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Chapter 9
9.1 Which of the following objects are chiral?
(a) A screwdriver (b) A screw (c) A bean stalk (d) A shoe (e) A hammer
Solution: chiral objects: screw, bean stalk, shoe.
9.2 Identify the chirality centers in the following molecules. Build molecular models if you need
help.
(a)
(b)
(c)
H3CO
H
CH2CH2CH3
N
CH3
HO
N CH3
H
H
H
Coniine
(poison hemlock)
menthol
(flavoring agent)
Solution:
chirality center
CH2CH2CH3
N
H
(a),
H
CH3
chirality center(R)
HO
H
chirality center(R) H
chirality center(S)
(b),
H3CO
chirality center(S)
R
N CH3
H
H
chirality center(S)
(c),
H
H
Dextromethorphan
(cough suppressant)
9.3 Alanine, an amino acid found in proteins, is chiral. Draw the two enantiomers of alanine using
the standard conversion of solid, wedged, and dashed lines.
NH2
CH3CHCO 2H
Alanine
Solution:
H
H
CO2H
H2N
HO2C
CH3
H3C
NH2
R
S
9.4 Identify the chirality centers in the following molecules.
(a)
HO
HO
H
H
C
C
O
C
H2
C
S
H
OH
(b)
R
F2
C
H2
C
H
C
F
O
H
Cl
9.5 A 1.50g sample of coiine, the toxic extract of poison hemlock,was dissolved in 10.0mL of ethanol
and placed in a sample cell with a 5.00 cm pathlength. The observed rotation at the sodium D line was
+1.21°. Calculate [α] D for coiine.
Solution:
Sample concentration: 1.50g/10.0mL=0.150g/mL
[α] D=+1.21°/(0.150 g/mL *5.00*0.1dm)=16.1°
9.6 Assign priorities to the following sets of substituents:
(a) ­H, ­Br, ­CH2CH3, ­CH2CH2OH
Solution:
­Br > ­CH2CH2OH > ­CH2CH3 > ­H
(b) ­CO2H, ­CO2CH3, ­CH2OH, ­OH
Solution:
­OH > ­CO2CH3 > ­CO2H > ­CH2OH
(c) ­CN, ­CH2NH2, ­CH2NHCH3, ­NH2
Solution:
­NH2 > ­CN >
­CH2NHCH3 > ­CH2NH2
(d) ­Br, ­CH2Br, ­Cl, ­CH2Cl
Solution:
­Br> ­Cl > ­CH2Br > ­CH2Cl
9.7 Orient each of the following drawings so that the lowest-priority group is toward the rear, and
the assign R or S configuration:
1
4
3
2
4
3
2
1
4
1
2
3
Solution:
1
4
3
2
4
1
4
3
3
2
2
1
4
1
4
3
2
3
3
4
2
1
1
S
R
S
2
9.8 Assign R or S configuration to the chirality center in each of the following molecules:
(a)
(b)
CH3
(c)
OH
H
C
H
Solution:
Br
NH2
CH3
C
COOH
H3C
H
COOH
CN
3
(a)
1
(b)
CH3
1
(c)
OH
H
C
4 H Br
1
C
3
COOH
H3C
2
4
2
H
NH2
CH3 3
COOH
CN
2
4
R
S
S
9.9 Draw a tetrahedral representation of (S)-2-pentanol (2-hydroxypentane).
Solution:
H
HO
9.10 Assign R or S configuration to the chirality center in the following molecular model of the
amino acid methionine(red=O, blue=N, yellow=S)
Solution:
First you can change the molecular model into the corresponding line-bond structure
following, and then assign R or S after identifying the priority of the atoms attaching to the
chirality center:
3
2
O
H2
C
S
H3C
C
H2
H
C
H2N
O
H
S
1
9.11 Assign R or S configuration to the chirality center in the following molecules. Which are
enantiomers, and which are diastereomers?
Br
CH3
H
CH3
H
Br
C
C
H
OH
CH3
Br
H
CH3
C
C
C
C
C
H
H
CH3
OH
(a)
Br
C
H3C
CH3
H
H3C
OH
(b)
OH
H
(c)
(d)
Solution:
First you can identify the priority of the group attached to the chirality center as following
Br 〉—OH 〉—CH3 〉H
And then you can assign R or S easily:
R
S
R
Br
H
CH3
CH3
H
C
CH3
Br
Br
H3C
CH3
Br
C
C
OH
H
H
C
C
H
S
C
C
H
H
C
CH3
OH
CH3
H3C
OH
OH
H
R
R
(a)
S
(b)
S
(c)
(d)
Obviously (a) and (d), (b) and (c) are enantiomers for their mirror relationships, however, (a) and
(d) are diastereomeric with (b) and (c).
