Unit 3 - Esters

Question 1

Esters

Answer 1

Esters are formed when an alcohol reacts with a carboxylic acid. This is an example of a reversible reaction.

All esters contain the ester link, -COO- (functional group).

Question 2

Naming Esters

Answer 2

The first word of the ester name comes from the parent alcohol and always ends in ‘yl’. The second word in the ester name comes from the parent carboxylic acid and always ends in ‘oate’.

C=O in its functional group, the C=O of the ester link is joined to the side that came from the carboxylic acid.

Question 3

Uses of Esters

Answer 3

Esters are used as flavouring and fragrances as many have pleasant, fruity smells e.g. in perfumes, confectionery. Esters are also used as solvents for non-polar compounds that don’t dissolve in water e.g. in nail varnishes and nail varnish removers.

Question 4

Formation of Esters

Answer 4

Esters are formed by a condensation reaction between an alcohol and a carboxylic acid.

In a condensation reaction, two molecules are joined together with the elimination of a small molecule. When an ester link is formed by the reaction between a hydroxyl group and a carboxyl group, the small molecule eliminated is water.

Question 5

Preparing Esters (Experimentally)

Answer 5

One way of preparing esters is to condense an alcohol with a carboxylic acid.

The reaction is slow at room temperature and the yield of ester is low. The rate can be increased by heating the reaction mixture and by using concentrated sulfuric acid to provide H+ ions which act as a catalyst. The presence of the concentrated sulfuric acid also increases the yield of ester by removing the other product, water.

The ester forms as an oily layer on the surface and has a distinctive smell.

Question 6

Properties of Esters - IFs and MP/BP

Answer 6

Esters are polar molecules and so have permanent dipole-permanent dipole interactions as well as London dispersion forces.

However, they do not form hydrogen bonds so their boiling points are lower than an acid with the same number of carbon atoms. This means that esters are more volatile (means we can smell them more easily)

Question 7

Properties of Esters - Solubility

Answer 7

Small esters are fairly soluble in water but solubility decreases as the carbon chain length increases.

Question 8

Properties of Esters - Hydrogen Bonding

Answer 8

Although esters can’t form hydrogen bonds with themselves, they can form hydrogen bonds with water molecules.

One of the slightly positive hydrogen atoms in a water molecule can be sufficiently attracted to one of the lone pairs on one of the oxygen atoms in an ester for a hydrogen bond to be formed.

Question 9

Hydrolysis of Esters

Answer 9

Esters can be hydrolysed to produce an alcohol and a carboxylic acid.

The breaking up of an ester can be achieved by heating the ester with an alkali such as sodium hydroxide.

In a hydrolysis reaction, a molecule reacts with water to break down into smaller molecules. Hydrolysis (break down of compound using H20) is the opposite of a condensation reaction.

Hydrolysis is reversible and therefore will be incomplete.

Acid + Alcohol ⇌ Ester + H20
Condensation is forward reaction - Hydrolysis is backward reaction.