Organic Chemistry

17.13 Soaps, Detergents, and Micelles

Fatsand oilsare triesters of glycerol. Glycerol contains three alcohol groups and

therefore can form three ester groups. When the ester groups are hydrolyzed in a basic

solution, glycerol and carboxylate ions are formed. The carboxylic acids that are

bonded to glycerol in fats and oils have long, unbranched R groups. Because they are

obtained from fats, long-chain unbranched carboxylic acids are called fatty acids. In

Section 26.3, we will see that the difference between a fat and an oil resides in the

structure of the fatty acids.

Soapsare sodium or potassium salts of fatty acids. Thus, soaps are obtained when

fats or oils are hydrolyzed under basic conditions. The hydrolysis of an ester in a basic

solution is called saponification—the Latin word for “soap”is sapo. The following

compounds are three of the most common soaps:

PROBLEM 26 SOLVED

An oil obtained from coconuts is unusual in that all three fatty acid components are identi-

cal. The molecular formula of the oil is What is the molecular formula of the

carboxylate ion obtained when the oil is saponified?

SOLUTION When the oil is saponified, it forms glycerol and three equivalents of car-

boxylate ion. In losing glycerol, the fat loses three carbons and five hydrogens. Thus, the

three equivalents of carboxylate ion have a combined molecular formula of

Dividing by three gives a molecular formula of for the carboxylate ion.

Long-chain carboxylate ions do not exist as individual ions in aqueous solution. In-

stead, they arrange themselves in spherical clusters called micelles, as shown in

Figure 17.4. Each micelle contains 50 to 100 long-chain carboxylate ions. A micelle

resembles a large ball, with the polar head of each carboxylate ion and its counterion

on the outside of the ball because of their attraction for water and the nonpolar tail

buried in the interior of the ball to minimize its contact with water. The attractive

forces of hydrocarbon chains for each other in water are called hydrophobic inter-

actions(Section 23.14). Soap has cleansing ability because nonpolar oil molecules,

which carry dirt, dissolve in the nonpolar interior of the micelle and are carried away

with the soap during rinsing.

C 14 H 27 O 2

C 42 H 81 O 6.

C 45 H 86 O 6.

sodium stearate

O

CH 3 (CH 2 ) 16 CO− Na+

sodium oleate

CH 3 (CH 2 ) 7 CH

O

CH(CH 2 ) 7 CO− Na+

sodium linoleate

CH 3 (CH 2 ) 4 CH CHCH 2 CH

O

CH(CH 2 ) 7 CO− Na+

a fat or an oil glycerol sodium salts of fatty acids

soap

CH 2 OC

O

R^1

CHO C

O

R^2

NaOH

CH 2 OC

O

R^3

R

1

C

O

O− Na+

R

2

C

O

O− Na+

R

3

C

O

O− Na+

+ H 2 O

CH 2 OH

CHOH

CH 2 OH

+

700 CHAPTER 17 Carbonyl Compounds I

3-D Molecules:

Sodium stearate;

Sodium oleate;

Sodium linoleate