9.12 Chloramphenicol, a powerful antibiotic isolated in 1949 form the Streptomyces venezuelae
bacterium, is active against a broad spectrum of bacterial infections and is particularly valuable
against typhoid fever. Assign R,S configurations to the chirality centers in chloramphenicol
H
OH
H2
C
OH
Cl
H
HN
C
O2N
CH
Cl
O
Solution:
1
R
H
OH
H2 3
OH
C
2
R
3
Cl
H
HN
C
CH
Cl
1
O2N
O
9.13 Assign R, S configuration to each chirality center in the following molecular model of the
amino acid isoleucine (red = O, blue = N):
Solution
S
S
9.14
Which of the following structures represent meso compounds?
(a)
H
OH
(b)
H
OH
(c)
CH3
H
(d)
Br
OH
OH
H
H
C
CH3
H
H3 C
C
H
Br
Solution: (a) (d)
9.15
Which of the following have a meso form?
(a) 2,3-Dibromobutane
(b) 2,3-Dibromopentane
Solution: (a) (c)
9.16
(c) 2,4-Dibromopentane
Does the following structure represent a meso compound? If so, indicate the symmetry
plane(red=O).
Solution: It is a meso compound. The symmetry plane is shown as following :
OH
9.17
How many chirality centers does morphine have? How many stereoisomers of morphine are
possible in principle?
CH3
N
H
*
*
*
*
O
HO
*
H
H
OH
morphine
There are five chirality centers in morphine .It has 25 stereoisomers in principle.
9.18
What stereoisomers would result from reaction of (
(S)-1-phenyl-ethylamine,and what is the relationship between them?
Solution:
)-lactic
acid
with
The products of the reaction of ( )-lactic acid with (S)-1-phenyl-ethylamine is a R,S salt
and a S,S salt. They are diastereomers.
9.19
What kinds of isomers are the following pairs?
(a) (S)-5-Chloro-2-hexene and chlorocyclohexane
(b) (2R,3R)-Dibromopentane and (2S,3R)-dibromopentane
Solution:
(a)
H
Cl
They are constitutional isomers.
(b)
Cl
Br
H
Br
H
R
s
R
R
H
Br
H
Br
They are diastereomers.
9.20
Addition of Br2 to an unsymmetrical alkene such as cis-2-hexene leads to racemic
2,3-dibromohexane, even though reaction of Br- ion with the unsymmetrical bromonium
intermediate is not equally likely at both ends. Make drawings of the intermediate and the
products, and explain the observed stereochemical result.
Solution:
cis-2-hexene
There are two intermediates leads to two products respectively.
Br
Br
R
H
H
Br
H
R
Br
S
S
H
H
H
major
Br
Br
Br
S
H
H
H
Br
R
S
Br
H
H
Br
R
H
major
The result is that there are racemic mixture of (2S,3S) and (2R,3R) 2,3-dibromoheane.
9.21
Predict the stereochemistry outcome of the reaction of Br2 with trans-2-hexene, and explain your
reasoning.
Solution:
There are two intermediates lead to two products respectively.
H
H
Br
Br
S
R Br
R
H
Br
H
H
H
Br
Br
H
S
R
H
Br
R
Br
S
Br
H
major
H
H
S
major
Br H
The outcome is that racemic mixture of (2S,3R) and (2R,3S).
The reason is that: the first step can occur equally well from either face of the double bond,
to give a 50:50 mixture of enantiomeric bromonium ions. Because the bromonium ions do
not have symmetry planes, reaction on the left and right is not equally likely, so an unequal
mix of (2S,3R) or (2R,3S) and (2R,3S) or (2S,3R) will be formed from each. The minor
product from one bromonium ion, however, will be the major product from the other, so
overall a 50:50(racemic)mixture of (2S,3R) and (2R,3S) dibromides will result.
9.22:
What products are formed from reaction of HBr with racemic (+)-4-methyl-hexene? What can you say
about the relative amounts of the products? Is the product mixture optically active?
Solution: Racemic mixture
Ratio
(1R,3S),(1S,3S) and (1S,3R),(1R,3R)
50:50
Optically
inactive
9.23:
What products are formed from reaction of HBr with 4-methyl-cyclopentene? What can you say about
the relative amounts of the products?
Solution: Racemic mixture
Ratio
(1R,3R),(1S,3R) and (1S,3R),(1S,3S)
not 50:50
9.24:
Identify the indicated hydrogens in the following molecules as pro-R or pro-S.
