Introduction to Organic Chemistry 2 ed William H. Brown

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17-1Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.

Introduction to Introduction to Organic Organic

ChemistryChemistry2 ed2 ed

William H. BrownWilliam H. Brown

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LipidsLipids

Chapter 17Chapter 17

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LipidsLipids• LipidsLipids: a heterogeneous class of naturally

occurring organic compounds classified together on the basis of common solubility properties• insoluble in water• soluble in aprotic organic solvents including diethyl

ether, methylene chloride, and acetone

• Lipids include• triglycerides, phospholipids, prostaglandins, and fat-

soluble vitamins• cholesterol, steroid hormones, and bile acids

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TriglyceridesTriglycerides• TriglycerideTriglyceride: an ester of glycerol with three fatty

acids

A triacylglycerol

(a triglyceride)

C H2

O - C R

O

O

O

C H2

O - C R ' '

1 . N a O H , H2

O

2 . H C l , H2

O

C H2

O H

C H2

O H

1,2,3-Propanetriol

(Glycerol, glycerin)

+

R C O2

H

R ' C O2

H

R ' ' C O2

H

Fatty acids

H O C H

R ' C O - C H

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Fatty AcidsFatty Acids• Fatty acidFatty acid: a long, unbranched chain carboxylic

acid, most commonly of 12 - 20 carbons, derived from hydrolysis of animal fats, vegetable oils, or the phospholipids of biological membranes

• In the shorthand notation for fatty acids• the number of carbons and the number of double

bonds in the chain are shown by two numbers, separated by a colon

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Fatty AcidsFatty Acids

20:4

18:3

18:2

18:1

16:1

20:0

18:0

16:0

14:0

12:0

Carbon Atoms/

Double Bonds

mp

(°C)

Common

Name

-49

-11

-5

16

1

77

70

63

58

44

arachidonic acid

linolenic acid

linoleic acid

oleic acid

palmitoleic acid

arachidic acid

stearic acid

palmitic acid

myristic acid

lauric acid

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Fatty AcidsFatty Acids• Among the fatty acids most abundant in plants

and animals• nearly all have an even number of carbon atoms, most

between 12 and 20, in an unbranched chain• the three most abundant are palmitic (16:0), stearic acid

(18:0), and oleic acid (18:1)• in most unsaturated fatty acids, the cis isomer

predominates; the trans isomer is rare• unsaturated fatty acids have lower melting points than

their saturated counterparts; the greater the degree of unsaturation, the lower the melting point

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Fatty AcidsFatty Acids• Stearic and linolenic acids

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TriglyceridesTriglycerides• Physical properties of triglycerides depend on

their fatty acid components• melting point increases as the number of carbons in

their hydrocarbon chains increases and as the number of double bonds decreases

• triglycerides rich in unsaturated fatty acids are generally liquid at room temperature and are called oilsoils

• triglycerides rich in saturated fatty acids are generally semisolids or solids at room temperature and are called fatsfats

• The lower melting points of triglycerides rich in unsaturated fatty acids are related to differences in their three-dimensional shape

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TriglyceridesTriglycerides• A saturated triglyceride

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TriglyceridesTriglycerides• Triglycerides rich in saturated fatty acids:

• saturated hydrocarbon chains can lie parallel and there are strong dispersion forces between their chains

• they pack into well-ordered, compact crystalline forms and have melting points above room temperature

• Triglycerides rich in saturated fatty acids:• because of the cis configuration of their double bonds,

their hydrocarbon chains have a less ordered structure• dispersion forces between hydrocarbon chains are

weaker; these triglycerides have melting points below room temperature

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Soaps and DetergentsSoaps and Detergents• Natural soaps are prepared by boiling lard or

other animal fat with NaOH, in a reaction called saponification (Latin, sapo, soap)

Sodium soaps

1,2,3-Propanetriol

(Glycerol; Glycerin)

