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3B Focus on Health & Medicine: Saturated and Unsaturated Fatty Acids

3B Focus on Health & Medicine: Saturated and Unsaturated Fatty Acids

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388



TABLE 13.1



UNSATURATED HYDROCARBONS



Common Saturated and Unsaturated Fatty Acids



Name



Structure



Mp (°C)



Stearic acid

(0 C C)



CH3CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2COOH

H



Oleic acid

(1 C C)



C



CH3CH2CH2CH2CH2CH2CH2CH2



C



HH

C



CH3CH2



C



H

C



C



CH2



CH3CH2CH2CH2CH2



C



C



–5

CH2CH2CH2CH2CH2CH2CH2COOH



HH



CH2



16

CH2CH2CH2CH2CH2CH2CH2COOH



HH

C



H



Linolenic acid

(3 C C)



H

C



H



Linoleic acid

(2 C C)



71



H

C



CH2



C



–11

CH2CH2CH2CH2CH2CH2CH2COOH



Table 13.1 lists the structure and melting point of four fatty acids containing 18 carbon atoms.

Stearic acid is one of the two most common saturated fatty acids, while oleic and linoleic acids

are the most common unsaturated ones.

Linoleic and linolenic acids are essential fatty acids, meaning they cannot be synthesized in the

human body and must therefore be obtained in the diet. A common source of these essential fatty

acids is whole milk. Babies fed a diet of nonfat milk in their early months do not thrive because

they do not obtain enough of these essential fatty acids.

One structural feature of unsaturated fatty acids is especially noteworthy.

• Generally, double bonds in naturally occurring fatty acids are cis.



The presence of cis double bonds affects the melting point of these fatty acids greatly.

• As the number of double bonds in the fatty acid increases, the melting point decreases.



The cis double bonds introduce kinks in the long hydrocarbon chain, as shown in Figure 13.2. This

makes it difficult for the molecules to pack closely together in a solid. The larger the number of

cis double bonds, the more kinks in the hydrocarbon chain, and the lower the melting point.

Fats and oils are organic molecules synthesized in plant and animal cells from fatty acids. Fats

and oils have different physical properties.

• Fats are solids at room temperature. Fats are generally formed from fatty acids having

few double bonds.

• Oils are liquids at room temperature. Oils are generally formed from fatty acids having a

larger number of double bonds.



Saturated fats are typically obtained from animal sources, while unsaturated oils are

common in vegetable sources. Thus, butter and lard are formed from saturated fatty acids,

while olive oil and safflower oil are formed from unsaturated fatty acids. An exception to this

generalization is coconut oil, which is composed largely of saturated fatty acids.

Considerable evidence suggests that an elevated cholesterol level is linked to increased risk of

heart disease. Saturated fats stimulate cholesterol synthesis in the liver, resulting in an increase

in cholesterol concentration in the blood. We will learn more about fats and oils in Section 13.7

and Chapter 19.



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INTERESTING ALKENES IN FOOD AND MEDICINE







FIGURE 13.2



389



The Three-Dimensional Structure of Four Fatty Acids



oleic acid



stearic acid



linoleic acid



PROBLEM 13.13



linolenic acid



Draw out a structure of arachidonic acid, indicating the arrangement of groups around the four

double bonds.

CH3(CH2)4CH



CHCH2CH



CHCH2CH



CHCH2CH



CHCH2(CH2)2CO2H



arachidonic acid



PROBLEM 13.14



You have two fatty acids, one with a melting point of 63 °C, and one with a melting point of

1 °C. Which structure corresponds to each melting point?

CH3(CH2)14COOH



CH3(CH2)5CH



palmitic acid



CH(CH2)7COOH



palmitoleic acid



13.4 INTERESTING ALKENES IN FOOD AND MEDICINE

In addition to the plant hormone ethylene, CH2 CH2 (Section 13.1), many useful compounds

contain one or more carbon–carbon double bonds.

Lycopene, the red pigment in tomatoes and watermelon, contains 13 double bonds. Lycopene

is an antioxidant, a compound that prevents an unwanted oxidation reaction from occurring.

Diets that contain a high intake of antioxidants like lycopene have been shown to decrease the

risk of heart disease and certain forms of cancer. Unlike other antioxidants that can be destroyed

when fresh fruits or vegetables are processed, even foods such as tomato paste, tomato juice, and

ketchup are high in lycopene.