Solution:
pro-S
pro-R
pro-R
pro-S
H
H
H
H
CO2
CHO
H3C
HO
(a)
HO
H
(b)
H3N
H
9.25
Identify the indicated faces in the following molecules as re or si:
re face
O
H3C
C
CH2OH
si face
(a) Solution:
re face
H
C
H3C
C
H
(b) Solution:
CH2OH
si face
9.26
Lactic acid buildup in tired muscles results from reduction of pyruvate. If the reaction occurs from the
re face, what is the stereochemistry of the product？
OH
O
CH
C
H3C
H3 C
CO2
CO2
OH
Solution: It’s
H3C
CO2
H
and is S-Lactate.
9.27
Which of the following structures are identical?
(a)
It’s R configuration.
(b)
It’s R configuration.
(c)
It’s S configuration.
(d)
It’s R configuration.
Solution: (a)(b)(d) are identical.
9.28:
Assign R or S configuration to the chirality centers in the following molecules(red=O, blue=N):
(a)
Serine
(b)
Adrenaline
O
N
H
H
OH
C (S)
H
HO
N
H
(R)
H
H
OH
OH
HO
H
Serine
Solution: (a)
(b)
Adrenaline
9.29:
Witch, if any, of the following structures represent meso compounds?(red=O. blue=N,
yellow-green=Cl.)
(b)
(a)
(c)
HO
OH
(R)
H
Solution:
(a)
(S)
H
There are two chirality centers which are of
different direction, and there is a symmetrical plane, so it is a meso-compound.
H2 N
(b)
H H2N
(R)
H
(S)
Just the same as problem (a), there are two chirality centers which are
of different direction, and there is a symmetrical plane, so it is a meso compound.
Cl
H
(R)
(c)
Cl
compound.
(R)
H As there are two chirality centers with the same direction, it is not a meso
9.30:
Assign R or S configuration to each chirality center in paeudophedrine, an over-the-counter
decongestant found in cold remedies(red=O, blue=N).
Solution:
HO
H
(R)
(S)
N
H
H
9.31
Polarimeters for measuring optical rotation are so sensitive that they can measure rotations to 0.0010, an
important fact when only small amounts of sample are available. Ecdysone, for example, is an insect
hormone that controls molting in the silkworm moth. When 7.00 mg ecdysone was dissolved in 1.00 ml
chloroform and the solution was placed in a cell with a 2.00 cm pathlength, an observed rotation of
+0.0870 was found. Calculate [α]D for ecdysone.
Solution:
C=7.00mg/1.00ml=0.007g/ml
[α]D=α/(l*C)= +0.0870/(2.00cm*0.007g/ml)=+62.140
9.32
Which of the following compounds are chiral? Draw them, and label the chirality centers.
(a) 2,4-Dimethylheptane
(b) 5-Ethyl-3,3-dimethylheptane
(c) cis-1,4-Dichlorocyclohexane
(d) 4,5-Dimethyl-2,6-octadiyne
Solution: (a) 2,4-Dimethylheptane and (d) 4,5-Dimethyl-2,6-octadiyne are chiral. Followings are
their structures.
CH3
∗
CH3
(a)
CH3
∗
∗
H
CH3
H
(d)
9.33
Draw chiral molecules that meet the following descriptions:
(a) A chloroalkane, C5H11Cl
(b) An alcohol, C6H14O
(c) An alkene, C6H12
(d) An alkane, C8H18
H3C
CH
H2
C
H3C
Cl
H
C
H2 H2
C C
Solution: (a)
H2
C
CH3
H2
C
CH3
OH
(b)
(c)
H2C
H2
C CH3
H
C
C
H
CH3
(d)
CH3
9.34
Eight alcohols have the formula C5H12O. Draw them. Which are chiral?
Solution:
Not chiral:
OH
OH
OH
OH
Chiral:
OH
*
*
OH
*
OH
OH
9.35
Draw the nine chiral molecules that have the formula C6H13Br
Solution:
Br
Br
*
*
*
Br
* *
*
Br
Br
CH2Br
Br
*
*
*
*
Br
Br
9.36
Draw compounds that fit the following descriptions:
(a) A chiral alcohol with four carbons.
(b) A chiral carboxylic acid with the formula C5H10O2
(c) A compound with two chirality centers
(d) A chiral aldehyde with the formula C3H5BrO
OH
C
CH
H
CH
C
2
H 3
Solution: (a) 3
COOH
C
H
H
CH
C
2
CH3
(b) 3
(c)
COOH
H
OH
HO
H
COOH
(d)
CHO
C H
H3C Br
9.37
Draw examples of the following:
(a)
A basketball
(b)
A fork
(c)
A wine glass
(d)
A golf club
(e)
A monkey wrench
(f)
A snowflake
Solution: (e) A monkey wrench is chiral.