+

saponification+

O

C H O H

C H2

O H

C H2

O H

3 N a O H

3 R C O-

N a+

A triacylglycerol

(a triglyceride)

C H2

O - C R

O

O

O

C H2

O - C R

R C O - C H

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Soaps and DetergentsSoaps and Detergents• Soaps clean by acting as emulsifying agents

• the long hydrophobic hydrocarbon chains of soaps are insoluble in water and tend to cluster in such a way as to minimize their contact with water

• the polar hydrophilic carboxylate groups tend to remain in contact with the surrounding water molecules

• driven by these two forces, soap molecules spontaneously cluster into micelles

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Soaps and DetergentsSoaps and Detergents• MicelleMicelle: a spherical arrangement of organic

molecules in water clustered so that their hydrophobic parts are buried inside the sphere and their hydrophilic parts are on the surface of the sphere and in contact with water

• When soap is mixed with water-insoluble grease, oil, and fat stains, the nonpolar parts of the soap micelles “dissolve” nonpolar dirt molecules and they are carried away in the polar wash water

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Soaps and DetergentsSoaps and Detergents• Soaps form water-insoluble salts when used in

water containing Ca(II), Mg(II), and Fe(III) ions (hard waterhard water)

+

+

A sodium soap

(soluble in water as micelles)

Calcium salt of a fatty acid

(insoluble in water)

2 C H3

( C H2

)1 4

C O2

- N a

+C a

2 +

[ C H3

( C H2

)1 4

C O2

-]

2C a

2 +2 N a

+

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Synthetic DetergentsSynthetic Detergents• The design criteria for a good detergent are

• a long hydrocarbon tail of 12 to 20 carbons• a polar head group that does not form insoluble salts

with Ca(II), Mg(II), or Fe(III) ions

• The most widely used synthetic detergents are the linear alkylbenzene sulfonates (LAS)

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Soaps and DetergentsSoaps and Detergents

• Also added to detergent preparations are• foam stabilizers• bleaches• optical brighteners

Dodecylbenzene

Sodium 4-dodecylbenzenesulfonate

(an anionic detergent)

C H3

( C H2

)1 0

C H2

C H3

( C H2

)1 0

C H2

S O3

- N a

+

1 . H2

S O4

2 . N a O H

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ProstaglandinsProstaglandins• ProstaglandinsProstaglandins: a family of compounds that have

the 20-carbon skeleton of prostanoic acid

Prostanoic acid

19

2018

17

16

15

14

13

7

89

10

1112

6

5

4

3

2

1

CO2

H

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ProstaglandinsProstaglandins• Prostaglandins are not stored in tissues as such,

but are synthesized from membrane-bound 20-carbon polyunsaturated fatty acids in response to specific physiological triggers

• one such polyunsaturated fatty acid is arachidonic acid

Arachidonic acid

1514

89

11 12

6 5

CO 2 H

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ProstaglandinsProstaglandins• among the prostaglandins synthesized from

arachidonic acid are

15

9

11

PGF2 α

C O 2 H

H O

H O

H O H

PGE2

11

9

15

H O

C O2

H

H O H

O

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ProstaglandinsProstaglandins• Research on the involvement of PGs in

reproductive physiology has produced several clinically useful derivatives• 15-Methyl-PGF2α is used as a therapeutic abortifacient

15

9

11

15- Methyl-PGF2 α

extra methyl group

-15at carbon

CO 2 H

HO

HO

HO CH 3

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ProstaglandinsProstaglandins• the PGE1 analog, misoprostol, is used for prevention of

ulceration associated with the use of aspirin-like NSAIDs

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Misoprostol

PGE 1

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CO2

CH3

HO

O

HO

CH3

HO HHO

O

CO2

H

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SteroidsSteroids• SteroidsSteroids: a group of plant and animal lipids that

have this tetracyclic ring structure

A B

C D

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Common FeaturesCommon Features

