524
CHAPTER
l7 Lipids
I7,I Woxes
AIM: To describethe generol structure,so.trce,ond useof waxes.
Focus
Waxes are naturers protective
coating.
Lipids are a large classof relatiuely water-insoluble compounds found in
nature. Waxesare among the simplest members of the lipid family. Waxes
are estersof long-chainfatty acids andlong-chain alcohok.
oo
-
I
Lipid
hpos (Greek): fatorlard
Someevergreensproduce longchain hydrocarbons rather than
waxes to protect their leaves
againstwater loss.
Fattyacid
iC
-u.,rl-------------l
-oH * no-L4.o,hol_l--rY.l
Fattyacid
rr
l------------l
lc-o-L!.ohol
You may recall from Section 14.1 that the fatty acids are straight-chain
aliphatic carboxylic acids (RCo2H). (Throughout the first three sections of
'this
chapteryou maywish to refer to Tables 14.1 and l4.2for fatty acid
structures.) In waxes, the hydrocarbon chains for both the acid and the
alcohol (R-OH) usually contain from 10 to 30 carbons.Waxesare lowmelting, stable solids that appear in nature in both plants and animals. A
wax coat protects surfacesof many plant leavesfrom water loss and attack
by microorganisms. Carnauba wax, a major ingredient in car wax and floor
polish, comes from the leaves of a South American palm tree. Beeswaxis
largely myricyl palmitate, the ester of myricyl alcohol and palmitic acid.
cH3(cH' 14-C-O-
(CHr)zgCHa
Myricyl palmitate
Waxes also coat skin, hair, and feathers and help keep them pliable and
waterproof.
17.2Triglycerides
AIMS: To chorocterizetriglycerides,fots, and oils by source,
structure,ond use,To distinguish betweenhydrolytic
roncidity ond oxidative roncidity ond exploin how edch
can be prevented.To describethe productionof soopby
soponificotion.
Focus
Tliglycerides are esters offatty
acids and glycerol.
Natural triglycerides-triesters of long-chain fatty acids (C12through C2a)
and glycerol-are the major components of animal lipids. The following
equation shows the general reaction for the formation of triglycerides:
o
cH2oH
HO-e-R
o
CHOH +HO-C-R+
o
o
tl
cH2-o-c-R
o
QH - O - C - R
CH2OH
HO-C-R
o
CH, -o-c-R
Glycerol
3 Fattyacid
molecules
Triglyceride
(triester of glycerol)
+ 3H2O
17.2Triglycerides
525
The bonding of the triglycerid-ebuilding blocks may be represented by a
diagram,as sho'vrm
in Figure 17.1.
The many different types of triglycerides vary with the positions and
identities of the fatty acids. Simple triglycerides are triesters made from
glycerol and three moleculesof one kind of fatty acid.For example, glycerol
and three molecules of stearic acid give a simple triglyceride called glyceryl
tri stearate,or tr isteari n:
o
Esterbond
Figutel7.l
Schematicdiagramof a triglyceride.
cHz-o-c-(cHtr6cH3
lo
tll
cH-o-c-(cHt16cH3
o
cHr-o-c-(cH2)16cH3
Glyceryl tristearate
(tristearin)
Simple triglycerides are rare. More often, natural triglycerides are
mixed triglycerides-triesters of glycerol with dffirent fatty acid components.The mixed triglycerides in butterfat, for example, contain at least 14
different fatty acids.Triglyceridesare the form in which fats are stored in the
human body. In the Casein Point earlier in this chapter, we questioned how
body fat is measuredand how obesityis treated.Answersto thesequestions
can be found in the Follow-up to the Casein Point below.
Sometimesreactionsof glycerolwith fatty acidsmay produce mono- or
diesters.For example,seeglycerolmonostearate,a commonly used emulsifier, in A CloserLook CosmeticCreams.
Forrow-up
ro rHECRsrrn Pornr:Obesity
The degee of obesity is measured by the excess of
body weight relative to height. Because everyone is
different, an ideal body weight (IBW) for different
heights has been established statistically. The IBW is
the weight that gives the lowest incidence of sickness and death in the statlstical sample of people of a
ceftain height. The terms overweight and obesity ate
based on the IBW. People with a relative weight up to
20% above the IBW are defined as being overweight.