9.38
Penicillin V is an important broad-spectrum antibiotic that contains three chirality centers. Identify
them:
H
H
H
N
O
S
C
*
O
*
N
*
CH3
Penicillin V
CH3
O
H
CO2 H
Solution:
9.39
Draw examples of the following:
(a) A meso compound with the formula C8H18
H
CH2CH3
CH3
C
C
H
CH3
CH2CH3
Solution:
(b) A meso compound with the formula C9H20
H
H
C
Solution:
(c)
H3CH2C
H3C
C
CH2CH3
CH2
CH3
A compound with two chirality centers, one R and the other S
CH3
H
Br
C
C
H3C
Solution:
H
OH
9.40
What is the relationship between the specific rotations of (2R,3R)-dichloropentane and
(2S,3S)-dichloropentane? Between (2R,3S)-dichloropentane and (2R,3R)-dichloropentane?
Solution: (2R,3R)-dichloropentane and (2S,3S)-dichloropentane are enantiomers.
(2R,3S)-dichloropentane and (2R,3R)-dichloropentane are diastereomers.
Cl
(R)
Cl
(R)
(R)
Cl
(S)
(S)
(S)
Cl
Cl
Cl
(2R,3R)-dichloropentane
(2R,3S)-dichloropentane
(2S,3S)-dichloropentane
9.41
What is the stereochemical configuration of the enantiomer of (2S,4R)-dibromooctane?
Solution: (2R,4S)-dibromooctane is the stereochemical configuration of the enantiomer of
(2S,4R)-dibromooctane.
Br
Br
Br
Br
(S)
(R)
(R)
(S)
(2R,4S)-dibromooctane
(2S,4R)-dibromooctane
9.42
What are the stereochemical configurations of the two diastereomers of (2S,4R)-dibromooctane?
Solution: (2S,4S)-dibromooctane and (2R,4R)-dibromooctane are the stereochemical
configurations of the two diastereomers of (2S,4R)-dibromooctane.
Br
Br
Br
Br
(S)
(R)
(S)
(R)
(2R,4R)-dibromooctane
(2S,4S)-dibromooctane
Br
Br
(R)
(S)
(2S,4R)-dibromooctane
9.43
Orient each of the following drawings so that the lowest-priority group is toward the rear , and then
assign R or S configuration:
(a).
G1
G2
Solution
G4
G3
G4
G3
G4
(b). G2
G1
(c).
G3
G1
G2
G4
G1
G2
G2
R
G3
(a).
G4
G4
G3
G3
R
G1
(b).
G1
R
G2
(c).
9.44
Assign Cahn-Ingold-Prelog priorities to the following sets of substituents:
(a).
C
H
CH2
CH(CH3)2
C
CH
C
H
CH2CH3
C(CH3)3
CH2
C(CH3)3
(b).
CO2CH3
(c).
C
(d).
Solution:
C
H
>
CH2CH3
CH2OCH3
CH2Br
N
C(CH3)3
(a).
COCH3
CH2CH2Br
>
CH2
CH(CH3)2
Br
>
CH2CH3
(b).
>
CO2CH3
(c).
Br
(d).
>
>
C
CH >
COCH3
>
CH 2Br
>
C(CH3)3
>
C
H
CH2OCH3 >
C
N
>
CH2
CH2CH3
CH 2CH 2 Br
9.45
Assign R or S configuration to the chirality centers in the following molecules
Cl
H
(a).
Solution:
H
H
OH
(b).
(c). HOH2C
OCH3
CO2H
Cl
H
S
OH
S
(a).
H
H
(c). HOH2C
(b).
OCH3
S
9.46
Assign R or S configuration to each chirality center in the following molecules:
(a)
OH
H
S
S
Cl
H
(b)
S
S
H
CH3
H3C
C
H2
H
(c)
R
HO
H3C
S
OH
CH3
9.47
Assign R or S configuration to each chirality center in the following biological molecules:
(a)
CO2H
Biotin
O
H
H
N
N
S
H
H
H
R
S
CH2CH2CH2CH2 CO2
S
(b)
Prostaglandin E1
O
R
H
CO2H
S
R
S
HO
H
H
HO
H
9.48
Draw tetrahedral representations of the following molecules:
(a) (S)-2-Butanol, CH3CH2CH(OH)CH3
OH
C
H3C
H
CH2CH3
(b) (R)-3-Chloro-1-pentene
Cl
H2 C
C
C
H
CH2CH3
H
9.49
Draw tetrahedral representations of the two enantiomers of the amine acid cysteine,
HSCH2CH(NH2)COOH, and identify each as R or S.
Solution:
H
H
C
COOH
HSH2C
HOOC
NH2
C
CH2SH
H2 N
R
S
9.50
Which of the following pairs of structures represent the same enantiomers, and which represent
different enantiomers?
