Organic Chemistry

Section 26.3 Fats and Oils 005

Triacylglycerols that are solids or semisolids at room temperature are called fats.
Fats are usually obtained from animals and are composed largely of triacylglycerols
with either saturated fatty acids or fatty acids with only one double bond. The saturat-
ed fatty acid tails pack closely together, giving the triacylglycerols relatively high
melting points, causing them to be solids at room temperature.

Liquid triacylglycerols are called oils. Oils typically come from plant products such
as corn, soybeans, olives, and peanuts. They are composed primarily of triacylglycerols
with unsaturated fatty acids that cannot pack tightly together. Consequently, they have
relatively low melting points, causing them to be liquids at room temperature. The
approximate fatty acid compositions of some common fats and oils are shown in
Table 26.2.
Some or all of the double bonds of polyunsaturated oils can be reduced by catalytic
hydrogenation (Section 4.11). Margarine and shortening are prepared by hydrogenat-
ing vegetable oils such as soybean oil and safflower oil until they have the desired con-
sistency. This process is called “hardening of oils.”The hydrogenation reaction must
be carefully controlled, however, because reducing all the carbon–carbon double
bonds would produce a hard fat with the consistency of beef tallow.

Vegetable oils have become popular in food preparation because some studies have
linked the consumption of saturated fats with heart disease. Recent studies have shown
that unsaturated fats may also be implicated in heart disease. However, one
unsaturated fatty acid—a 20-carbon fatty acid with five double bonds, known as EPA
and found in high concentrations in fish oils—is thought to lower the chance of devel-
oping certain forms of heart disease. Once consumed, dietary fat is hydrolyzed in the
intestine, regenerating glycerol and fatty acids. We have seen that the hydrolysis of fats
under basic conditions forms glycerol and salts of fatty acids that are commonly
known as soap(Section 17.13).

H 2

RCH CHCH 2 CH CHCH 2 CH CH Pt RCH 2 CH 2 CH 2 CH CHCH 2 CH 2 CH 2

a fat an oil

glycerol

O

O

O

CH 2 OH

CH 2 OH

CH OH

CH 2 O

CH 2 O

CH O

fatty acids

R^1 C OH

R^3 C OH

R^2 C OH

O

O

O

a triacylglycerol

a fat or an oil

C R^1

C R^3

C R^2

This puffin’s diet is high in fish oil.