Chapter 26 - ChemConnections

advertisement
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.
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
•
Lipids are naturally occurring compounds that are extracted from cells using
_______________ solvents.
Complex lipids readily undergo ______________, while simple lipids do not.
_________ are high molecular weight esters that are constructed from carboxylic
acids and alcohols.
___________________ are the triesters formed from glycerol and three longchain carboxylic acids, called fatty acids. The resulting triglyceride is said to
contain three fatty acid _____________.
For saturated fatty acids, the melting point increases with increasing __________
____________. 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
hydrogenation. During the hydrogenation process, some of the double bonds can
isomerizes to give _______ π bonds
In the presence of molecular oxygen, triglycerides are particularly susceptible to
oxidation at the ____________ position to produce hydroperoxides.
Transesterification of triglycerides can be achieved either via _____ catalysis or
______ catalysis to produce biodiesel.
________________ are similar in structure to triglycerides except that one of the
three fatty acid residues is replaced by a phosphoester group.
The structures of steroids are based on a tetracyclic ring system, involving three
six-membered rings and one ______-membered ring.
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.
Terpenes are a class of naturally occurring compounds that can be thought of as
being assembled from _________ units.
A terpene with 10 carbon atoms is called a _______________, while a terpene
with 20 carbon atoms is called a __________________.
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
O
O
O
O
O
O
O
26.2 Identifying the Products of Triglyceride Hydrolysis
DRAW THE PRODUCTS OBTAINED WHEN THE FOLLOWING TRIGLYCERIDE IS TREATED WITH AQUEOUS SODIUM HYDROXIDE.
O
O
O
NaOH
O
O
O
26.3 Drawing a Mechanism for Transesterification of a Triglyceride
PROTON
TRANSFER
NUCLEOPHILIC
ATTACK
A __________
__________ IS
PROTONATED
THE ______________
FUNCTIONS AS A
NUCLEOPHILE AND
ATTACKS THE
PROTOANTED
CARBONYL GROUP
PROTON
TRANSFER
THE RESULTING
THE _____________
__________________
___________ IS
INTERMEDIATE
PROTONATED
IS DEPROTOANTED,
THEREBY REMOVING
THE POSITIVE
CHARGE
26.4 Identifying Isoprene Units in a Terpene
IDENTIFY THE
ISOPRENE UNITS IN
THE FOLLOWING
TERPENE
O
PROTON
TRANSFER
LOSS OF A
LEAVING GROUP
PROTON
TRANSFER
THE __________
____________ IS
REGENERATED VIA
EXPULSION OF THE
LEAVING GROUP
DEPROTONATION
YIELDS THE
PRODUCT
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
O
O
O
O
O
O
O
O
Na
O
O
O
OH
O
O
+
OH
O
O
Na
O
OH
O
O
Na
O
O
O
MeO
O
OH
O
O
O
O
OH
+
MeO
OH
O
MeO
O
692
CHAPTER 26
Solutions
26.1
O
O
O
CH3(CH2)14
(CH2)29CH3
hydrolysis
CH3(CH2)14
OH
+
HO
(CH2 )29 CH3
26.2
O
O
26.3.
a) trimyristin°
b) triarachadin
c) triolein
d) tristearin
26.4.
tripalmitolein, tripalmitin, and tristearin
26.5. The fatty acid residues in triarachadin have more carbon atoms than the fatty acid
residues in tristearin. Therefore, triarachadin is expected to have a higher melting point.
It should be a solid at room temperature, and should therefore be classified as a fat, rather
than an oil. Therefore, triglycerides made from lauric acid will also have a low melting
point.
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.
O
O
O
O
O
O
a)
b) Tristearin
c) The melting point of tristearin is higher than triolein.
d) Stearic acid
CHAPTER 26
26.8.
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
O
26.9.
O
Na
O
OH
O
OH
Na
O
OH
Na
O
O
26.10.
O
O
O
O
O
O
26.11
O
O
O
O
Not a
chirality center
O
O
693
694
CHAPTER 26
26.12. Each of the three ester moieties is hydrolyzed via the following mechanism:
H
H
O
O
O
Me
H
Me
Me
O
HO
H
O
O
O
H
Me
O
H
O
HO
O
Me
O
H
Me
O
Me
OH
H
Me
O
H
O
Me
OH
O
HO
O
O
H
Me
O
O
H
26.13.
OH
O
+
OH
O
OH
(three equivalents)
26.14.
a) Hydroxide functions as a catalyst by establishing an equilibrium in which some
ethoxide ions are present.
HO
+
H
H2 O
OEt
OEt
+
ethoxide
Then, each ester moiety undergoes transesterification via the following mechanism:
O
O
OEt
O
O
O
OEt
O
Et
O
H
O
O
H
OH
Et
O
b) Hydroxide could function as a nucleophile and triglyceride would undergo hydrolysis
rather than transesterification.
26.15.
O
O
O
O
O
O
a)
O
O
P
O O
O
O
N
b)
O
O
P
O O
N
c) No. The C2 position would no longer be a chirality center.
CHAPTER 26
695
26.16.
O
O
O
O
O
O
O
O
P
O O
O
O
O
NH3
a)
O
P
O O
NH3
b) Yes. The C2 position would still be a chirality center.
26.17.
O
O
R
O
O
O
O
O
P
O
R
O
O
O
R
O
O
O
O
R
O
O
O
R
O
P
O
O
O
R
O
P
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.