(a)
(b)
Br
COOH
CN
H3C
H
Br
CN
H
H
NC
CH3
(c)
Br
H
HOOC
Br
CN
(d)
OH
CH3
H
H3CH2C
H
H3C
OH
CH3
H
H2N
CH2CH3
COOH
COOH
H3C
H2N
H
Solution: (a) is different and (b), (c), (d) are the same.
9.51
Assign R or S configuration to each chirality center in the following molecules:
(a)
(b)
H
H
Br
H
NH2
H
H3C
COOH
CH3
H
OH
Br
H
Solution: (a)
(b)
H
H
Br
H
NH2
H
R
S
H3C
S
H
CH3
H
Br
COOH
S
OH
9.52
Draw tetrahedral representations of the following molecules.
(a) The 2S,3R enantiomer of 2,3-dibromopentane
(b) The meso form of 3,5-dichloroheptane.
(a)
H Br
Br
9.53
(b)
H
H Cl
H Cl
Draw the meso form of eachof the following molecules, and indicate the plane of
symmetry in each.
H OH
(a)
HO H
(c)
(b)
OH
9.54
The plane of symmetry
The plane of symmetry
The plane of symmetry
Assign R or S configurations to the chirality centers in ascorbic acid (vitamin c)
9.55:
Assign R or S sterochemistry to the chirality centers in the following Newman projections:
Cl
(a)
CH3
H
H3 C
H3C
(b)
H
H
H3C
OH
CH3
H
H
S
R
9.56:
Xylose is a common sugar found in many types of wood, including maple and cherry. Because it
is much less prone to cause tooth decay than sucrose, xylose has been used in candy and chewing
gum. Assign R or S configuration to the chirality center in xylose.
HO H
HO
S
R
OHC
HO
H
R
CH2OH
H
9.57:
Ribose, an essential part of ribonucleic acid (RNA), has the following structure:
H H
H
OH
CHO
HO
HO
(a)
H
HO
H
How many chirality centers does ribose have? Identify them
H H
H
∗
∗
HO
HO
Solution: Three chirality centers.
OH
H
CHO
∗
H
HO
(b) How many stereoisomers of ribose are there?
Solution: There are eight stereoisomers of ribose.
(c) Draw the structure of the enantiomer of ribose.
H H
HO
H
S
HO
CHO
S
S
H
OH
OH
H
Solution:
(d) Draw the structure of a diastereomer of ribose.
Solution:
H H
HO
H H
H
R
HO
HO
HO
HO
HO
H
H H
H
H
OH
H H
HO
CHO
HO
H
H
H
HO
OH
R
OH
CHO
R
H
S
HO
H
OH
H H
R
OH
S
S
H
H
OH
R
H
HO
CHO
S
H
CHO
R
HO
S
HO
H
S
R
HO
OH
H
OH
R
R
HO
H
H H
CHO
S
S
H
S
H
CHO
OH
9.58 On catalytic hydrogenation over a platinum catalyst, ribose (Problem 9.57) is converted into ribitol.
Is ribitol optically active or inactive? Explain.
H
H
H
H
OH
H H
OH
CHO
CH2OH
HO
HO
HO
H
H HO
HO
H HO H
Ribitol
Ribose
S
H
OH H OH
S
C
H
H HO H
H H
R
HO
HO
Ribitol
Solution: The product ribitol is optically inactive , because it is Meso compound.
9.59 Hydroxylation of cis-2-butene with OsO4 yields butane-2,3-diol. What steeochemistry do you expect
for the product?
O
O
CH3
H3C
C
C
H
Os
OsO4
Pyridine
H
O
O
C
C
H
H3C
NaHSO3
H2O
OH
HO
R
H
CH3
C
S
C
H
H3C
H
CH3
9.60:
Hydroxylation of cis-2-butene with OsO4 yields butane-2,3-diol. What steeochemistry do you expect
for the product?
Solution:
O
O
H
H3C
C
H
C
CH3
Os
OsO4
Pyridine
O
O
C
H
H3C
NaHSO3
H2O
C
H
CH3
OH
HO
R
C
H
H3C
C
H
R
CH3
9.61
Alkenes undergo reaction with peroxycarboxylic acids (RCO3H) to give threemembered-ring cyclic
ethers called epoxices. For example, 4-octene reacts with a peroxyacid to yield 4,5-epoxyoctane:
O
CH3CH2CH2 CH
CHCH2CH2CH3
RCO3H
CH3CH2CH2CH
4-Octene
CHCH2CH2CH3
4,5-Epoxyoctane
Assuming that this epoxidation reaction occurs with syn stereochemistry, draw the structure obtained
from epoxidation of cis-4-octene. Is the product chiral? How many chirality centers does it have? How
would you describe it stereochemically?
