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