CONCEPT MAP: THE GENERATION OF BIOCHEMICAL ENERGY
23.2 Fatty Acids and Their Esters
Naturally occurring fats and oils are triesters formed between glycerol and fatty acids. Fatty acids are long, unbranched hydrocarbon chains with a carboxylic acid group at one end. Most have even numbers of carbon atoms. Fatty acids may or may not contain carbon–carbon double bonds. Those without double bonds are known as saturated fatty acids; those containing some double bonds are known as unsatu- rated fatty acids. If double bonds are present in naturally occurring fats and oils, the double bonds are usually cis rather than trans.
A saturated fatty acid (palmitic acid)
A cis unsaturated fatty acid (linolenic acid)
CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2C
CH3CH2CH
OH O
OH O
OH O CHCH2CH CHCH2CH CHCH2CH2CH2CH2CH2CH2CH2C Saturated fatty acid A long-chain
carboxylic acid containing only car- bon–carbon single bonds.
Unsaturated fatty acid A long-chain carboxylic acid containing one or more carbon–carbon double bonds.
Wax A mixture of monoesters of long- chain carboxylic acids with long-chain alcohols.
Recall that an ester, RCOOR⬘, is formed from a carboxylic acid and an alcohol (Section 17.4).
In the cis configuration, the groups attached to the double-bond carbons are on the same side of the double bond (Section 13.3).
Some of the common fatty acids are listed in Table 23.1. Chemists use a short- hand nomenclature for fatty acids that avoids using the common names. The notation uses C for carbon followed by the number of carbon atoms present in the fatty acid, a colon, and the number of unsaturated bonds present. Thus, lauric acid, which contains 12 carbon atoms and no double bonds, is represented by C12:0. Palmitic acid (C16:0) and stearic acid (C18:0) are the most common saturated acids; oleic (C18:1) and linoleic acids (C18:2) are the most common unsaturated ones. Oleic acid is monounsaturated, that is, it has only one carbon–carbon double bond. The polyunsaturated fatty acids have more than one carbon–carbon double bond.
Two of the polyunsaturated fatty acids, linoleic and linolenic, are essential in the human diet because the body does not synthesize them, even though they are needed for the synthesis of other lipids. Infants grow poorly and develop severe skin lesions if fed a diet lacking these acids. Adults usually have sufficient reserves in body fat to avoid such problems. A deficiency in adults can arise, however, after long-term intra- venous feeding that contains inadequate essential fatty acids or among those surviving on limited and inadequate diets. Malnutrition in the developed world also results from many other causes; two common ones are unusual weight-loss diets and anorexia.
Waxes
The simplest fatty acid esters in nature are waxes. A wax is a mixture of fatty acid–
long-chain alcohol esters. The acids usually have an even number of carbon atoms, generally from 16 to 36 carbons, whereas the alcohols have an even number of carbon atoms, ranging from 24 to 36 carbons. For example, a major component in beeswax Polyunsaturated fatty acid A long-
chain carboxylic acid that has two or more carbon–carbon double bonds.
S E C T I O N 2 3 . 2 Fatty Acids and Their Esters 725
is the ester formed from a 30-carbon alcohol (triacontanol) and a 16-carbon acid (palmitic acid). The waxy protective coatings on most fruits, berries, leaves, and ani- mal furs have similar structures. Aquatic birds have a water-repellent waxy coating on their feathers. When caught in an oil spill, the waxy coating dissolves in the oil and the birds lose their buoyancy.
C(CH2)14CH3 CH3(CH2)28CH2 O
O
Triacontanyl hexadecanoate (from beeswax) Example of a wax
Long-chain alcohol
Long-chain acid
Triacylglycerols
Animal fats and vegetable oils are the most plentiful lipids in nature. Although they appear different—animal fats like butter and lard are solid, whereas vegetable oils like corn, olive, soybean, and peanut oil are liquid—their structures are closely related. All fats and oils are composed of triesters of glycerol (1,2,3-propanetriol, also known as glycerine) with three fatty acids. They are named chemically as triacylglycerols, but are often called triglycerides.
CH2 O C R
R′
O
CH
R″
C O
O
CH2 O C O CH2OH RC
O
CHOH R′C O
CH2OH R″C O
Fatty acid
Fatty acid
Fatty acid Glycerol
Triacylglycerols
OH
OH
OH +
Glycerol Fatty acids
Triacylglycerol (triglyceride) A triester of glycerol with three fatty acids.