Solution:
S
R
O
RCO3H
cis-2,3-Dipropyl-oxirane
cis-Oct-4-ene
9.62
Answer Problem 9.61, assuming that the epoxidation reaction is carried out on trans-4-octene.
Solution:
R
R
O
RCO3H
trans-Oct-4-ene
trans-2,3-Dipropyl-oxirane
9.63
Identify the indicated hydrogens in the following molecules as pro-R or pro-S:
H
H
H
COOH
H
H
H3 CS
CO2
CO2
HS
HOOC
(a)
HO
H
(b)
H
Solution:
pro-R
pro-S
H
H
HO2C
CO2H
a)
H
HO
H
H
H3N
H
(c)
H3N
H
por-R
pro-S
H
H
H3CS
CO2
H
b)
H
H3N
H
pro-S
pro-R
pro-R
pro-S
H
H
HS
CO2
H3N
H
c)
9.64
Identify the indicated faces in the following molecules as re or si:
O
H
C
C
H3C
O 2C
CO2
(b)
(a)
CH3
C
H
Solution:
re
O
C
H3C
CO2
si
(a)
si
H
C
O2C
CH3
C
H
(b)
re
9.65.
Write the products of following reaction and indicate the stereochemistry obtained in each
instance:
(a)
Br2,H2O
DMSO
?
(b)
Br2
CH2Cl2
?
OsO4
NaHSO3
(c)
?
Solution:
H
Br2,H2O
DMSO
(a)
Br
OH
H
H
Br2
CH2Cl2
(b)
Br
Br
H
H
OsO4
(c)
NaHSO3
OH
H
OH
9.66.
Draw all possible stereoisomers of cyclobutane-1,2-dicarboxylic acid, and indicate the
interrelationships. Which, if any, are optically active? Do the sane for
cyclobutane-1,3-dicarboxylic acid.
Solution: the stereoisomers of cyclobutane-1,2-dicarboxylic acid:
HOOC
H
H
H
COOH
HOOC
H
H
COOH
1
COOH
2
1 and 2 are optically active.
the stereoisomers of cyclobutane-1,3-dicarboxylic acid:
COOH
H
3
COOH
H
HOOC
H
COOH
H
H
COOH
H
H
COOH
COOH
They are all optically inactive.
9.67
Compound A, C7H12, was found to be optically active. On catalytic reduction over a palladium
catalyst, 2 equivalents of hydrogen were absorbed, yielding compound B, C7H16. On ozonolysis of
A, two fragments were obtained. One fragment was identified as acetic acid. The other fragment,
compound C, was an optically active carboxylic acid, C5H10O2.Write the reactions, and draw
structures for A, B, C.
Solution:
CH3
CH3
CH3
A. CH3C
CCHC2H5
B. C3H7CHC2H5
CH3
C. C2 H5CHCO2H
CH3
2H2
CH3C
CCHC2H5
Pd
C3H7CHC2 H5
CH3
CH3
O3
CH3C
+
CH3CO2H
C2H5CHCO2H
CCHC2H5
9.68
Compound A, C11H16O, was found to be an optically active alcohol. Despite its apparent
unsaturation, no hydrogen was absorbed on catalytic reduction over a palladium catalyst. On
treatment of A with dilute sulfuric acid, dehydration occurred and an optically inactive alkene B,
C11H14, was produced as the major product. Alkene B, on ozonolysis, gave two products. One
product was identified as propanal, C2H5CHO. Compound C, the other product, was shown to be a
ketone, C8H8O. How many degrees of unsaturation does A have? Write the reactions, and identify
A, B, and C.
Solution:
The degree of unsaturation of A is 4.
Ph
Ph
C2H5CHCHCH3
A.
OH
B. C2H5 CH
CCH3
O
C. CH3C
Ph
Ph
Ph
H2SO4
C2H5CHCHCH3
C2H5CH
CCH3
OH
Ph
C2H5 CH
O
O
O3
CCH3
CH3 C
C2H5CH
+
Ph
9.69
One of the steps in fat biosynthesis is the hydration of crotonate to yield 3-hydroxybutyrate. The
reaction occurs by addition of —OH to the si face at C3, followed by protonation at C2, also from the
si face. Draw the product of the reaction, showing the stereochemistry of each step.
3
OH
CO2
H3C
H3 C
2
CHCH2CO2
Solution:
H
3
H3C
CO2
H3C
H3C
H
2
H
H
CO2
CO2
OH
H
OH
9.70
The dehydration of citrate to yield cis-aconitate, a step in the citric acid cycle, involves the pro-R “arm”
of citrate rather than the pro-S arm. Which of the following two products is formed?
HO
O2C
CO2
CO2
CO2
CO2
O2C
CO2
or
O2C
CO2
CO2
Solution: This product is formed:
O2C
CO2
9.71
Hydration of cis-aconitate yields (2R, 3S)-isocitrate. Show the stereochemistry of the product, and
tell whether the addition of the OH group takes place on the re or si face of cis-aconitate.
