Chapter 26
Lipids
Review of Concepts
Fill in the blanks below. To verify that your answers are correct, look in your textbook at
the end of Chapter 26. Each of the sentences below appears verbatim in the section
entitled Review of Concepts and Vocabulary.
____________. The presence of a _____ double bond causes a decrease in the
melting point.
• Triglycerides that are solids at room temperature are called ______, while those
that are liquids at room temperature are called _______.
• Triglycerides containing unsaturated fatty acid residues will undergo
• The ring fusions are all _______ in most steroids, giving steroids their rigid
geometry.
• All steroids, including cholesterol, are biosynthesized from ____________.
• Prostaglandins contain twenty carbon atoms and are characterized by a ______-
membered ring with two side chains.
690
CHAPTER 26
Review of Skills
Fill in the blanks and empty boxes below. To verify that your answers are correct, look
in your textbook at the end of Chapter 26. The answers appear in the section entitled
SkillBuilder Review.
26.1 Comparing Molecular Properties of Triglycerides
CIRCLE THE TRIGLYCERIDE BELOW THAT IS EXPECTECD TO HAVE A HIGHER MELTING POINT.
O
O
O
O
O
OO
O
O
O
O
O
26.2 Identifying the Products of Triglyceride Hydrolysis
26.3 Drawing a Mechanism for Transesterification of a Triglyceride
A __________
__________ IS
PROTONATED
NUCLEOPHILIC
ATTACK
LOSS OF A
LEAVING GROUP
PROTON
TRANSFER
PROTON
TRANSFER
PROTON
TRANSFER
PROTON
TRANSFER
THE ______________
FUNCTIONS AS A
NUCLEOPHILE AND
ATTACKS THE
PROTOANTED
CARBONYL GROUP
THE RESULTING
__________________
INTERMEDIATE
IS DEPROTOANTED,
THEREBY REMOVING
THE POSITIVE
CHARGE
THE _____________
___________ IS
PROTONATED
THE __________
____________ IS
REGENERATED VIA
EXPULSION OF THE
LEAVING GROUP
DEPROTONATION
YIELDS THE
PRODUCT
26.4 Identifying Isoprene Units in a Terpene
CHAPTER 26
691
Review of Reactions
Identify the reagents necessary to achieve each of the following transformations. To
verify that your answers are correct, look in your textbook at the end of Chapter 26. The
answers appear in the section entitled Review of Reactions.
O
O
O
O
O
OO
O
O
O
O
O
692
CHAPTER 26
Solutions
26.1
26.2
O
O
26.3.
a) trimyristin° b) triarachadin c) triolein d) tristearin
26.6.
a) All three fatty acid residues are saturated, with either 16 or 18 carbon atoms, so the
triglyceride is expected to have a high melting point. It should be a solid at room
temperature, so it is a fat.
b) All three fatty acid residues are unsaturated, so the triglyceride is expected to have a
low melting point. It should be a liquid at room temperature, so it is an oil.
26.7.
CHAPTER 26
693
26.8.
O
O
O
O
O
O
O
O
O
O
O
O
26.9.
O
O
O
O
O
O
OH
OH
OH
Na
Na
Na
26.11
O
O
O
O
O
O
Not a
chirality center
694
CHAPTER 26
26.12. Each of the three ester moieties is hydrolyzed via the following mechanism:
O
OH O
HOMe
O
OH
O
HO O
Me
H
H
Me HOMe
O
HO O Me
26.13.
O
O
OH
OH
OH
+
(three equivalents)
26.14.
a) Hydroxide functions as a catalyst by establishing an equilibrium in which some
ethoxide ions are present.
26.15.
O
O
O
O
O
O
O
O
26.16.
O
O
O
O
O
O
26.17.
O
O
R
O
O
R
P
O
O
O
O
O
O
R
O
O
R
P
O
O
O
O
O
O
R
O
O
R
P
O
O
O
O
26.18. Octanol has a longer hydrophobic tail than hexanol and is therefore more efficient
at crossing the nonpolar environment of the cell membrane.
26.20. A ring-flip is not possible for trans-decalin because one of the rings would have
to achieve a geometry that resembles a six-membered ring with a trans-alkene,
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CHAPTER 26
26.21.
a)
Me
Me
MeMe
axial
equatorial
x
i
l
equatorial
c)
axial
axial
x
i
a
l
H
OH H
CH
3
H
H
CH
3
O
26.22.
26.23.
O
H
HH
OH
HO
OO
O
CHAPTER 26
697
26.24.
a) PGE
1
b) PGF
1α
26.26.
a) Yes, it has 10 carbon atoms, which are comprised by the joining of two isoprene units.
b) No, it has 11 carbon atoms.
c) No, it has 11 carbon atoms.
d) No. It has 10 carbon atoms, but the branching pattern cannot be achieved by joining
two isoprene units.
26.27.
OPP
OPP
OPP
–
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CHAPTER 26
26.28.
OPP
OPP
OPP
OPP
OPP
H
B
–
α
αα
α
-farnesene
26.29.
a) steroid
26.30.
a)
O
O
O
O
O
O
CHAPTER 26
699
26.31. Both compounds are chiral:
O
O
O
26.32. The fatty acid residues in this triglyceride are saturated, and will not react with
molecular hydrogen.
O
O
O
O
O
O
26.33.
a) not a lipid
b) a lipid
26.34.
HO
O
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CHAPTER 26
26.37. Water would not be appropriate because it is a polar solvent, and terpenes are
nonpolar compounds. Hexane is a nonpolar solvent and would be suitable.
26.38.
a) saturated
26.39. Arachidonic acid
26.40.
a) No. It is an oil.
26.41.
a) Yes. It is a fat.
26.42.
O
O
20 CARBON ATOMS 30 CARBON ATOMS
26.43. Trimyristin is expected to have a lower melting point than tripalmitin because the
former is comprised of fatty acid residues with fewer carbon atoms (14 instead of
16).
26.44. Each of the three ester moieties is hydrolyzed via the following mechanism:
O
OH
O
H
O
OHH O
O
HO O H
O
HO O
H O
26.45. See the solution to Problem 26.14.
26.46.
O
O
O
O
O
O
26.48.
HO
CH
3
H
CH
3
H
HH
H
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CHAPTER 26
26.49.
a)
bisabolene
b)
f
l
e
x
i
i
l
e
e
c)
m
l
e
e
26.50.
a)
OH
OPO
O
O
N
NH
O
R
Hydrophobic tails Polar Head
b) Yes, they have one polar head and two hydrophobic tails.
26.51.
CH
3
H
CH
3
H
CHAPTER 26
703
26.52.
HO
H
OH
HH
Br
d)
O
H
O
HH
O
O
26.53. The compound is chiral.
26.54.
a) H
2
, Ni
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CHAPTER 26
26.55.
a) Limonene is comprised of 10 carbon atoms and is, therefore, a monoterpene.
b) The compound does not have any chirality centers and is, therefore, achiral:
Br
Br
26.56.
OH
OH
OH
NaBH
4
MeOH
OHHO
O
H
H
3
O
+
OO
O
H
Cl
O
py
trimyristin
Excess
26.57.
a) Fats and oils have a glycerol backbone connected to three fatty acid residues.
OH
OH
OH
HO
O
H
O
HO
O