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10 Focus on Health & Medicine: Histamine and Antihistamines

10 Focus on Health & Medicine: Histamine and Antihistamines

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CHAPTER HIGHLIGHTS



HEALTH NOTE



563



is also responsible for the symptoms of allergies, including a runny nose and watery eyes. In the

stomach, histamine stimulates the secretion of acid.

Antihistamines bind to the H1 histamine receptor, but they evoke a different response. An antihistamine like chlorpheniramine or diphenhydramine, for example, inhibits vasodilation, so it

is used to treat the symptoms of the common cold and environmental allergies. These antihistamines also cause drowsiness by crossing the blood–brain barrier and binding to H1 receptors

in the central nervous system. Newer antihistamines like fexofenadine (trade name: Allegra)

also bind to H1 receptors, but they do not cause drowsiness because they cannot cross the

blood–brain barrier and bind to the H1 receptors in the central nervous system.



Scombroid fish poisoning, associated with facial flushing, hives, and

general itching, is caused by the

ingestion of inadequately refrigerated fish, typically mahimahi and

tuna. Bacteria convert the amino

acid histidine to histamine, and when

consumed in large amounts, this

clinical syndrome results.



Cl



CHOCH2CH2N(CH3)2



CHCH2CH2N(CH3)2

N



diphenhydramine

(antihistamine)



chlorpheniramine

(antihistamine)

OH



CH3



C



N



(CH2)3CH



C



CH3



COOH



OH



fexofenadine

(nonsedating antihistamine)



Drugs that bind to the H2 receptor produce a different effect. For example, cimetidine (trade

name: Tagamet) binds to the H2 histamine receptor and, as a result, reduces acid secretion in the

stomach. Thus, cimetidine is an effective anti-ulcer medication.

N

CH2SCH2CH2NHCNHCH3

HN



N



CN



CH3

cimetidine

(Trade name: Tagamet)

anti-ulcer drug



PROBLEM 18.30



Draw a complete structure for cimetidine including all lone pairs on heteroatoms.



CHAPTER HIGHLIGHTS

KEY TERMS

Alkaloid (18.4)

Amine (18.1)

Ammonium salt (18.6)

Antihistamine (18.10)

Anti-ulcer drug (18.10)

Axon (18.8)

Dendrite (18.8)



smi26573_ch18.indd 563



Heterocycle (18.1)

Hormone (18.9)

Neuron (18.8)

Neurotransmitter (18.8)

Postsynaptic neuron (18.8)

Presynaptic neuron (18.8)

Primary (1°) amine (18.1)



Quaternary ammonium ion (18.1)

Secondary (2°) amine (18.1)

SSRI (18.8)

Synapse (18.8)

Tertiary (3°) amine (18.1)

Vesicle (18.8)



1/5/10 3:17:31 PM



564



AMINES AND NEUROTRANSMITTERS



KEY REACTIONS

Amines react as bases with inorganic and organic acids (18.6).

H

R



N



H



+



H



Cl



H



R



N+ H



+



Cl−



H



KEY CONCEPTS

❶ What are the characteristics of amines? (18.1, 18.3)

• Amines are organic nitrogen compounds, formed by

replacing one or more hydrogen atoms of NH3 by alkyl

groups.

• Primary (1°) amines have one C N bond; 2° amines have

two C N bonds; 3° amines have three C N bonds.

• An amine has a lone pair of electrons on the N atom, and is

trigonal pyramidal in shape.

• Amines contain polar C N and N H bonds. Primary (1°)

and 2° amines can hydrogen bond. Intermolecular hydrogen

bonds between N and H are weaker than those between O

and H.

• Tertiary (3°) amines have lower boiling points than 1° and

2° amines since they cannot hydrogen bond.

❷ How are amines named? (18.2)

• Primary (1°) amines are identified by the suffix -amine.

• Secondary (2°) and 3° amines with identical alkyl groups

are named by adding the prefix di- or tri- to the name of the

1° amine.

