17
Introduction to
Organic
Chemistry
2 ed
William H. Brown
17-1
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17
Lipids
Chapter 17
17-2
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Lipids
• Lipids: 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 fatsoluble vitamins
• cholesterol, steroid hormones, and bile acids
17-3
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Triglycerides
• Triglyceride: an ester of glycerol with three fatty
acids
O
O
CH2 O- CR
R'CO- CH
O
CH2 O- CR''
A triacylglycerol
(a triglyceride)
1 . Na OH, H2 O
2 . HCl, H2 O
RCO2 H
CH2 OH
HOCH
CH2 OH
+
R'CO2 H
R''CO2 H
1,2,3-Propanetriol Fatty acids
(Glycerol, glycerin)
17-4
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Fatty Acids
• Fatty 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
17-5
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Fatty Acids
Unsaturated
Saturated
Carbon Atoms/
Double Bonds
12:0
14:0
16:0
18:0
20:0
16:1
18:1
18:2
18:3
20:4
Common
Name
lauric acid
myristic acid
palmitic acid
stearic acid
arachidic acid
palmitoleic acid
oleic acid
linoleic acid
linolenic acid
arachidonic acid
mp
(°C)
44
58
63
70
77
1
16
-5
-11
-49
17-6
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Fatty 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
17-7
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Fatty Acids
• Stearic and linolenic acids
17-8
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Triglycerides
• 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 oils
• triglycerides rich in saturated fatty acids are generally
semisolids or solids at room temperature and are called
fats
• The lower melting points of triglycerides rich in
unsaturated fatty acids are related to differences
in their three-dimensional shape
17-9
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Triglycerides
• A saturated triglyceride
17-10
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Triglycerides
• 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
17-11
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Soaps and Detergents
• Natural soaps are prepared by boiling lard or
other animal fat with NaOH, in a reaction called
saponification (Latin, sapo, soap)
O
O
CH2 O- CR
RCO- CH
+
3 Na OH
saponification
O
CH2 O- CR
A triacylglycerol
(a triglyceride)
CH 2 OH
CHOH
O
+
CH 2 OH
1,2,3-Propanetriol
(Glycerol; Glycerin)
-
+
3 RCO Na
Sodium soaps
17-12
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Soaps 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
17-13
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Soaps and Detergents
• Micelle: 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
17-14
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Soaps and Detergents
• Soaps form water-insoluble salts when used in
water containing Ca(II), Mg(II), and Fe(III) ions
(hard water)
-
+
2 CH3 ( CH2 ) 1 4 CO2 Na + Ca
A sodium soap
(soluble in water as micelles)
2+
-
2+
[ CH3 ( CH2 ) 1 4 CO2 ] 2 Ca
Calcium salt of a fatty acid
(insoluble in water)
+
2 Na
+
17-15
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Synthetic 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)
17-16
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Soaps and Detergents
CH3 ( CH2 ) 1 0 CH2
Dodecylbenzene
1 . H2 SO4
2 . Na OH
CH3 ( CH2 ) 1 0 CH2
SO3
-
Na
+
Sodium 4-dodecylbenzenesulfonate
(an anionic detergent)
• Also added to detergent preparations are
• foam stabilizers
• bleaches
• optical brighteners
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17-17
17 Prostaglandins
• Prostaglandins: a family of compounds that have
the 20-carbon skeleton of prostanoic acid
9
7
5
1
3
8
6
4
2
12
14
16
18
CO2 H
10
11
13
15
17
19
20
Prostanoic acid
17-18
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Prostaglandins
• Prostaglandins are not stored in tissues as such,
but are synthesized from membrane-bound 20carbon polyunsaturated fatty acids in response to
specific physiological triggers
• one such polyunsaturated fatty acid is
9
8
6 5
arachidonic acid
CO2 H
11 12
14
15
Arachidonic acid
17-19
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Prostaglandins
• among the prostaglandins synthesized from
arachidonic acid are
O
9
CO2 H
15
11
HO
HO H
PGE2
HO
9
CO2 H
HO
15
11
HO H
PGF 2
17-20
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Prostaglandins
• Research on the involvement of PGs in
reproductive physiology has produced several
clinically useful derivatives
• 15-Methyl-PGF2 is used as a therapeutic abortifacient
extra methyl group
at carbon-15
HO
9
CO 2 H
HO
11
15
HO CH 3
15-Methyl-PGF 2 
17-21
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Prostaglandins
• the PGE1 analog, misoprostol, is used for prevention of
ulceration associated with the use of aspirin-like
NSAIDs
O
CO2 H
PGE1
15 16
HO
O
HO H
CO2 CH 3
HO
HO
15
CH 3
16
Misoprostol
17-22
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Steroids
• Steroids: a group of plant and animal lipids that
have this tetracyclic ring structure
C
A
D
B
17-23
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Common Features
CH 3
H
H
CH 3
H
H
• 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
17-24
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Cholesterol
H3 C
H3 C
H
H
H
HO
17-25
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Androgens
• Androgens - male sex hormones
• synthesized in the testes
• responsible for the development of male secondary sex
characteristics
H3 C
H3 C
OH
H
H
H
H3 C
H3 C
H
O
O
H
H
H
HO
Testosterone
Androsterone
17-26
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Anabolic Steroids
• Among the synthetic anabolic steroids are
H3 C
H3 C
2
OH
CH 3
17
H
1
H
H3 C
H
O
H3 C
Methandrostenolone
N
A
N
H
OH
CH3
H
H
H
Stanozolol
H
17-27
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Estrogens
• Estrogens - female sex hormones
• synthesized in the ovaries
• responsible for the development of female secondary
sex characteristics and control of the menstrual cycle
CH 3
H3 C
H 3 C C=O
H
H3 C
H
H
H
O
H
Progesterone
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
H
HO
Estrone
O
H
17-28
17 Synthetic Estrogens
• Progesterone-like analogs are used in oral
HO C CH
contraceptives
H C
"Nor" refers to the absence
of a methyl group here.
