Organic Chemistry

acetic acid

702 CHAPTER 17 Carbonyl Compounds I

which means “to wipe off.”After the initial introduction of detergents into the market-

place, it was discovered that straight-chain alkyl groups are biodegradable, whereas

branched-chain alkyl groups are not. To prevent nonbiodegradable detergents from

polluting rivers and lakes, detergents should have straight-chain alkyl groups.

Compounds that lower the surface tension of water are called surfactants. Surfac-

tants have a polar head group and a long-chain nonpolar tail. Soaps and detergents are

surfactants—that is why soap and detergent solutions feel slippery. Lowering the

surface tension enables the soap or detergent to penetrate the fibers of a fabric, thus

enhancing its cleaning ability. Because the polar head of a soap or a detergent is

negatively charged, it is called an anionicsurfactant. Cationicsurfactants are used

widely as fabric softeners and hair conditioners.

17.14 Reactions of Carboxylic Acids

Carboxylic acids can undergo nucleophilic acyl substitution reactions only when they

are in their acidic forms. The basic form of a carboxylic acid does not undergo nucleo-

philic acyl substitution reactions because the negatively charged carboxylate ion is

resistant to nucleophilic attack (Section 17.12). Thus, carboxylate ions are even less

reactive toward nucleophilic acyl substitution reactions than are amides.

Carboxylic acids have approximately the same reactivity as esters—the leav-

ing group of a carboxylic acid has approximately the same basicity as the leaving

group of an ester. Therefore, like esters, carboxylic acids do not react with halide ions

or with carboxylate ions.

Carboxylic acids react with alcohols to form esters. The reaction must be carried

out in an acidic solution, not only to catalyze the reaction but also to keep the car-

boxylic acid in its acid form so that it will react with the nucleophile. Because the

tetrahedral intermediate formed in this reaction has two potential leaving groups of ap-

proximately the same basicity, the reaction must be carried out with excess alcohol to

drive it toward products. Emil Fischer (Section 5.5) was the first to discover that an

ester could be prepared by treating a carboxylic acid with excess alcohol in the pres-

ence of an acid catalyst, so the reaction is called a Fischer esterification.

Carboxylic acids do not undergo nucleophilic acyl substitution reactions with amines.

Because a carboxylic acid has a lower pKathan a protonated amine, the carboxylic

+

CH 3

+

HCl

methyl alcohol

excess

acetic acid methyl acetate

CH 3 OH H 2 O

OCH 3

C

O

CH 3 OH

C

O

RO-

HO-

RNHOH >>R 2 R O−

relative reactivities toward nucleophilic acyl substitution

C

O

C

O

C

O

most reactive least reactive

a cationic surfactant

a germicide

CH 3 Cl–

CH 3

an anionic surfactant

a detergent

O

O

a sulfonic acid

O

O

O–^ Na+ CH 2 N(CH 2 ) 15 CH 3

+

R S OH CH 3 (CH 2 ) 11 S