COO
OOC
COO
COO
H2O
OOC
COO
cis-Aconitate
OH
Isocitrate
Solution: the stereochemistry of the product as follow
OH
O
O
H
O
-O
OH
-O
And the OH group takes place on the re face of the cis-aconitate
9.72
The so-called tetrahedranes are an interesting class compounds, the first example of which was
synthesized in 1979. Make a model of a substituted tetrahedrane with four different substituents. Is
it chiral? Explain.
X
W
Z
Y
Solution:
It is chiral.because
X
X
W
Z
I
Y
Y
Z
W
II
II is mirror-image of I, and they are enantiomers. Therefore I is chiral.
9.73
llenes are compounds with adjacent carbon-carbon double bonds. Many allenes are chiral, even though
they don’t contain chirality centers. Mycomycin, for example, a naturally occurring antibiotic
isolated from the bacterium Nocardia acidophilus, is chiral and has [α]D = -130º. Explain why
mycomycin is chiral. Making a molecular model should be helpful.
HC
C
C
C
C
H
C
C
H
C
H
C
H
C
H
C
H
CH2CO 2H
Mycomycin (an allene)
Solution:
According to the molecular model, we can see the atoms are arranged symmetrically
besides the carboxylic group. Except C1, there isn’t any symmetric element in the
molecule, so the molecule is chiral.
9.74
Long before chiral allenes were known , the resolution of 4-methylcyclohexylideneacetic acid
into two enantiomers had been carried out. Why is it chiral? What geometric similarity does it
have to allenes?
CO2H
H
C
H3C
H
methylcyclohexylideneacetic acid
Solution:
The geometry of the acid is similar to the allene, the atoms are arranged symmetrically besides the
carboxylic group. Except C1, there isn’t any symmetric element in the molecule, so it is also chiral.
9.75
Suppose that racemic lactic acid reacts with methnol, CH3OH, to yield the ester, methyl lactate.
What stereochemistry would you expect the products to have? What is the relationship of one product
to another?
Solution:
H
H
C
HO
H3C
50%
50%
S
OH
H
H
C
HO
CH3
C
HO
HOOC
R
COOCH3
H3 C
H3 C
S
C
HO
COOH
CH3
H3COOC
R
50%
50%
9.76
Suppose that (S)-lactic reacts with (R)-2-butanol to form an ester. What stereochemistry
would you expect the products to have?
Solution:
H
C
HO
H3C
S
H
H
COOH
C
H3 CH2C
CH3
R
OH
O
C
HO
C
H3C
H
O
S
C
R
CH2 CH3
CH3
9.77
Suppose that racemic lactic acid reacts with (S)-2-butanol to form an ester (Problem 9.76). What
stereochemistry does the product(s) have? What is the relationship of one product to another?
Assuming that esters can be converted back into carboxylic acids, how might you use this reaction to
resolve (±)-lactic acid?
Solution：
(1) The products are:
HO
H
HO
H
CH3
R
H3C
O
H3C
CH3
S
O
CH3
S
H
and
O
O
H
CH3
S
(2) They are diastereomers.
(3)
The diastereomers have different boiling point, so they can be separated by distillation. So we can
let the racemic lactic acid reacts with (S)-2-butanol to form these two esters, then separate them
by distillation, at last we can convert them back to (±)-lactic acid.
9.78
(S)-1-Chloro-2-methylbutane undergoes light-induced reaction with Cl2 by a radical mechanism to
yield a mixture of products. Among the products are 1,4-dichloro-2-methylbutane and
1,2-dichloro-2-mehylbutane.
(a)Write the reaction, showing the correct stereochemistry of the reactant.
(b)One of the two products is optically active, but the other is optically inactive. Which is which?
(c)What can you conclude about the stereochemistry of radical chlorination reactions?
Solution:
(a)
H3C
H3C
H
CH3
+
Cl2
hv
Cl
Cl
Cl
CH3
CH3
+ Cl
CH3
ClH2C
H3C
+
Cl
H
CH2Cl
(b)
1,2-dichloro-2-mehylbutane is optically inactive, because it is racemic.
(c)
Conclusion: when forming 1,2-dichloro-2-mehylbutane,half inversion occurs.
9.79
Draw the structure of a meso compound that has five carbons and three chirality centers.
Solution:
Sorry, I don’t know. I am wondering how can a meso compound which has odd chirality centers.
（my solution:
CH3
H
OH
H
OH
H
OH
CH3
S
S
R
CH3
HO
H
H
OH
HO
H
CH3
R
R
S
Both of them are meso-cpds,
because of the symmetrical plane
(C3-H-OH).