TABLE23.1 Structures of Some Common Fatty Acids
Name Typical
Source Number
of Carbons
Number of Double
Bonds Condensed Formula Melting
Point 1ⴗC2 Saturated
Lauric Coconut oil 12 0 CH31CH2210COOH 44
Myristic Butter fat 14 0 CH31CH2212COOH 58
Palmitic Most fats and oils
16 0 CH31CH2214COOH 63
Stearic Most fats and oils
18 0 CH31CH2216COOH 70
Unsaturated
Oleic Olive oil 18 1 CH31CH227CH“CH1CH227COOH1cis2 4
Linoleic Vegetable oils 18 2 CH31CH224CH“CHCH2CH“CH1CH227COOH1all cis2 -5 Linolenic Soybean and
canola oils
18 3 CH3CH2CH“CHCH2CH“CHCH2CH“CH1CH227COOH1all cis2 -11 Arachidonic Animal fat 20 4 CH31CH2241CH“CHCH224CH2CH2COOH1all cis2 -50
▲ This grebe is coated with oil spilled by a tanker that sank off Brittany on the northwest coast of France. If the oil is not removed from its feathers, the bird will perish.
TABLE23.2 Approximate Composition of Some Common Fats and Oils*
Unsaturated Fatty
Saturated Fatty Acids (%) Acids (%)
Source
C12:0 Lauric
C14:0 Myristic
C16:0 Palmitic
C18:0 Stearic
C18:1 Oleic
C18:2 Linoleic Animal Fat
Lard — 1 25 15 50 6
Butter 2 10 25 10 25 5
Human fat 1 3 25 8 46 10
Whale blubber — 8 12 3 35 10
Vegetable Oil
Corn — 1 8 4 46 42
Olive — 1 5 5 83 7
Peanut — — 7 5 60 20
Soybean — — 7 4 34 53
*Where totals are less than 100%, small quantities of several other acids are present, with cholesterol also present in animal fats.
The three fatty acids of any specific triacylglycerol are not necessarily the same, as is the case in the molecule below.
CH2 O C CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3
CH2CH2CH2CH2CH2CH2CH2CH O
CH CHCH2CH2CH2CH2CH2CH2CH2CH3
CH2CH2CH2CH2CH2CH2CH2CH CHCH2CH C
O O
CH2 O C O
Example of a triacylglycerol
CHCH2CH2CH2CH2CH3 Palmitic acid
Oleic acid
Linoleic acid
Furthermore, the fat or oil from a given natural source is a complex mixture of many different triacylglycerols. Table 23.2 lists the average composition of fats and oils from several different sources. Note particularly that vegetable oils consist almost entirely of unsaturated fatty acids, whereas animal fats contain a much larger percent- age of saturated fatty acids. This difference in composition is the primary reason for the different melting points of fats and oils, as explained in the next section.
PROBLEM 23.2
One of the constituents of the carnauba wax used in floor and furniture polish is an ester of a 32-carbon straight-chain alcohol with a C20:0 straight-chain carboxylic acid. Draw the structure of this ester. (Use subscripts to show the numbers of con- nected CH2 groups.)
PROBLEM 23.3
Draw the structure of a triacylglycerol whose components are glycerol and three oleic acid acyl groups.
PROBLEM 23.4
Which one should you choose for a treat—a small dipped ice cream cone from kiosk A or two oatmeal cookies from kiosk B? Some of the nutrition facts for these choices are listed in the table below. To decide, consider which snack would best help you
S E C T I O N 2 3 . 3 Properties of Fats and Oils 727 stay within the nutrition guidelines regarding daily intake of total fat and saturated
fat in the diet. (See Chemistry in Action: Lipids in the Diet on page 728.) Food
Total
Calories Total Fat (g)
Saturated Fat (g)
% Calories from Fat
Carbohydrates (g)
% Calories from Carbohydrates
Cone 340 17 9 45 42 49
2 Cookies 300 12 2 36 46 61
KEY CONCEPT PROBLEM 23.5
(a) Which animal fat has the largest percentage of saturated fatty acids?
(b) Which vegetable oil has the largest percentage of polyunsaturated fatty acids?
(c) Which fat or oil has the largest percentage of the essential fatty acid linoleic acid?