Those with a relative weight g"reaterthan 20% above
the IBW are defined as being obese. Another measure of overweight and obesity is the body mass
index (BMI). This measure is defined as body mass in
kilograms divided by height squared (height is in
meters). Overweight is defined as a BMI of 25 to 30
kg/mz. Obesity is a BMi of >30 kg/m2.
The causes of most obesity are not very well
understood. A few diseases, such as hypothyroldism,
tumors of the hypothalamus, and Cushing's disease,
can cause obesity, but these djseases are very rare.
Inherited and environmental factors may be
involved, since about 80% of the offspring of obese
parents wi-llbe obese.Only about 15% of the offspring
of normal-weight parents are obese.
The treatment of obesity usually involves
restricting dietary caloric intake and increasing exercise. However, many people go on "crash'l diets and
lose weight only to gain it back. Permanent reductions in weight often requrre that, besides diet and
exercise, eatlng habits be changed. No medications
are currently avai1able that are effective in the long
term. Surgery in which the size of the stomach is
restricted can be part of a reducing program for
l
patients who weigh double their IBW.
526
CHAPTER
17 Lipids
-'.
f-* :""Cosmetic Creams
Skincareproducts such as cold creams,moisturizers,lotions, sunscreens,and cleansingcreamsare
basically emulsions of oil and water (seefigure).
They are used to protect the skin from the dryrng
effects of detergents and exposure to wind and
sun. Emulsifiers are used to stabilize oil-water
mixtures.One commonly used emulsifier is the oil
glycerolmonostearate.
cHzoH
CHOH
I
cH2o-c-(cH2)16cH3
6
Glycerolmonostea-rate
Cold creamsare oil-in-water emulsions.That
is, there is more water than oil in the emulsion.As
the water evaporates,it produces a cooling effect
on the skin. The thin film of oil that remains is
quickly absorbed by the outer skin layers.Waterin-oil emulsions, which contain more oil than
water, are a-lsoused in cosmetics. Water-in-oil
emulsions are sometimes called warm emulsions
becausethere is little cooling effect due to water
Glycerolmonostearate
is an emulsifiercommonlyused
in cosmeticproducts,
evaporation.These emulsions permit the oil to
contact the skin almost immediately. Cleansing
creams,which dissolveoilv materials on the surface of the skin, and moisiurizrnglotions, which
soften the skin and help replace lost water, are
water-in-oil emulsions.
PRACTICE
EXERCISE
I7.I
Write one structure for the mixed triglyceride that is made from glycerol, stearicacid, palmitic acid, and oleic acid.
PRACTICE
EXERCISE
I7.2
Name the following triglyceride:
o
cH2-o-c-(cH2)14cH3
o
cH-o-c-(cH2)14cH3
o
ll
cH2-o-c-(cHt14cH3
17.2 Triglycerides
of fats and oils
Hydrogenadon
Animal fats tend to be rich in saturated fatty acids; vegetable oils are
rich in unsaturated fatty acids.
TWopolyunsaturated fatty acids
with 18 carbon atoms, linolenic
and linoleic acid, are essential
fatty acids that cannot be slmthesized by the body. These fatty acids
are found in fish oils and in many
plants. Some studies suggestthat
the consumption of linolenic acid
may reduce the risk of heart disease.
!t27
The distinction between fats and oils is usually based on melting p oint. At
room temperature, fats are solid triglycerides and oils are liquid triglycerides.The melting point of a fat or oil depends on its structure, usually
increasing with the number of carbons. The number of carbon-carbon
double bonds in the fatty acid component also has an effect:Triglycerides
rich in unsaturated fatty acids, such as oleic and linoleic, are generally
oils; triglycerides rich in saturated fatty acids, such as stearic and
palmitic, are generally fats. The triglycerides in liquid olive oil contain
mostly unsaturated oleic acid, for example,but those of solid beef fat, or
tallow, contain mostly saturated stearic acid. Hydrogenation (see Sec.