)
9.80
How many stereoisomers of 2,4-dibromo-3-chloropentane are there? Draw them, and indicate
which are optically active.
Solution:
Br
Br
H
CH3
H3C
H
Br
Br
CH3
H
H
CH3
Cl
Cl
(2R,4R)-Dibromo-3-chloro-pentane
Br
Br
H
H
H3C
(2S,4S)-Dibromo-3-chloro-pentane
CH3
Cl
(2R,4S)-Dibromo-3-chloro-pentane
The first two of them are optically active.
9.81
Draw both cis- and trans-1,3-dimethylcyclohexane in their most stable chair conformations.
(a) How many stereoisomers are there of cis-1,4-dimethylcyclohexane, and how many of
trans-1,4-dimethylcyclohexane?
(b) Are any of the structures chiral?
(c) Whar are the stereochemical relationships among the various stereoisomers of
1,4-dimethylcyclohexane?
Solution:
CH3
CH3
CH3
H3C
CH3
CH3
trans
cis-1,4-dimethylcyclohexane
trans-1,4-dimethylcyclohexane
(a) There
is
only
one
trans-1,4-dimethylcyclohexanes.
cis-1,4-dimethylcyclohexane
and
two
(b) Because of the existence of the symmetrical axes in cis or trans-1,4-dimethylcyclohexane,
neither of them have the structures chiral.
(c) They are diastereomers, in more detailed description, they are configurations and
Cis-trans diastereomers.
9.82
Draw both cis- and trans-1,3-dimethylcyclohexane in their most stable chair conformations.
(a) How many stereoisomers are there of cis-1,3-dimethylcyclohexane, and how many of
trans-1,3-dimethylcyclohexane?
(b) Are any of the structures chiral?
(c) Whar are the stereochemical relationships among the various stereoisomers of
1,3-dimethylcyclohexane?
Solution:
H 3C
CH3
cis-1,3-dimethylcyclohexane
H3C
CH3
CH3
CH3
(1S,3S)-1,3-dimethylcyclohexane
(1R,3R)-1,3-dimethylcyclohexane
(a) There are one cis-1,3-dimethylcyclohexane and two trans-1,3-dimethylcyclohexanes.
(b) They are all chiral.
(c) They are diastereomers, in more detailed description, they are configurations and
Cis-trans diastereomers.
9.83
How can you explain the observation that cis-1,2-dimethylcyclohexane is optically inactive even it
has two chirality centers?
My Solutions :
H
ring flip
H
H
ring flip
H
CH3
H
H
H 3C
H
H3C
CH 3
CH3
CH3
H
ring flip
CH3
CH3
A
B
H
H
H
H 3C
CH3
racemic pairs
H
CH3
CH3
B
A
The resolution of them is impossible, for the ring flip is
very fast.
9.84
An alkyl halide reacts with a nucleophile to give a substitution product by a mechanism that
involves inversion of stereochemistry at carbon:
Nu
C
X
Nu
C
+
X
Formulate the reaction of (S)-2-bromobutane with HS- ion yield butane 2-thiol ,
CH3CH2CH(SH)CH3 .What is the stereochemistry of the product?
H
HS
Br
H
HS
S
R
My Solution:
9.85
Grignard reagents, RMgX, react with aldehaydes to yield alcohols. For example, the reaction of
methylmagnesium bromide with propanal yields 2-butanol:
O
OH
1.CH3MgBr
H3CH2C
C
H
H3CH2C
C
CH3
2.H3O+
H
(a) Is the product chiral? Is it optically active?
(b) How many stereoisomers of butanol are formed, what are their stereochemical relationships,
and what are their relative amounts?
Solution:
(a) The product id chiral and it’s optically active.
(b) A pair of enatiomers.
OH
OH
C2H5
H
H
C 2H 5
CH3
The ratio is
CH3
(R)- 2-butanol
1
:
(S)- 2-butanol
1
9.86
Imagine that another Grignard reaction similar to that in Problem 9.85 is carried out between
methylmagnesium bromide and (R)-2-phenylpropanal to yield 3-phenyl-2-butanol:
H
CH3
C
OH
O
1.CH3MgBr
C
H3C
H
C
C
H
CH3
2.H3 O+
H
(a) Is the product chiral? Is it optically active?
(b) How manystereoisomers of 3-phenyl-2-butanol are formed, what are their stereochemical
relationships ,and what are their relative amounts?
Solution:
(a) chiral product, optically active
(b)
H
H
Ph
CH3
OH
H
Ph
H
OH
CH3
CH3
（2S,3R）-3-Phenyl-2-butanol
The ratio:
1
Meso-compound
CH3
:
(2R,3R)-3-Phenyl-2-butanol
1
chiral product