• Secondary (2°) and 3° amines with different alkyl groups

are named as N-substituted 1° amines.

❸ What are alkaloids? Give examples of common alkaloids.

(18.4, 18.5)

• Alkaloids are naturally occurring amines isolated from

plant sources.

• Examples of alkaloids include caffeine (from coffee and

tea), nicotine (from tobacco), morphine and codeine (from

the opium poppy), quinine (from the cinchona tree), and

atropine (from the deadly nightshade plant).

❹ What products are formed when an amine is treated with

acid? (18.6)

• Amines act as proton acceptors in water and acid. For

example, the reaction of RNH2 with HCl forms the watersoluble ammonium salt RNH3+ Cl–.

❺ What are the characteristics of ammonium salts and how

are they named? (18.6, 18.7)

• An ammonium salt consists of a positively charged

ammonium ion and an anion.

• An ammonium salt is named by changing the suffix -amine

of the parent amine to the suffix -ammonium followed by

the name of the anion.



smi26573_ch18.indd 564



• Ammonium salts are water-soluble solids.

• Water-insoluble amine drugs are sold as their ammonium

salts to increase their solubility in the aqueous environment

of the blood.

➏ What are neurotransmitters and how do they differ from

hormones? (18.8, 18.9)

• A neurotransmitter is a chemical messenger that transmits a

nerve impulse from a neuron to another cell.

• A hormone is a compound produced by an endocrine gland

that travels through the bloodstream to a target tissue or

organ.

❼ What roles do dopamine and serotonin play in the body?

(18.8)

• Dopamine affects movement, emotions, and pleasure.

Too little dopamine causes Parkinson’s disease. Too much

dopamine causes schizophrenia. Dopamine plays a role in

addiction.

• Serotonin is important in mood, sleep, perception, and

temperature regulation. A deficiency of serotonin causes

depression. SSRIs are antidepressants that effectively

increase the concentration of serotonin.

❽ Give examples of important derivatives of

2-phenylethylamine. (18.9)

• Derivatives of 2-phenylethylamine contain a benzene ring

bonded to a two-carbon chain that is bonded to a nitrogen

atom.

• Examples of 2-phenylethylamine derivatives include

epinephrine, norepinephrine, amphetamine, and

methamphetamine. Albuterol and salmeterol are also

derivatives of 2-phenylethylamine that are used to treat

asthma.

❾ What is histamine, and how do antihistamines and antiulcer drugs work? (18.10)

• Histamine is an amine with a wide range of physiological

effects. Histamine dilates capillaries, is responsible for the

runny nose and watery eyes of allergies, and stimulates the

secretion of stomach acid.

• Antihistamines bind to the H1 histamine receptor and

inhibit vasodilation, so they are used to treat the symptoms

of colds and allergies.

• Anti-ulcer drugs bind to the H2 histamine receptor and

reduce the production of stomach acid.



12/16/08 10:45:00 AM



PROBLEMS



565



PROBLEMS

Selected in-chapter and end-of-chapter problems have brief answers provided in Appendix B.



Structure and Bonding

18.31



NHCH3



Classify each amine as 1°, 2°, or 3°.

CH3



a.



CH3CHCH2CH2NHCH3



c.



CH3



18.32



N



CH3CH2CH2CHCH2CH3



f.



CH3CHCH2CH2CH2CH2CH3

NHCH2CH3



NH2



18.38



CH3



CH3



b.



C



e.



CH3



Give an acceptable name for each amine.

a. CH3(CH2)6NH2

b.



Classify each amine as 1°, 2°, or 3°.



CH3CH2CH2CH2CH2N(CH3)2



c.



NHCH2CH3



CH3



a.



CH3



C



CH2NH2



c.



N(CH3)2



d.



N



CH2CH3



CH3

Cl



b.

18.33



18.34



18.35



18.36



e.



