Chemistry

UNIT 8: ESTERS, ACID ANHYDRIDES, AMIDES AND ITRILES.

Key unit competency:

To be able to relate the functional groups of esters, acid anhydrides, amides and

nitriles to their reactivity, preparation methods and uses. 

8.1. Structure and nomenclature of esters 

8.1.1. Structure of esters

In unit 7, the reactions of carboxylic acids were discussed. The reactions of carboxylic acids produce the derivatives of acids such as esters, acid halides, acid anhydrides and amides.

The general molecular formula of esters is Cn H₂nO₂  and their general structural

formula is: RCOOR’ or

Esters are known for their distinctive odor and they are commonly responsible for

the characteristic of food (fruits) aroma, flowers and fragrances. Esters are found in

nature but they can be also synthesized. Both natural and synthetic esters are used

in perfumes and as flavoring agents.

8.1.2. Nomenclature of esters

The nomenclature of esters follows some steps. When naming esters the alkyl

group R’ is named followed by the name of RCOO- group.

The group name of the alkyl or aryl portion is written first and is followed by the

name of the acid portion. In both common and International Union of Pure and

Applied Chemistry (IUPAC) nomenclature, the -ic ending of the corresponding acid

is replaced by the suffix –ate. Some examples of names of esters are given in Table

8.1.

8.1.3. Physical properties and uses of Esters

B. Comparing boiling points of alcohols, carboxylic acids and esters

Materials and Chemicals

Propan-1-ol, propanoic acid and methyl ethanoate, test tubes, test tube holders

(lacks), heaters, and thermometers.

Procedure

1. Put 10 mL of each substance in a labeled test tube.

2. Boil carefully substances are volatile and flammable

3. Use a thermometer to measure the boiling point of each substance.

4. Record the results and compare them. Suggest a reason for the difference

in boiling points of the three substances.

Conclusion: Esters have lower boiling points than alcohols and carboxylic

acids because they lack hydrogen bonds. A compound having hydrogen bonds

has a high boiling point because, to break that bond requires higher energy. 

Other physical properties of esters

i. Lower esters have sweet fruity smells

ii. Melting and boiling points of esters increase as the molecular mass

increases.

iii. Small esters are fairly soluble in water but the solubility decreases as the

length of the chain increases

8.1.4. Uses of Esters

Esters find various uses:

i. They are used as organic solvent

ii. Due to their aroma, they are used as constituent of fragrance, essential oils,

food flavoring and cosmetics.

iii. They are used to manufacture soaps, detergents and glycerol.

iv. They are used to provide energy in the body

v. Polyesters are used to produce plastics etc.

8.2. Preparation and chemical properties of esters

8.2.1. Preparation of Esters

The preparation of esters involves different types of reaction such as esterification,

reaction of an acid chloride with an alcohol and the reaction of acid anhydrides

with alcohols.

1.Esterification reaction

In units five and seven, it is mentioned that esters can be produced by a reaction

between alcohols and carboxylic acids in strong acidic medium acting as a catalyst.

The acid is commonly a concentrated sulphuric acid, under reflux (Figure 8.3). The

reaction is generally called “Esterification” (a condensation reaction which involves

the addition of the alcohol and acid molecules followed by an elimination of a

water molecule).