of a strong acid such as concentrated sulphuric acid which acts as a catalyst. The ester is formed in a condensation reaction where two molecules react together to form a larger molecule with the elimination of a smaller molecule, in this case water.
- the reaction is reversible at every step and forms an equilibrium mixture of reactants and products unless the forward direction is forced. We can do this by adding an excess of the alcohol, by removing water as it forms with a dehydrating agent such as anhydrous magnesium sulphate or by distilling off the ester as it forms (it is more volatile than either of the reactants).
- for those who are interested, the mechanism is shown below:
2. with a phenol
In practice we find that carboxylic acids are fairly unreactive so esterification reactions involving phenols must replace the carboxylic acid with an acyl chloride such as ethanoyl chloride, CH3COCl.
When phenol and ethanol chloride are warmed in the presence of a base, NaOH, the phenol reacts with the OH– ions forming the phenoxide ion and water.
The phenoxide ion behaves as a nucleophile and attacks the 𝛿+ carbon in the acyl chloride. The products of the reaction are phenyl ethanoate and sodium chloride.
Breaking esters
We can hydrolyse an ester with either an acid catalyst or a base.
1. acid hydrolysis
Reagents / conditions: either reflux with dilute aqueous acid or use a strong acid at room temperature.
Once again, this method leaves us with an equilibrium mixture of reactants and products ☹️. The mechanism is shown below for the super-interested!
2. alkaline hydrolysis (saponification)
Reagents / conditions: reflux with an aqueous alkali such as sodium hydroxide.
The carboxylic acid formed is immediately neutralised by the base e.g. NaOH(aq) to form a carboxylate salt, hence removing it from the reaction mixture.
No equilibrium forms, this reaction is irreversible and importantly, the base (OH–) is NOT behaving as a catalyst as it is not regenerated, it is simply a reactant.
Fats, oils and soap
Animal fats and vegetable oils contain triesters (triglycerides) formed from the condensation reaction between glycerol (propane-1,2,3-triol) and fatty acids. These molecules are long chain carboxylic acids which are saturated in the case of animal fats and unsaturated in the case of vegetable oils.
Heating animal fats or vegetable oils with NaOH or KOH causes the esters to hydrolyse forming the sodium or potassium salts of the fatty acids, and glycerol. The salts are soaps, molecules with polar carboxylate groups that can form ion-dipole bonds with water and non-polar fatty acid chains that will dissolve in oils. The soap molecules arrange themselves into micelles which are easily washed away when clothes etc. are rinsed.
Practice questions
- (a) The ester shown below can be hydroylsed by refluxing with aqueous potassium hydroxide. Draw and name the structures of the products of this reaction.
(b) The same ester can also be hydrolysed by refluxing with dilute sulphuric acid. Compare and contrast these two methods of hydrolysis.
- (a) Deduce the structure of the carboxylic acid and alcohol that react together in the presence of concentrated sulphuric acid to form the ester shown below:
(b) Name this type of reaction.
- The cyclic ester shown below is formed when molecule X reacts in the presence of concentrated sulphuric acid.
(a) Draw the structure of molecule X.
(b) Deduce the by-product of this reaction.
Answers
- (a)
(b) Whether hydrolysis occurs by reaction with an acid (H2SO4(aq)) or a base (KOH(aq)) the same alcohol is produced. However, whereas acid hydrolysis is a reversible reaction, alkaline hydrolysis is irreversible and with alkaline hydrolysis, the OH– is a reactant not a catalyst as in the case of the H+ in acid hydrolysis. With acid hydrolysis the second product is a carboxylic acid but with alkaline hydrolysis it is a carboxylate salt.
2. (a)
(b) esterification / condensation
3. (a)
(b) water
Note: it would be worth checking you could answer this question the other way around – commonly examiners ask you to draw the ester given the starting molecule X without the hint that the product is cyclic or that it has an ester functional group.