It is present in ethindrone
3
H
H
H
O
H
Norethindrone
17-29
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Glucorticoid Hormones
•
•
•
•
synthesized in the adrenal cortex
regulate metabolism of carbohydrates
decrease inflammation
involved in the reaction to stress
CH 2 OH
H3 C
O
H3 C
C=O
OH
H
H
CH 2 OH
C=O
H3 C
OH
HO
H3 C
H
O
H
H
H
O
Cortisone
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
Cortisol
17-30
17 Mineralocorticoid Horm.
• synthesized in the adrenal cortex
• regulates blood pressure and volume by stimulating the
kidneys to absorb Na+ , Cl-, and HCO3-
OH
O
H3 C
C= O
H
H
O
CH
CH2 OH
H
Aldosterone
17-31
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Bile 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
H3 C
H
H
HO
CO2 H
H
OH
H
Cholic acid
17-32
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Biosynthesis 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
bile acids
(e.g., cholic acid)
Cholesterol
sex hormones
(e.g., testosterone and estrone)
mineralocorticoid homones
(e.g., aldosterone)
glucocorticoid hormones
(e.g., cortisone)
17-33
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Phospholipids
• 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)
17-34
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Phospholipids
• A phosphatidic acid
stearic acid
O
CH 2 -O- P- O O
OO CH
O CH 2
palmitic acid
O
glycerol
• Further esterification with a low-molecular weight
alcohol gives a phospholipid
• among the most common of these low-molecularweight alcohols are
17-35
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Phospholipids
Name and Formula
Name of Phospholipid
ethanolamine
HOCH 2 CH 2 NH 2
choline
+
HOCH 2 CH 2 N( CH 3 ) 3
serine
HOCH 2 CH CO 2
NH 3
inositol
HO
HO
phosphatidylethanolamine
(cephalin)
phosphatidylcholine
(lecithin)
phosphatidylserine
+
OH
HO
OH
OH phosphatidylinositol
17-36
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Phospholipids
choline
• A lecithin
O
stearic acid
O
+
O P OCH 2 CH 2 N( CH 3 ) 3
O
CH
2
O CH
O CH 2
palmitic acid
glycerol
O
• in aqueous solution, phospholipids spontaneously form
into a lipid bilayer, with a back-to-back arrangement of
lipid monolayers
17-37
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 A Lecithin
17-38
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Biological Membranes
• Fluid mosaic model: a biological membrane
consists of a phospholipid bilayer with proteins,
carbohydrates, and other lipids embedded on the
surface and in the bilayer
• fluid signifies that the protein components of
membranes “float”in the bilayer and can move freely
along the plane of the membrane
• mosaic signifies that the various components of the
membrane exist side-by-side, as discrete units rather
than combining to form new molecules and ions
17-39
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Fat-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
17-40
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin A
• Vitamin A, or retinol, occurs only in the animal
world
CH 3
CH 3
CH 3
CH 2 OH
CH 3
CH 3
Retinol (Vitamin A)
• 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 b-carotene followed by
reduction gives two molecules of vitamin A
17-41
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin A
CH 3
CH 3
site of
cleavage
CH 3
H3 C
CH 3
b- Carotene
CH 3
H3 C
CH 3
CH 3
CH 3
enzyme-catalyzed cleavage
and reduction in the liver
CH 3
CH 3
CH 3
CH 2 OH
CH 3
CH 3
Retinol (Vitamin A)
17-42
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin 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
17-43
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin A
CH 3
CH 3
11-12 cis configuration
11
12
CH 3
CH 3
CH=N-opsin
H3 C
light
CH 3
CH3
CH 3
CH3
11
CH=N-opsin
12
CH 3
17-44
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin 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
HO
Vitamin D3
17-45
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin E
• Vitamin E: a group of compounds of similar
structure
• the most active is -tocopherol
OH
H3 C
CH3
Four isoprene units, joined
head-to-tail, beginning here
and ending at the aromatic ring
H3 C
CH3
O
H3 C
CH3
CH3
CH3
Vitamin E ( -Tocopherol)
17-46
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin 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
17-47
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17 Vitamin K
• The name of this vitamin comes from the German
word Koagulation, signifying its important role in
the blood-clotting process
isoprene units
O
CH33
O
Vitamin K1
2
O
CH33
Menadione
O
(a synthetic
vitamin K analog)
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.
17-48
17
Lipids
End Chapter 17
17-49
Copyright © 2000 by John Wiley & Sons, Inc. All rights reserved.