NH



Draw the structure of a compound that fits each

description:

a. a 1° amine with molecular formula C5H13N

b. a 2° amine with molecular formula C6H15N

c. a 3° amine with molecular formula C6H13N

d. a quaternary ammonium ion with molecular formula

C8H20N+

Draw the structure of a compound of molecular formula

C4H11NO that fits each description:

a. a compound that contains a 1° amine and a 1° alcohol

b. a compound that contains a 2° amine and a 2° alcohol

c. a compound that contains a 1° amine and a 3° alcohol

d. a compound that contains a 3° amine and a 1° alcohol

What is the difference between a 3° amine and a 3°

alcohol? Give an example of each compound that

contains six carbons.

What is the difference between a 3° amine and a 3°

amide? Give an example of each compound that contains

six carbons.



CH3CHCH3



f.



Give an acceptable name for each amine.



18.39



18.40



18.41



18.42



H



a.



CH3CH2



N



CH2CH3



b.



NH2



c.



NCH2CH3



18.43



18.44

CH3

NH2



d.



smi26573_ch18.indd 565



CH3CH2CHCH2CH2CH2CH2CH3



N



CH2CH3



CH3



Nomenclature

18.37



NH2



18.45



Draw the structure corresponding to each name.

a. 1-decanamine

b. tricyclohexylamine

c. p-bromoaniline

d. 3-aminobutanoic acid

e. N,N-dipropyl-2-octanamine

f. N-ethylhexylamine

Draw the structure corresponding to each name.

a. cyclobutylamine

b. 4-nonanamine

c. N-pentylaniline

d. 3-amino-2-butanone

e. 3-methyl-1-butanamine

f. N-ethyl-N-propylcyclopentylamine

What is the difference between N,N-dimethylaniline

and 2,4-diethylaniline? Draw structures to illustrate the

difference.

What is the difference between N-butylaniline and

o-butylaniline? Draw structures to illustrate the difference.

Draw the structure of each ammonium salt.

a. dipropylammonium chloride

b. butylammonium bromide

c. ethyldimethylammonium hydroxide

Draw the structure of each ammonium salt.

a. cyclohexylammonium fluoride

b. triethylammonium iodide

c. ethylmethylammonium chloride

Draw the structures for the four amines of molecular

formula C3H9N. Give the systematic name for each amine.



12/16/08 10:45:00 AM



566



18.46



AMINES AND NEUROTRANSMITTERS



(a) Draw the structures for the eight constitutional

isomers of molecular formula C4H11N. (b) Give the

systematic name for each amine. (c) Identify the chirality

center present in one of the amines.



18.57



Draw the products of each acid–base reaction.

a. CH3CH2CH2N(CH3)2 + HCl

NH2



b.



+



CH3CH2CHCH2CH3



H2SO4



Physical Properties and Intermolecular Forces

18.47



18.48

18.49



Explain why pyridine is more water soluble than benzene.

How would you expect the boiling point of pyridine and

benzene to compare?

Explain why CH3CH2CH2NH2 is water soluble but

CH3CH2CH2CH3 is not water soluble.

Which compound in each pair has the higher boiling

point?

a. CH3CH2CH2NH2 or CH3(CH2)7NH2

b.



CH3(CH2)6OH



c.



or



+



c.



H



d.



+



CH3CH2 N



+



CH2CH3



NaOH



CH2CH3



18.58



Draw the products of each acid–base reaction.

+



a.



CH3(CH2)6NH2



CH2N(CH3)2



HBr



N



HCl



N



or



b.



CH2CH2CH2NH2



+



CH3CH2CHCH2CH3



H2SO4



NHCH3



18.50



Which compound in each pair has the higher boiling

point?

a. (CH3CH2)2C O or (CH3CH2)2CHNH2



c.

d.

18.59



b.

Cl



c.

18.51

18.52

18.53



18.54



NH



or



+



CH3CHCH2CHCH3



or



Acid–Base Reactions of Amines

18.55



18.56



Draw the acid–base reaction that occurs when each amine

dissolves in water: (a) CH3CH2NH2; (b) (CH3CH2)2NH;

(c) (CH3CH2)3N.

