Organic Chemistry

PROBLEM 3

Do all triacylglycerols have the same number of asymmetric carbons?

PROBLEM 4

Which has a higher melting point, glyceryl tripalmitoleate or glyceryl tripalmitate?

Organisms store energy in the form of triacylglycerols. A fat provides about six

times as much metabolic energy as an equal weight of hydrated glycogen because fats

are less oxidized than carbohydrates and, since fats are nonpolar, they do not bind

water. In contrast, two-thirds of the weight of stored glycogen is water (Section 22.18).

Animals have a subcutaneous layer of fat cells that serves as both an energy source

and an insulator. The fat content of the average man is about 21%, whereas the fat con-

tent of the average woman is about 25%. Humans can store sufficient fat to provide for

the body’s metabolic needs for two to three months, but can store only enough carbo-

hydrate to provide for its metabolic needs for less than 24 hours. Carbohydrates, there-

fore, are used primarily as a quick, short-term energy source.

Polyunsaturated fats and oils are easily oxidized by by means of a radical chain

reaction. In the initiation step, a radical removes a hydrogen from a methylene group

that is flanked by two double bonds. This hydrogen is the most easily removed one

O 2

006 CHAPTER 26 Lipids

OLESTRA: NONFAT WITH FLAVOR

Chemists have been searching for ways to reduce

the caloric content of foods without decreasing

their flavor. Many people who believe that “no fat”is synony-

mous with “no flavor”can understand this problem. The federal

Food and Drug Administration (Section 30.13) approved the

limited use of Olestra as a substitute for dietary fat in snack

foods. Procter and Gamble spent 30 years and more than $2

billion to develop this compound. Its approval was based on the

results of more than 150 studies.

Olestra is a semisynthetic compound. That is, Olestra does

not exist in nature, but its components do. Developing a com-

pound that can be made from units that are a normal part of our

diet decreases the potential toxic effects of the new compound.

Olestra is made by esterifying all the OH groups of sucrose

with fatty acids obtained from cottonseed oil and soybean oil.

Therefore, its component parts are table sugar and vegetable

oil. Olestra works as a fat substitute because its ester linkages

are too hindered to be hydrolyzed by digestive enzymes. As a

result, Olestra tastes like fat, but it has no caloric value since it

cannot be digested.

CO

CO

CO

OC

COCH 2

CH 2 OC

CH 2

O

O

O

2

1

OC

O

O

O

O

O

O

O

OC

O

olestra

3-D Molecule:

Olestra