• the fusion of rings is trans and each atom or group of atoms at a ring junction is axial

• the pattern of atoms or groups of atoms along the ring junctions is nearly always trans-anti-trans-anti-trans

• the steroid system is nearly flat and quite rigid• many have axial methyl groups at C-10 and C-13

H

CH3

H

H

CH3

H

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CholesterolCholesterol

H3

C

HO

H3

C

H H

H

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AndrogensAndrogens• Androgens - male sex hormones

• synthesized in the testes• responsible for the development of male secondary sex

characteristics

AndrosteroneTestosterone

O

H

OH

H

H 3 C H

H

H3

C H3

C

H

HH 3 C

H

O

HO

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Anabolic SteroidsAnabolic Steroids• Among the synthetic anabolic steroids are

Stanozolol

Methandrostenolone

17

2

1

A

O

C H3

O H

H

H3

C H

H

H3

C

H3

C

H

HH3

C

H

O H

C H3

N

N

HH

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EstrogensEstrogens• Estrogens - female sex hormones

• synthesized in the ovaries• responsible for the development of female secondary

sex characteristics and control of the menstrual cycle

Progesterone Estrone

H3

C

H

HH3

C

H

C=O

H

O

CH3

O

HO

H

H

H

H3

C

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Synthetic EstrogensSynthetic Estrogens• Progesterone-like analogs are used in oral

contraceptives"Nor" refers to the absence

of a methyl group here.

It is present in ethindrone

Norethindrone

H3

C

H

HH

O

H

H OC C H

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Glucorticoid HormonesGlucorticoid Hormones• synthesized in the adrenal cortex• regulate metabolism of carbohydrates• decrease inflammation• involved in the reaction to stress

CortisolCortisone

C=O

CH2

OH

O

OH

H

H3

C H

H

H3

C

O HO

H3

C

H

HH3

C

H

C=O

OH

O

CH2

OH

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Mineralocorticoid Horm.Mineralocorticoid Horm.• synthesized in the adrenal cortex• regulates blood pressure and volume by stimulating the

kidneys to absorb Na+ , Cl-, and HCO3-

Aldosterone

C = O

C H2

O H

O

H

H3

C H

H

C HO

O H

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Bile AcidsBile Acids• synthesized in the liver, stored in the gallbladder, and

secreted into the intestine where • their function is emulsify dietary fats and aid in their

absorption and digestion

H3

C

HO

H3

C

CO2

H

H H

H

HOH

Cholic acid

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Biosynthesis of SteroidsBiosynthesis of Steroids• all carbon atoms of cholesterol are derived from the

two carbon acetyl group of acetyl-CoA• cholesterol is, in turn, the starting material for the

synthesis of these classes of compounds

Cholesterol

bile acids

(e.g., cholic acid)

sex hormones

(e.g., testosterone and estrone)

mineralocorticoid homones

(e.g., aldosterone)

glucocorticoid hormones

(e.g., cortisone)

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PhospholipidsPhospholipids• Phospholipids are the second most abundant

group of naturally occurring lipids• they are found almost exclusively in plant and animal

membranes, which typically consist of 40% -50% phospholipids and 50% - 60% proteins

• the most abundant phospholipids are derived from phosphatidic acid, a molecule in which glycerol is esterified with two molecules of fatty acid and one of phosphoric acid

• the three most abundant fatty acids in phosphatidic acids are palmitic acid (16:0), stearic acid (18:0), and oleic acid (18:1)

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PhospholipidsPhospholipids• A phosphatidic acid

• Further esterification with a low-molecular weight alcohol gives a phospholipid• among the most common of these low-molecular-

weight alcohols are

C H 2

C H

C H2

- O - P - O-

O

O

O

O

O

O-

glycerol

palmitic acid

stearic acid

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PhospholipidsPhospholipidsName and Formula Name of Phospholipid

ethanolamine

+choline

phosphatidylcholine

(lecithin)

phosphatidylethanolamine

(cephalin)

serine

phosphatidylserine

inositol

phosphatidylinositol

N H 3

+

H O

H OO H

O H

O HH O

H O C H 2 C H C O 2

-

H O C H 2 C H 2 N ( C H 3 ) 3

H O C H 2 C H 2 N H 2

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PhospholipidsPhospholipids• A lecithin

