Chapter 9 – Stereochemistry
For the following questions MATCH each definition to a term from the list below. Place the letter of the term
in the blank to the left of the definition.
a.
b.
c.
d.
e.
1.
racemates
chirality center
chirality
diastereomers
enantiomers
g
is the property of “handedness”; the property of an object that causes it to be nonsuperimposable on
its mirror image.
Answer:
3.
c
are stereoisomers that are not mirror images.
Answer:
4.
d
is an atom in a molecule that is bonded to four different atoms or groups of atoms.
Answer:
5.
b
are molecules which contain chirality centers and a plane of symmetry.
Answer:
6.
meso compounds
optically active
prochirality center
optically inactive
achiral
____ describes organic molecules which rotate plane-polarized light.
Answer:
2.
f.
g.
h.
i.
j.
f
_____ describes an sp3-hybridized atom that can become a chirality center by changing one of its attached
groups.
Answer: h
7.
The specific rotation of a compound is denoted by the symbol:
a.
b.
c.
d.
Answer:
68
R
S
α
α
D
d
Chapter 9: Stereochemistry
Place asterisks at all the chirality centers in each molecule below.
8.
COOH
O
HO
OH
prostacyclin
Answer:
COOH
O
* *
*
*
*
HO
OH
9.
CH3O
O
H
N
H
CH3
HO
H
codiene
Answer:
CH3O
O
H
*
*
HO
*
*
*
H
N
CH3
H
Test Items for McMurry’s Organic Chemistry, Seventh Edition
69
10.
OH
NHCH3
ephedrine
Answer:
OH
*
*
NHCH3
O
C6H5CHCNH
11.
S
NH2
N
CH3
O
cephalexin
CO2H
Answer:
O
*
C6H5CHCNH
S
* *
NH2
N
CH3
O
CO2H
12.
H3C
HO
HO
O
CCH2OH
CH3
H3C
F
O
70
betamethasone
Chapter 9: Stereochemistry
Answer:
H3C
HO
H3C
*
*
*
*
*
*
O
HO
CCH2OH
*
*
CH3
F
O
13.
COOH
O
O
thromboxane A2
OH
Answer:
*
O *
*
O
*
COOH
*
OH
Assign R, S configurations to each indicated chirality center in the molecules below.
14.
OH
15.
H
CH2NH2
HO
COOH
C
H2N
H
HO
norepinephrine
CH3
alanine
O
17.
16.
OH
HO2C
H
HO
H
H
CH3
CO2H
14. The configuration of this carbon atom is
CH2
dihydrocarvone
.
R
15. The configuration of this carbon atom is
Answer:
18.
H2C
tartaric acid
Answer:
H
.
S
Test Items for McMurry’s Organic Chemistry, Seventh Edition
71
16. The configuration of this carbon atom is
Answer:
.
R
17. The configuration of this carbon atom is
Answer:
.
S
18. The configuration of this carbon atom is
Answer:
.
S
Consider the structure of streptimidone to answer the following questions.
O
O
H
*
H3C
NH
OH
*
O
streptimidone
H
19. Assign R or S configuration to each chirality center indicated in streptimidone.
Answer:
O
O
S
*
H3C
H
OH
*
R
NH
O
H
20. Based on the number of chirality centers, how many stereoisomers of streptimidone are possible?
Answer:
Streptimidone has two chirality centers so there are 22, or 4, possible stereoisomers.
21. Will streptimidone have a meso stereoisomer? Explain your answer.
Answer:
72
No. Meso compounds are compounds that contain chirality centers, but possess a plane of
symmetry. Because the substituents on the chiral carbons are different in streptimidone it is
impossible for any stereoisomer to have a plane of symmetry.
Chapter 9: Stereochemistry
Label each pair of stereoisomers below as:
a.
b.
c.
enantiomers
diastereomers
identical
Place the letter of the correct answer in the blank to the left of the pair of stereoisomers.
H
H3C
22.
H3C
OH
HO
Answer:
H
HO
CH2OH
H
HO
Answer:
H
CH2OH
a
H
CO2H
23.
OH
HO
H2OC
H
CH3
CH3
c
HO2C
24.
HO2C
Answer:
CO2H
CO2H
b
H
25.
OH
HOH2C
HO
CHO
OHC
H OH HO H
Answer:
H
CH2OH
H
OH HO H
b
Br
Br
26.
Br
H3C
Answer:
H3C
Br
c
H
27.
CH3CH2
Answer:
CH2CH3
C
Cl
CH3
H
C
Cl
CH3
a
Test Items for McMurry’s Organic Chemistry, Seventh Edition
73
O
O
H
28.
CH3
H
H
H3C
H
H2C
CH2
CH2
Answer:
CH2
b
29. Draw a wedge-dash projection of (2R,3S)-dibromobutane.
Answer:
CH3 H
Br
H
H3C
Br
30. Draw a Newman projection of the most stable conformation of (2R,3S)-dibromobutane sighting down the
C2-C3 bond.
Answer:
Br
H
CH3
H
CH3
Br
31. (2R,3S)-Dibromobutane is:
a.
b.
c.
d.
Answer:
optically active.
racemic.
dextrorotatory.
a meso compound.
d
32. The alkane formed by hydrogenation of (S)-4-methyl-1-hexene is optically active while the one formed by
hydrogenation of (S)-3-methyl-1-pentene is not. Explain.
