Properties and uses of esters

Due to their pleasant, and usually sweet, smell, esters are commonly used as food flavourings. For example:

• ethyl methanoate is used for raspberry flavouring
• ethyl butanoate is used for pear flavouring

This sweet smell can also used to detect whether carboxylic acids and alcohols are present. A sweet smell is produced when esters are:

• heated with ethanol and a strong acid carboxylic acid
• heated with ethanoic acid and a strong acid alcohol

Another use for esters is as plasticisers. Adding them to polymers makes the material more flexible and less rigid as the intermolecular forces between the polymer chains are weakened.

Esters are also used for organic compounds as solvents. The solute stays while the ester evaporates. One example is nail varnish.

Hydrolysis of esters

Acid hydrolysis

Due to the fact that the esterification reaction is reversible, when an ester is heated under reflux with concentrated H₂SO₄, an alcohol and carboxylic acid are formed with a reversible reaction:

R₁-COOR₂ + H₂O ? R₁-COOH + R₂OH

This reaction is known as ester hydrolysis. Completion is not reached and equilibrium achieved when only 30% of the ester has been hydrolysed. Therefore, there is not a good yield of either the carboxylic acid or the alcohol.

Alkaline hydrolysis

When as ester is heated with a strong alkali, like NaOH, under reflux, the products formed are a carboxylate salt and an alcohol:

R₁-COOR₂ + NaOH ? R₁-COO^–Na⁺ + R₂OH

This reaction is known as alkaline hydrolysis or saponification. As it is not readily reversible, the yield of carboxylate and alcohol is better than acid hydrolysis using the same ester.

The equilibrium shifts to the right with the use of excess NaOH because NaOH is a reactant, not a catalyst.

Therefore, saponification is more efficient at hydrolysing an ester and produces a better yield than acid hydrolysis.

Once cooled, the carboxylate salt can be easily converted to carboxylic acid by adding acid to the mixture:

R-COO^–(aq) + H⁺(aq) ? R-COOH(aq)

Therefore, to ensure that carboxylic acid is the final product the ester should be hydrolysed in alkaline conditions initially and then the mixture should be acidified afterwards.

Naturally occurring fats and oils

Fats and oils that occur naturally are esters and have the general formula:

R = C₁₃H₂₇, C₁₅H₃₁, C₁₇H₃₅ or C₁₉H₃₉

And the following functional group:

Fats and oils consist of three ester groups. In the acidic conditions of the stomach these esters are hydrolysed and can then be broken down into their constituent alcohol and acid. It is also possible to hydrolyse them artificially in alkaline conditions.

Acid hydrolysis of fats

In the stomach, some fats are broken down in the presence of the enzyme, lipase. The products are:

• propan-1,2,3-tri-ol (glycerol)
• carboxylic acids made up of three long chains (fatty acids)

Glycerol is an important energy source and is used by the body to form glucose for respiration. Fatty acids are important for healthy tissue growth as they are used to build cell membranes.

Alkaline hydrolysis of fats

Commercially, fats and oils are hydrolysed in alkaline conditions to produce:

• glycerol
• the salts of the fatty acids

Glycerol, a by-product, can be used for pharmaceuticals and cosmetics.

The salts, known as soaps, are used to mix oil and water together. The carboxylate end is polar and attracts water as well as other hydrophilic groups. The long-chain hydrocarbon end is non-polar and attracts oils as well as other hydrophobic groups. Therefore, the same molecule is able to attract both aqueous and oily species. This means that soap molecules can attract oil from the skin and into water.

Making biodiesel

Biodiesel is composed of long chain fatty acids. It is produced through the hydrolysis of naturally occurring fats or oils. The resulting fatty acids are then mixed with methanol and a suitable catalysis to create a methyl ester. This reaction is an example of esterification.

Due to the fact that it is made from vegetable oil from crops like rape, biodiesel is a renewable fuel.