What ammonium salt is formed when each amine is

treated with HCl?

NH2



b.



CH2NHCH3



c.



N(CH2CH3)2



smi26573_ch18.indd 566



+



CH3(CH2)5NH3



N

H



NaOH



CH2CH2CH3



coniine

HO



b.



O

N

CH3



HO

morphine



18.60



What ammonium salt is formed when each biologically

active amine is treated with H2SO4?

a.



CH2CHNH2

CH3

amphetamine

HO



b.

a.



CH3COOH



What ammonium salt is formed when each alkaloid is

treated with HCl?



CH3CH2CH2CHCH3



Draw the hydrogen bonding interactions that occur

between two molecules of diethylamine [(CH3CH2)2NH].

Draw the hydrogen bonding interactions that occur

between diethylamine [(CH3CH2)2NH] and water.

Explain why a 1° amine and a 3° amine having the same

number of carbons are soluble in water to a similar

extent, but the 1° amine has a higher boiling point.

Which compound has the higher water solubility:

CH3(CH2)5NH2 or CH3(CH2)5NH3+ Cl–? Explain your

choice.



+

+



a.



NH3 Cl−



NH2



CH3NH2



HO



CH2CH2NH2

dopamine



12/16/08 10:45:01 AM



PROBLEMS



567



Neurotransmitters, Alkaloids, and Other

Biologically Active Amines

18.61

18.62



18.63

18.64

18.65

18.66



18.67

18.68

18.69



18.70



18.71

18.72

18.73



18.74

18.75



What type of nitrogen heterocycle occurs in both coniine

and morphine (Problem 18.59)?

Only one of the N atoms in nicotine has a trigonal

pyramidal shape. Identify which N is trigonal pyramidal

and explain why the other N atom has a different

molecular shape.

What physiological effects does caffeine produce on the

body?

Why should caffeine intake be limited in pregnant and

nursing mothers?

Why are aqueous solutions of an alkaloid slightly basic?

For each alkaloid, list its natural source and give one

of its uses in medicine: (a) morphine; (b) quinine;

(c) atropine.

What is the difference between a presynaptic neuron and

a postsynaptic neuron?

Besides the cell body, what are the important parts of a

neuron?

What are the physiological effects of dopamine? What

conditions result when dopamine levels are too high or

too low?

Explain why patients with Parkinson’s disease cannot be

treated by giving them dopamine, even though the disease

results from low dopamine levels.

What are the physiological effects of serotonin? What

condition results when there is too little serotonin?

Give two examples of SSRIs and explain how they

combat depression.

In the body, what amino acid is used to synthesize each

compound: (a) dopamine; (b) norepinephrine;

(c) serotonin; (d) histamine?

What is the difference between a neurotransmitter and a

hormone?

Which of the following amines are derivatives

of 2-phenylethylamine? Label the atoms of

2-phenylethylamine in those amines that contain this unit.



18.76



CH3 CH3



a.



C



CH2N(CH3)2



18.77



b.

HN

CH2



b.



CH2NH



c.



CH2CH2



CH3



N



Locate the atoms of 2-phenylethylamine in each drug.

a.



CH2CNH2

CH3

phentermine

(one component of fen–phen,

a banned diet drug)



b.



CH2CH2



N



N



COCH2CH3

fentanyl

(a narcotic pain reliever)



18.78



Locate the atoms of 2-phenylethylamine in each drug.

OH



a.



HO

HOCH2



CHCH2NHC(CH3)3



albuterol

(asthma drug)

CH3



b.



N



N



CH2CH2



COCH2CH3

3-methylfentanyl

(street drug, 3,000 times

more potent than morphine)



18.79



18.80



Give an example of an antihistamine. Explain how an

antihistamine relieves the runny nose and watery eyes of

a cold or allergies.

Give an example of an anti-ulcer drug, and explain how it

is used to treat a stomach ulcer.



General Questions

18.81



c.



