Chemistry, Third edition

338 18 · COMMON CLASSES OF ORGANIC COMPOUNDS

Naming carboxylic

acids

Name the following acids:

(i) CH 3 CH 2 CH 2 COOH

(ii)CH 3 (CH 2 ) 5 COOH

(iii)COOH

|

CH 2

|

CH 2

|

COOH

Exercise 18I

Carboxylic acids

These compounds contain a carboxylgroup. The carboxyl group is written as

• COOH or –CO 2 H and has the structure

—C==O

|

OH

The systematic names of the acids end in ‘oic acid’. Many of the acids, however,

are still referred to by their old names. Some simple carboxylic acids are shown in

Table 18.3.

The prefixes ‘di’, ‘tri’ etc. are used to show more than one carboxylic acid group.

Ethanedioic acid (common name oxalic acid) is found in rhubarb leaves:

COOH

|

COOH

oxalic acid

18.4

Table 18.3Some carboxylic acids and their common names

Formula Systematic name Common name Where found

HCOOH methanoic acid formic acid ant or nettle stings

CH 3 COOH ethanoic acid acetic acid vinegar

CH 3 CH 2 COOH propanoic acid propionic acid milk, butter and cheese

C 6 H 5 COOH benzoic acid benzoic acid preservatives

Physical properties of carboxylic acids

Pure ethanoic (acetic) acid is a liquid, at room temperature, and is referred to as

glacial ethanoic acid. The melting and boiling points of carboxylic acids are rela-

tively high because of hydrogen bonding. Pairs of carboxylic acids molecules link up

to produce hydrogen bonded dimers:

Because of their ability to hydrogen bond, the lower molecular mass carboxylic acids

are soluble with water.

Methanoic, ethanoic and propanoic acids all have sharp, ‘vinegary’ smells. The

smell of butanoic acid is detected in rancid butter and human sweat. Each person’s

sweat glands produce a characteristic blend of carboxylic acids, and the ability of

dogs to track humans stems from the fact that the animals can detect and distinguish

these different combinations of carboxylic acids.

Acid strengths of carboxylic acids

Carboxylic acids are weak acids compared with dilute sulfuric, nitric or hydrochloric

acids:

CH 3 COOH(l)H 2 O(l)\==\CH 3 COO(aq)H 3 O(aq)