Chemistry, Third edition

CARBOXYLIC ACIDS 339

If an electronegative species, such as chlorine, is present in the acid molecule, close

to the carboxyl group, chlorine pulls electrons towards it. This stabilizes the anion

produced when the acid ionizes. For example, consider the reaction of chloro-

ethanoic acid, CH 2 ClCOOH, with water:

Cl—CH 2 COOHH 2 O\==\Cl—CH 2 COO H 3 O

The Cl—CH 2 COO ion is more stable than CH 3 COO, so Kafor chloroethanoic

acid is much greater than that for ethanoic acid. Additional electron-withdrawing

groups increase this effect, but the influence of the electron-withdrawing group on

acid strength falls off with distance.

Some reactions of carboxylic acids

1.Carboxylate salts are formed when a carboxylic acid reacts with base:

CH 3 COOH(aq)NaOH(aq)CH 3 COONa(aq)H 2 O(l)

sodium ethanoate

2.Carboxylic acids react with alcohols in an acid catalysed, reversible reaction. The

compounds formed are called esters. An ester contains the –COOR (or –CO 2 R)

group:

—C==O

|

OR

In a typical example of an esterification reaction, glacial ethanoic acid is heated

with ethanol in the presence of concentrated sulfuric acid, which acts as the cata-

lyst. The sweet-smelling ester ethyl ethanoate is formed:

CH 3 COOH(l)CH 3 CH 2 OH(l)\==\CH 3 COOCH 2 CH 3 (l)H 2 O(l)

ethyl ethanoate

The general reaction is

RCOOHROH\==\RCOORH 2 O

Note that these reactions are reversible. In order to maximize the yield of ester,

excess of one of the reactants is used (usually the cheaper one!). Concentrated

sulfuric acid, besides acting as the catalyst, removes the water produced, so shift-

ing the equilibrium composition so that the concentration of ester increases.

Because the reaction is reversible an ester can be hydrolysed, or reacted with

water, in the presence of an acid catalyst, to make the parent alcohol and acid.

3.Carboxylic acids are reduced to primary alcohols using lithium aluminium hydride:

LiAlH 4

CH 3 COOH —CH 3 CH 2 OH

Strength of

carboxylic acids

Arrange the following in

order of increasingacid

strength:

CHCl 2 COOH, CF 3 COOH,

CH 3 COOH, CCl 3 COOH,

CH 2 BrCOOH, CH 2 ClCOOH.

Exercise 18J

Esterification

(i) Name and write the

structural formulae of

the esters produced

from the reactions of the

following acids and alco-

hols:

(a)ethanoic acid and

methanol

(b)octanoic acid and

ethanol

(c)methanoic acid and

methanol

(d)oxalic acid and

ethanol.

(ii)Which alcohols and

acids would be obtained

on hydrolysis of the

following esters?

(a)methyl propanoate

(b)ethyl methanoate

(c)CH 3 COOCH 2 CH 2 CH 3

(d)CH 2 ClCOOCH 3.

Exercise 18K

BOX 18.2

Fatty acids

Carboxylic acids with long hydrocarbon chains can be obtained from hydrolysis of fats or oils. They

are called fatty acids. Stearic acidis a fatty acid and its sodium salt (sodium stearate) is used as

soap. The hydrocarbon chain in a fatty acid may be saturated, as in stearic acid, or it may be

unsaturated(contain a double bond). Oleic acid, obtained from corn oil, contains one double

bond, whereas fatty acids with more than one double bond can be prepared from vegetable oils.

Common name Formula Systematic name

Stearic acid CH 3 (CH 2 ) 16 COOH octadecanoic acid

Oleic acid CH 3 (CH 2 ) 7 CH=CH(CH 2 ) 7 COOH octadec-9-enoic acid