C



CH3



CH3

CH2CH2CH2NH2



C



CH3 NHCH3



CH3



a.



Which of the following amines are derivatives

of 2-phenylethylamine? Label the atoms of

2-phenylethylamine in those amines that contain this unit.



Benzphetamine (trade name: Didrex) is a habitforming diet pill with many of the same properties as

amphetamine.

CH3

CH2CHNCH2

CH3

benzphetamine



smi26573_ch18.indd 567



12/16/08 10:45:02 AM



568



AMINES AND NEUROTRANSMITTERS



a.

b.

c.

d.

e.

f.

g.

18.82



Label the amine as 1°, 2°, or 3°.

Label the chirality center in benzphetamine.

Draw the two enantiomers of benzphetamine.

Draw a constitutional isomer that contains a 1° amine.

Draw a constitutional isomer that contains a 3° amine.

Draw the structure of benzphetamine hydrochloride.

What products are formed when benzphetamine is

treated with acetic acid, CH3COOH?

Phentermine is one component of the banned diet drug

fen–phen.



a.

b.

c.

d.

e.

f.



Identify the functional groups.

Label the amine as 1°, 2°, or 3°.

Label any alcohol as 1°, 2°, or 3°.

Label any 4° carbon.

Locate any chirality centers.

Draw the structure of terfenadine hydrochloride.



Applications

18.85



CH3

CH2CNH2



The antihistamine in the over-the-counter product

Chlortrimeton is chlorpheniramine maleate. Draw the

structure of the amine and carboxylic acid that are used to

form this ammonium salt.

H



CH3

Cl



phentermine



a.

b.

c.

d.

e.

f.

18.83



Label the amine as 1°, 2°, or 3°.

Indicate the molecular shape around each atom.

Draw a constitutional isomer that contains a 1° amine.

Draw a constitutional isomer that contains a 2° amine.

Draw the structure of phentermine hydrobromide.

What products are formed when phentermine is treated

with benzoic acid, C6H5COOH?

Ritalin is the trade name for methylphenidate, a drug used

to treat attention deficit hyperactivity disorder (ADHD).



COO−



HOOC



+



CHCH2CH2N(CH3)2



C

H



C

H



N

chlorpheniramine maleate



18.86



Dextromethorphan hydrobromide is a cough

suppressant used in many over-the-counter cough and

cold medications. Given the structure of the parent

amine dextromethorphan, what is the structure of its

hydrobromide salt?



HN

CH3O



CH

COOCH3

methylphenidate

(Trade name: Ritalin)



18.84



N

CH3



a. Identify the functional groups.

b. Label the amine as 1°, 2°, or 3°.

c. Locate the chirality centers.

d. Locate the atoms of 2-phenylethylamine.

e. Draw the structure of methylphenidate hydrochloride.

Seldane is the trade name for terfenadine, an

antihistamine once used in the United States but

withdrawn from the market because of cardiac side

effects observed in some patients.

OH

(CH3)3C



CHCH2CH2CH2



dextromethorphan



18.87



18.88

18.89



OH

N



C



terfenadine

(Trade name: Seldane)



18.90



What is the difference between a vasodilator and a

bronchodilator? Give an example of a drug that fits each

category.

Why do some antihistamines cause drowsiness while

others do not?

In Section 17.7 we learned that aspirin exists in different

forms (neutral or ionic) in the stomach and the intestines.

Similar behavior occurs with amines that are ingested,

since they undergo acid–base reactions. In what form

does the drug albuterol (Section 18.9B) exist in the

stomach? In the intestines?

Explain why eating a poppy seed bagel can produce a

positive test in a drug screening.



CHALLENGE QUESTIONS

18.91

18.92



Compare the structures of morphine and heroin. Suggest

a method to convert morphine to heroin.

Heroin and the morphine from which it is made are both

odorless solids. Drug-sniffing dogs can sometimes locate

heroin because of a biting odor that is present in a small



smi26573_ch18.indd 568



enough concentration that humans cannot detect, but the

keen nose of a canine can. Suggest what compound might

cause the odor that allows for heroin detection. (Hint:

What products would be formed by the hydrolysis of

heroin in moist air?)