26.19. No. Glycerol has three OH groups (hydrophilic) and no hydrophobic tail. It
cannot cross 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,
which is not possible. The ring fusions of cholesterol all resemble the ring fusion
in trans-decalin, so cholesterol cannot undergo ring-flipping.
Hypothetical ring flip
696
CHAPTER 26
26.21.
axial
Me
equatorial
Me
Me
equatorial
Me
axial
a)
axial
Me
Me
axial
Me
equatorial
Me
axial
b)
axial
axial
OH
O
H
H
H
CH3
H
CH3
c) axial
26.22.
OH
H
HO
H
O
H
H
H
H
oxymetholone
H
norgestrel
26.23.
O
O
HO
O
OH
H
O
C
O
H
H
OH
Et
H
C
H
697
CHAPTER 26
26.24.
a) PGE1
b) PGF1α
26.25.
O
OH
OH
a)
menthol
b)
grandisol
carvone
c)
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
OPP
OPP
B
H
698
CHAPTER 26
26.28.
OPP
- OPP
OPP
OPP
H
B
OPP
- OPP
OPP
geranyl
pyrophosphate
H
OPP
OPP
B
H
farnesyl
pyrophosphate
B
α-farnesene
26.29.
a) steroid
b) terpene
c) triglyceride
d) phospholipid
e) prostaglandin
f) wax
26.30.
O
O
O
O
O
a)
O
CHAPTER 26
OH
OH
b)
699
O
+
Na
O
(three equivalents)
OH
26.31. Both compounds are chiral:
O
O
O
O
O
O
O
O
P
O O
O
O
O
NH3
O
P
O O
NH3
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
c) a lipid
d) a lipid
e) a lipid
f) not a lipid
g) a lipid
h) not a lipid
26.34.
O
HO
26.35. The fatty acid residues of tristearin are saturated and are therefore less susceptible
to auto-oxidation than the unsaturated fatty acid residues in triolein.
26.36.
a < b < c
700
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
b) saturated
c) unsaturated
d) saturated
e) unsaturated
f) unsaturated
26.39. Arachidonic acid
26.40.
a) No. It is an oil.
b) No. It is reactive towards molecular hydrogen in the presence of Ni.
c) Yes. It undergoes hydrolysis to produce unsaturated fatty acids.
d) Yes. It is a complex lipid because it undergoes hydrolysis.
e) No. It is not a wax.
f) No. It does not have a phosphate group.
26.41.
a) Yes. It is a fat.
b) Yes. It is unreactive towards molecular hydrogen in the presence of Ni.
c) No. It undergoes hydrolysis to produce fatty acids that are saturated.
d) Yes. It is a complex lipid because it undergoes hydrolysis.
e) No. It is not a wax.
f) No. It does not have a phosphate group.
26.42.
20 CARBON ATOMS
30 CARBON ATOMS
O
O
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).
O
O
O
O
O
O
O
O
O
O
O
O
trimyristin
tripalmitin
701
CHAPTER 26
26.44. Each of the three ester moieties is hydrolyzed via the following mechanism:
H
O
O
O
H
H
H
O
HO
O
O
O
O
H
H
O
HO
O
O
H
O
H
O
OH
O
H
O
OH
O
O
HO
O
H
26.45. See the solution to Problem 26.14.
26.46.
O
O
O
O
O
O
26.47.
O
O
O
O
O
O
26.48.
H
CH3
H
CH3
H
HO
H
H
O
H
702
CHAPTER 26
26.49.
bisabolene
a)
flexibilene
b)
humulene
c)
OH
OH
Vitamin A
d)
geraniol
e)
sabinene
f)
26.50.
Hydrophobic tails
R
Polar Head
O
N
H
O
O
OH
O
P
O
N
a)
b) Yes, they have one polar head and two hydrophobic tails.
26.51.
H
H
CH3
CH3
H
CH3
H
a)
HO
H
CH3
H
H
H
HO
O
H
H
O
b) The methyl group (C19) provides steric hindrance that blocks one side of the π bond,
and only the following is obtained:
H
CH3
H
CH3
H
HO
O
H
H
CHAPTER 26
26.52.
OH
H
Br
H
H
HO
a)
Br
O
H
H
b)
H
HO
OEt
H
H
c)
H
EtO
O
O
H
O
H
d)
H
O
26.53. The compound is chiral.
R
O
O
O
O O
R
R
O
O
O
O
O
CH2O
O
O
O
R
O
O
R
O
R
O
O
R
R
26.54.
a) H2, Ni
b) H2, Ni, followed by NaOH, followed by EtI.
c) H2, Ni, followed by LAH, followed by H2O
d) O3, followed by DMS, followed by Na2Cr2O7 and H2SO4
e) H2, Ni, followed by PBr3 and Br2, followed by H2O
703
704
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
c)
O
H
O
O
+
O
H
26.56.
O
H3O
O
+
HO
OH
OH
NaBH4
O
OH
MeOH
H
O
OH
H
Excess
O
Cl
trimyristin
py
26.57.
a) Fats and oils have a glycerol backbone connected to three fatty acid residues.
Plasmalogens also have a glycerol backbone, but it is only connected to two fatty acid
residues. The third group is not a fatty acid residue.
b)
O
R"
O
Na
HO
O
O
c)
O
HO
OH
O
OH
OH
R'
HO
OH
O
H
Na
O
R
Download