I2.2) convertsvegetableoils into fats. It is commercially important in the
manufacture of margarine and lard substitutes. Powdered metallic nickel
(which is removed later) is dispersed in the hot oil as a catalyst. Hydrogen
adds to some of the double bonds in the unsaturated fatty acid carbon
chains, saturating them and thereby converting the oil to a fat. For example, the hydrogenation of triolein (melting point -17 "C) produces tristearin (melting point 55 'C):
o
o
II
cH2- o -c (cH)7cH:cH(cH,
CH - O
il-.
cHz-o-c-(cHt16cH3
7cH3
o
o
il
-C
(CHi zCH : CH(CH2)7CH3
H2,heat
>
Pressure+
catalyst (Ni)
cH2-o -c (cH, 7cH: cH(cH2hcH3
Triolein (an oil)
cH-o-c-(cHrr6cH3
o
cH2-o-c-(cHt16cH3
.
Tristearin (afat)
The consistency of the fat is controlled by the degreeof hydrogenation; the
higher the degreeof saturation of the fat by hydrogenation, the harder is the
product at room temperature.
EXERCISE
t7.r
PRACTTCE
!
A mixed triglyceride with the following structure is hydrogenated.lVhat
is the hydrogenation product?
o
CH2-O ---C- (CHz)zCH:CHCH2CH:CH (CHr)4eH3[t]
o
h-"
S(cHtzcH:cH(cH,)?cH3\ b
o
CH2-O - C- (CH2)zCH:C}ICH2CH:CHCH2CH
: CHCH2CH3
\B
528
CHAPTER
17 Lipids
Rancidlty
Triglycerides soon become rancid, deuelopingan unpleasanr odor and flauor on exposure to nxoist air at room ternperature.The release of volatile
fatty acids (particularly butyric acid) from butterfat causesthe disagreeable
odor of rancid butter. These acids are formed either by the hydrolysis of
ester bonds or by the oxidation of double bonds. The hydrolysis of a fat or
oil is often catalyzedby en4rmes, called lipases,present in airborne bacteria. Hydrolytic rancidity is prevented or delayed by storing food in a refrigerator.The unwelcome odor of sweatresultswhen bacteriallipasescatalyze
the hydrolysis of oils and fats on the skin.
Oxidative processes,however, not hydrolysis, are the major cause of
rancidity in food. Warmth and exposureto air induce oxidative rancidity. In
oxidatiue rancidity, double bonds in the unsaturated fatty acid components
of triglycerides rupture, forming low-formula-weight aldehydeswith obj ectionable odors. The aldehydesthen oxidize to the equally offensive low-formula-weight fatty acids. Oxidative rancidity shortens the shelf life of cookies and similar foods. Antioxidants are compounds that delay the onset of
oxidative rancidity. TWo naturally occurring substances often used as
antioxidants are ascorbic acid (vitamin C) and cr-tocopherol (vitamin E).
Saponifrcadon
Saponification
sapon(Latin): soap
of fats and oils
Fats and oils are simply esters.Like other esters,they are easily hydrolyzed
in the presence of acids and bases (Sec. 14.8). Hydrolysis of oik or fats by
boiling them with aqueoussodium hydroxide is calledsaponification. This
processis used to make soap.Soapsare the alkali metal (Na, K, or Li) salts of
fatty acids.
Soap is made by heating beef tallow or coconut oil in large kettles with
an excessof sodium hydroxide. \fhen sodium chloride is added to the
saponified mixture, the sodium salts of the fatty acids separate as a thick
curd of crude soap.If the fat is tristearin, the soap is sodium stearate:
o
cH2-o-c-(cHrr6cH3
cH2-oH
o
cH-o-c-(cH2)16cH3
3NaOH
(Boil)
CH-OH
c
il
+ 3CH3(CH)16C-O-Na*
o
cH2-o-c-(cHrr6cH3
Tristearin
(a triester)
cH2-oH
Glycerol
Sodium stearate
(soap)
Glycerol is an important by-product of the reaction. It is recoveredby evaporating the water layer.The crude soap is then purified, and coloring agents
and perfumes are added according to market demands.
..
,
PRACTICE
EXERCISE
I7.4
VVhatare the products when a mixed triglyceride with the following
structure is saponified?