12/16/08 10:45:02 AM



19

CHAPTER OUTLINE

19.1



Introduction to Lipids



19.2



Fatty Acids



19.3



Waxes



19.4



Triacylglycerols—Fats and Oils



19.5



Hydrolysis of Triacylglycerols



19.6



Phospholipids



19.7



Cell Membranes



19.8



FOCUS ON HEALTH & MEDICINE:

Cholesterol, the Most Prominent

Steroid



19.9



Steroid Hormones



19.10 FOCUS ON HEALTH & MEDICINE:

Fat-Soluble Vitamins

19.11 FOCUS ON HEALTH & MEDICINE:

Prostaglandins and Leukotrienes



CHAPTER GOALS

In this chapter you will learn how to:

❶ Describe the general characteristics

of lipids

❷ Classify fatty acids and describe the

relationship between melting point

and the number of double bonds

❸ Draw the structure of a wax and

identify the carboxylic acid and alcohol

components

❹ Draw the structure of triacylglycerols

and describe the difference between

a fat and an oil

❺ Draw the hydrolysis products of

triacylglycerols

❻ Identify the two major classes of

phospholipids

❼ Describe the structure of a cell

membrane, as well as different

mechanisms of transport across the

membrane

❽ Recognize the main structural

features of steroids like cholesterol,

and describe the relationship

between blood cholesterol level and

cardiovascular disease

❾ Define what a hormone is and list

several examples of steroid hormones

❿ Identify fat-soluble vitamins

⓫ Discuss the general structural

features and biological activity of

prostaglandins and leukotrienes



Elevated levels of cholesterol, a prominent steroid that belongs to the lipid family of biological

molecules, are associated with coronary artery disease.



LIPIDS

CHAPTER 19 is the first of four chapters that deal with the chemistry of biomolecules,

organic molecules found in biological systems. Chapter 19 focuses on lipids, biomolecules that contain many carbon–carbon and carbon–hydrogen bonds, making

them soluble in organic solvents and insoluble in water. Chapter 20 discusses carbohydrates, the largest group of biomolecules in nature, while Chapter 21 focuses on

proteins and the amino acids that compose them. Finally, the properties of DNA, the

polymer responsible for the storage of genetic information in the chromosomes of cells,

is presented in Chapter 22. These compounds are all organic molecules, so many of the

principles and chemical reactions that you have already learned will be examined once

again. But, as you will see, each class of compound has its own unique features that we

will discuss as well.



569



smi26573_ch19.indd 569



12/16/08 11:25:18 AM



570



LIPIDS



19.1 INTRODUCTION TO LIPIDS

• Lipids are biomolecules that are soluble in organic solvents and insoluble in water.



Lipids are unique among organic molecules because their identity is defined on the basis of a

physical property and not by the presence of a particular functional group. Because of this, lipids

come in a wide variety of structures and they have many different functions. Common lipids

include triacylglycerols in vegetable oils, cholesterol in egg yolk, and vitamin E in leafy greens.

CH3

CH3 CH(CH2)3CH(CH3)2

CH3



cholesterol



HO



[The hydroxyl (OH) is labeled in blue.]



CH3

HO



CH3



O

CH3



O

CH2 O



C



O



C



CH3



CH3



CH3



vitamin E

[The phenol hydroxyl (OH in blue) and

ether oxygen (in green) are labeled.]



CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3



O

CH



(CH2)3CH(CH2)3CH(CH2)3CH(CH3)2



CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH2CH3



O

CH2 O



C



CH2CH2CH2CH2CH2CH2CH2CH



CHCH2CH2CH2CH2CH2CH2CH2CH3



triacylglycerol

[The three ester groups are labeled in red.]