More examples of antioxidants are

given in Section 13.12.



CH3



CH3



CH3



H



C



C



C



CH3



H



C



C



CH3



H



C



C



H



H



C



H



C



H



C



H



H



C



C



C



CH3



C(CH2)2



C



C



C



C



C



C



C



C



C



C



C(CH2)2



C



H



H



H



H



H



H



H



CH3



H



CH3



H



CH3



CH3



lycopene



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390



UNSATURATED HYDROCARBONS



Lycopene’s red color is due to the 11 double bonds that are each separated by one single bond

(labeled in red). These double bonds absorb some (but not all) wavelengths of visible light. When

a compound absorbs visible light, it takes on the color of those wavelengths of light it does not

absorb. Since lycopene absorbs blue-green light, it appears red because it does not absorb the red

light of the visible spectrum.



HEALTH NOTE



Lycopene absorbs this part of the visible region.



visible region

This part of the spectrum is not absorbed.

Lycopene appears red.



Processed tomato products contain

the antioxidant lycopene.



Tamoxifen is a potent anticancer drug that contains a carbon–carbon double bond in addition to

other functional groups. Tamoxifen is used widely in the treatment of certain breast cancers that

require the female sex hormone estrogen for growth. Tamoxifen binds to estrogen receptors, and

in this way it inhibits the growth of breast cancers that are estrogen dependent. Tamoxifen is sold

under the trade name of Novaldex.

OCH2CH2N(CH3)2



C



C



tamoxifen



CH3CH2



PROBLEM 13.15



Identify all of the functional groups in tamoxifen.



13.5 FOCUS ON HEALTH & MEDICINE

ORAL CONTRACEPTIVES

The development of synthetic oral contraceptives in the 1960s revolutionized the ability to

control fertility. Prior to that time, women ingested all sorts of substances—iron rust, gunpowder,

tree bark, sheep’s urine, elephant dung, and others—in the hope of preventing pregnancy.

Synthetic birth control pills are similar in structure to the female sex hormones estradiol and

progesterone, but they also contain a carbon–carbon triple bond. Most oral contraceptives

contain two synthetic hormones that are more potent than these natural hormones, so they can be

administered in lower doses.

O



CH3 OH



CH3 C



CH3



CH3



HO



O

estradiol



smi26573_ch13.indd 390



progesterone



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FOCUS ON HEALTH & MEDICINE: ORAL CONTRACEPTIVES







FIGURE 13.3



391



How Oral Contraceptives Work



CH3 OH

C



CH



A

HO

ethynylestradiol

(a synthetic estrogen)

pituitary gland

CH3 OH

C



CH



uterus

O



norethindrone

(a synthetic progesterone)



ovary



B



[The carbon–carbon triple bond is drawn in red.]



cervix



Monthly cycles of hormones from the pituitary gland cause ovulation, the release of an egg from an ovary. To prevent pregnancy, the

two synthetic hormones in many oral contraceptives have different effects on the female reproductive system. A: The elevated level

of ethynylestradiol, a synthetic estrogen, “fools” the pituitary gland into thinking a woman is pregnant, so ovulation does not occur.

B: The elevated level of norethindrone, a synthetic progesterone, stimulates the formation of a thick layer of mucus in the cervix,

making it difficult for sperm to reach the uterus.



Two common components of birth control pills are ethynylestradiol and norethindrone. Ethynylestradiol is a synthetic estrogen that resembles the structure and biological activity of estradiol.

Norethindrone is a synthetic progesterone that is similar to the natural hormone progesterone.

These compounds act by artificially elevating hormone levels in a woman, and this prevents pregnancy, as illustrated in Figure 13.3.

Two other synthetic hormones with triple bonds are RU 486 and levonorgestrel. RU 486 blocks

the effects of progesterone, and by this means, prevents implantation of a fertilized egg. RU 486

is used to induce abortions within the first few weeks of pregnancy. Levonorgestrel interferes

with ovulation, and thereby prevents pregnancy if taken within a few days of unprotected sex.

(CH3)2N

CH3 OH

C



O



C



CH3CH2 OH

C



CH3



CH



O

RU 486

(Trade name: Mifepristone)



levonorgestrel

(Trade name: Plan B)



[The carbon–carbon triple bond is drawn in red.]



PROBLEM 13.16



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Identify the functional groups in RU 486 and levonorgestrel.



12/8/08 10:53:15 AM



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