Answer:
74
Each starting alkene is optically active because it contains a stereogenic center—a carbon with
four different groups bonded to it. The alkane formed by the hydrogenation of (S)-4-methyl-1hexene is optically because carbon four is still chiral. The product of hydrogenation of (S)-3methyl-1-pentene is not optically active because two of the groups on carbon three are the same
now—they are ethyl groups.
Chapter 9: Stereochemistry
33. Estriol, a potent estrogenic hormone, has been isolated from the urine of pregnant women. When 40 mg of
estriol is dissolved in 1.0 mL of dioxane and placed in a sample tube with 1 dm path length a rotation of
+2.32° is observed. Calculate the specific rotation for estriol.
α
Answer:
D
=
+2.32°
= +58°
1 dm × 0.040 g mL
34. Compound A, C9 H16 , was found to be optically active. On catalytic reduction over a palladium catalyst, 2
equivalents of hydrogen were absorbed to yield compound B. Ozonolysis of A gave two compounds. One
was identified as acetaldehyde, CH 3CHO . The other compound, C, was an optically active dialdehyde,
C5H8O2 . Formulate the reactions and draw structures for compounds A, B and C.
Answer:
CH3
CH3CH2CH2CHCH2CH2CH2CH3
*
B
*
CHCHCH
2CH
CH3CH
A
CHCH3
CH3
H2
Pd/C
O3, then Zn, HOAc
O
CH3CH
O
O
*
+ HCCHCH2CH
CH3
C
Refer to the structure below to answer the following questions.
CO2H
H
HOH2C
C
(S)-(-)-Serine
NH2
35. (S)-(−)-Serine:
a.
b.
c.
d.
is dextrorotatory
rotates plane-polarized light in a counterclockwise direction
rotates plane-polarized light in a clockwise direction
is racemic
Answer:
b
36. Draw the enantiomer of (S)-(−)-serine in a wedge-dash projection.
Answer:
CO2H
H2N
C
H
CH2OH
Test Items for McMurry’s Organic Chemistry, Seventh Edition
75
37. Give the complete name of the enantiomer of (S)-(−)-serine.
Answer:
(R)-(+)-serine
A natural product having [α]D = +40.3° has been isolated and purified
38. This information indicates that the natural product:
a.
b.
c.
d.
Answer:
is racemic.
does not rotate plane-polarized light.
is levorotatory.
is dextrorotatory.
d
39. Two structures have been proposed for this natural product. Circle the structure that is consistent with the
information presented and briefly explain your choice.
CO2H
HO H HO H
OH
HO
or
HO
OH
HO H
HO
OH
HO H
Answer:
CO2H
HO
HO
OH
OH
HO H HO H
or
HO
OH
HO H
HO H
The information presented indicates that the natural product is optically active. To be optically
active molecules must be chiral - that is, they must not have a plane of symmetry. The cyclic
structure, although is has chirality centers, has a plane of symmetry, indicated by the dashed line
on the structure, and can, therefore, not be optically active. The circled structure has four chirality
centers, and is not symmetric. We would expect it to be optically active.
76
Chapter 9: Stereochemistry
40. The enzyme aconitase catalyzes the hydration of the alkene functional group of aconitic acid to give two
products, citric acid and isocitric acid. Isocitric acid is optically active; citric acid is not optically active.
Based on your knowledge of alkene hydration and optical activity, the structure of citric acid is:
O
O
COH
HOCCH2
C
aconitase
C
HOC
citric acid
+
isocitric acid
(optically active)
(optically inactive)
H
O
aconitic acid
a. HO2CCH2
OH
H
C
H
C
c. HO2CCH2
CO2H
CH2CO2H
HO
CO2H
d. cannot be determined
H
OH
C
Answer:
C
CO2H
HO2C
b. HO2CCH2
C
CO2H
H
b
Identify the indicated hydrogens in the following molecules as pro-R or pro-S.
41.
H
H
CO2
2OC
H
H
succinate
Answer:
pro-S
H
H
CO2
2OC
H
H
pro-R
Test Items for McMurry’s Organic Chemistry, Seventh Edition
77
42.
H
H
CO2
O
phenylpyruvate
Answer:
pro-R
pro-S
H
H
CO2
O
phenylpyruvate
Identify the indicated faces in the following molecules as re or si.
43.
Ph
C
O
2OC
phenylpyruvate
Answer:
re
Ph
C
O
2OC
si
44.
3OPOH2C
C
O
HOH2C
dihydroxyacetone phosphate
Answer:
si
3OPOH2C
C
O
HOH2C
re
78
Chapter 9: Stereochemistry
45. Acetoacetate synthase catalyzes the addition of pyruvate to β-ketobutyrate to yield α-aceto-αhydroxybutyrate. If the addtion occurs from the si face of β-ketobutyrate, what is the stereochemistry of
the product?
O
O
CH3CCO2
+ CH3CH2CCO2
O
acetoacetate synthase
thiamine pyrophosphate
CH3C
OH
CCO2
+
CO2
CH2CH3
α-aceto-α-hydroxybutyrate
Answer: Addition of the aceto group to the si face results in the S enantiomer of α-aceto-αhydroxybutyrate.
addition
-OOC
-OOC
C
O
H3CH2C
Test Items for McMurry’s Organic Chemistry, Seventh Edition
H3CH2C
COCH3
S
OH
79