Lipids contain a large number of nonpolar carbon–carbon and carbon–hydrogen bonds. In addition, most lipids have a few polar bonds that may be found in a variety of functional groups. For

example, a triacylglycerol contains three esters, cholesterol possesses a hydroxyl group, and

vitamin E has both a phenol (an OH group on a benzene ring) and an ether. The size of these

functional groups is small compared to the overall size of the molecules. As a result, lipids are

nonpolar or weakly polar molecules that are very soluble in organic solvents like hexane (C6H14)

and carbon tetrachloride (CCl4), and insoluble in a polar medium like water.



The word lipid comes from the Greek

word lipos for fat.



SAMPLE PROBLEM 19.1



Which compounds are likely to be lipids? Sorbitol is a sweetener used in sugar-free mints and

gum. β-Carotene is the orange pigment in carrots, and the precursor of vitamin A.

CH2OH



a.



H



C



OH



HO



C



H



H



C



OH



H



C



OH



CH2OH



CH3



CH3 H

C



b.



CH3



H



C



C



CH3



H



C



C



H



H



C



CH3

H



H



C



C



C



C



C



C



C



C



C



C



C



C



H

CH3



H



H



H



H



CH3



H



CH3



H CH3 CH3



β-carotene



sorbitol



smi26573_ch19.indd 570



12/16/08 11:25:26 AM



INTRODUCTION TO LIPIDS



571



ANALYSIS

SOLUTION



PROBLEM 19.1



Lipids contain many nonpolar C C and C H bonds and few polar bonds.

a. Sorbitol is not likely to be a lipid since it contains six polar OH groups bonded to only six

carbon atoms.

b. β-Carotene is likely to be a lipid since it contains only nonpolar C C and C H bonds.

Which compounds are likely to be lipids?

CH3 OH



CH3

CH3



a.



HO2CCH2



C



b.



CH2CH2OH



c.

OH



OH

mevalonic acid



CH(CH3)2



HO



menthol



TABLE 19.1 Summary

of Lipid Chemistry Prior

to Chapter 19

Topic



Section



Vitamin A



11.7A



Fatty acids



13.3B



Oral contraceptives



13.5



Margarine and butter



13.7



Aspirin and prostaglandins



17.4B



Soap



17.6C



Olestra, a synthetic fat



17.9C



estradiol



Because lipids share many properties with hydrocarbons, several features of lipid structure and

properties have been discussed in previous chapters, as summarized in Table 19.1.

Lipids can be categorized as hydrolyzable or nonhydrolyzable.

1. Hydrolyzable lipids can be converted into smaller molecules by hydrolysis with water. We

will examine three subgroups: waxes, triacylglycerols, and phospholipids.

Hydrolyzable

lipids



Waxes



Triacylglycerols



Phospholipids



Most hydrolyzable lipids contain an ester. In the presence of acid, base, or an enzyme, the C O

bond of the ester is cleaved, and a carboxylic acid and an alcohol are formed.

O



+



C

R



O



This bond is broken.



OR'

ester



H



OH



H+ or −OH

or enzyme



+



C



H



OR'



R



OH

carboxylic acid



alcohol



2. Nonhydrolyzable lipids cannot be cleaved into smaller units by aqueous hydrolysis. Nonhydrolyzable lipids tend to be more varied in structure. We will examine three different

types: steroids, fat-soluble vitamins, and eicosanoids.

Nonhydrolyzable

lipids



Steroids



Fat-soluble vitamins



Eicosanoids



Lipids have many important roles in biological systems. Since lipids release over twice the amount

of energy per gram than carbohydrates or proteins (9 kcal/g for lipids compared to 4 kcal/g for carbohydrates and proteins, Section 6.1), lipids are an excellent source of energy. Moreover, lipids are



smi26573_ch19.indd 571



12/16/08 11:25:28 AM



572



LIPIDS



key components of the cell membrane, and they serve as chemical messengers in the body. A lipid

coating protects a leaf’s surface and keeps it from drying out. Ducks are insulated by a protective

layer of lipids on their feathers.