• in aqueous solution, phospholipids spontaneously form into a lipid bilayer, with a back-to-back arrangement of lipid monolayers

C H2

C H

C H2

O

O

O

O

O P O C H2

C H2

N ( C H3

)3

O

O-

+

palmitic acid

stearic acid

glycerol

choline

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A LecithinA Lecithin

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Biological MembranesBiological Membranes• Fluid mosaic modelFluid mosaic model: a biological membrane

consists of a phospholipid bilayer with proteins, carbohydrates, and other lipids embedded on the surface and in the bilayer• fluidfluid signifies that the protein components of

membranes “float”in the bilayer and can move freely along the plane of the membrane

• mosaicmosaic signifies that the various components of the membrane exist side-by-side, as discrete units rather than combining to form new molecules and ions

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Fat-Soluble VitaminsFat-Soluble Vitamins• Vitamins are divided into two broad classes on

the basis of their solubility • those that are fat-soluble (and hence classified as lipids• those that are water-soluble

• The fat-soluble vitamins include A, D, E, and K

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Vitamin AVitamin A• Vitamin A, or retinol, occurs only in the animal

world

• Vitamin A is found in the plant world in the form of a provitamin in a group of pigments called carotenes (tetraterpenes)• enzyme-catalyzed cleavage of -carotene followed by

reduction gives two molecules of vitamin A

CH3

CH3

CH3

CH3

CH3

CH2

OH

Retinol (Vitamin A)

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Vitamin AVitamin Asite of

cleavage

- Carotene

enzyme-catalyzed cleavage

and reduction in the liver

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

CH3

H3

C

H3

C

CH3

CH3

CH3

CH3

CH2

OH

Retinol (Vitamin A)

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Vitamin AVitamin A• The best understood role of Vitamin A is its

participation in the visual cycle in rod cells• the active molecule is retinal (vitamin A aldehyde),

which forms an imine with an -NH2 group of the protein opsin to form the visual pigment called rhodopsin

• the primary chemical event of vision in rod cells is absorption of light by rhodopsin followed by isomerization of the 11-cis double bond to the 11-trans configuration

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Vitamin AVitamin A

CH3

CH3

CH3

CH 3CH 3

CH=N-opsin11

12

CH3

CH3

CH3

H3

C

CH3

CH=N-opsin

11

12

11-12 cis configuration

light

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Vitamin DVitamin D• A group of structurally related compounds that

play a role in the regulation of calcium and phosphorus metabolism• the most abundant form in the circulatory system is

vitamin D3

V itamin D3

HO

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Vitamin EVitamin E• Vitamin E: a group of compounds of similar

structure• the most active is α-tocopherol

F our isoprene units, joined

head-to-tail, beginning here

and ending at the aromatic ring

V itamin E ( α - )Tocopherol

H3

C

H3

C

H3

C

C H3

O H

OC H

3C H

3

C H3

C H3

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Vitamin EVitamin E• In the body, vitamin E functions as an antioxidant;

it traps peroxy radicals of the type HOO• and ROO• formed as a result of oxidation by O2 of unsaturated hydrocarbon chains in membrane phospholipids

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Vitamin KVitamin K• The name of this vitamin comes from the German

word Koagulation, signifying its important role in the blood-clotting process

Vitamin K1

isoprene units

2

O

O

CH3

O

O

CH3

2

O

O

CH3

O

O

CH3

Menadione

(a synthetic

vitamin K analog)

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LipidsLipids

End Chapter 17End Chapter 17

Introduction to Organic Chemistry 2 ed William H. Brown

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