Making and breaking esters

Making esters

Esters are a derivative of carboxylic acids and formed from the reaction between a carboxylic acid and an alcohol – this is known as esterification.

• 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:

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)

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

1. (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.

2. (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.

Answers

1. (a)

(b) Whether hydrolysis occurs by reaction with an acid (H₂SO_4(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