PROBLEM 19.2



In which solvents or solutions will a lipid be soluble: (a) CH2Cl2; (b) 5% aqueous NaCl

solution; (c) CH3CH2CH2CH2CH3?



19.2 FATTY ACIDS

Hydrolyzable lipids are derived from fatty acids, carboxylic acids that were first discussed in

Section 13.3B.

• Fatty acids are carboxylic acids (RCOOH) with long carbon chains of 12–20 carbon atoms.



Because they have many nonpolar C C and C H bonds and few polar bonds, fatty acids are

lipids, making them soluble in organic solvents and insoluble in water. Palmitic acid is a common

16-carbon fatty acid whose structure is given in condensed, skeletal, and three-dimensional

representations.

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The nonpolar part of the molecule (comprised of C C and C H bonds) is not attracted to water,

so it is said to be hydrophobic (water fearing). The polar part of the molecule is attracted to water,

so it is said to be hydrophilic (water loving). In a lipid, the hydrophobic portion is always much

larger than the hydrophilic portion.

Naturally occurring fatty acids have an even number of carbon atoms. There are two types of

fatty acids.



Recall from Section 13.3 that a cis

alkene has two alkyl groups on the

same side of the double bond, while

a trans alkene has two alkyl groups

on opposite sides of the double bond.



smi26573_ch19.indd 572



• Saturated fatty acids have no double bonds in their long hydrocarbon chains.

• Unsaturated fatty acids have one or more double bonds in their long hydrocarbon

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



Three-dimensional models for stearic acid, a saturated fatty acid, and oleic acid, an unsaturated

fatty acid with one cis double bond, are shown. Table 19.2 lists the structures of the most common

saturated and unsaturated fatty acids.



12/16/08 11:25:28 AM



FATTY ACIDS



TABLE 19.2

Number of C’s



573



Common Fatty Acids

Number of C=C’s



Structure



Name



Mp (°C)



Saturated Fatty Acids

12



0



CH3(CH2)10COOH



Lauric acid



44



14



0



CH3(CH2)12COOH



Myristic acid



58



16



0



CH3(CH2)14COOH



Palmitic acid



63



18



0



CH3(CH2)16COOH



Stearic acid



71



20



0



CH3(CH2)18COOH



Arachidic acid



77



Unsaturated Fatty Acids

16



1



CH3(CH2)5CH CH(CH2)7COOH



Palmitoleic acid



1



18



1



CH3(CH2)7CH CH(CH2)7COOH



Oleic acid



16



18



2



CH3(CH2)4CH CHCH2CH CH(CH2)7COOH



Linoleic acid



–5



18



3



CH3CH2CH CHCH2CH CHCH2CH CH(CH2)7COOH



Linolenic acid



–11



20



4



CH3(CH2)4(CH CHCH2)4(CH2)2COOH



Arachidonic acid



–49



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The most common saturated fatty

acids are palmitic and stearic acid.

The most common unsaturated

fatty acid is oleic acid. Linoleic and

linolenic acids are called essential

fatty acids because humans cannot

synthesize them and must acquire

them in our diets.

Oils formed from omega-3 fatty acids

may provide health benefits to individuals with cardiovascular disease,

as discussed in Section 19.4.



smi26573_ch19.indd 573



Unsaturated fatty acids are sometimes classified as omega-n acids, where n is the carbon at which

the first double bond occurs in the carbon chain, beginning at the end of the chain that contains the

CH3 group. Thus, linoleic acid is an omega-6 acid and linolenic acid is an omega-3 acid.

first C

CH3CH2CH2CH2CH2CH



C at C6



CHCH2CH



an omega-6 acid



CHCH2CH2CH2CH2CH2CH2CH2COOH



linoleic acid

1



2



3



4



5



first C

CH3CH2CH



6

C at C3



CHCH2CH



an omega-3 acid



CHCH2CH



CHCH2CH2CH2CH2CH2CH2CH2COOH



linolenic acid

1



2



3



12/16/08 11:25:29 AM



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