Camosun College
Chemistry 230
Assignment # 4
Stereochemistry and Chirality
1. Draw the cis- and trans-diastereomers for the following compounds:
a.
b.
c.
d.
1-ethyl-3-methylcyclobutane
3,4-dimethyl-3-heptene
1-bromo-4-chlorocyclohexane
1,3-dibromocyclobutane
2. Which is the more stable diastereomer: cis-2-butene or trans-2-butene? Explain.
3. Assign these compounds as the E or Z diastereomers:
Cl
H
b.
a.
F
OCH 3
NC
c.
d.
CO2H
4. Determine whether each of the following compounds contains a stereocenter. If so, draw the two
enantiomers of the compound.
Br
a.
b.
HO
NH2
c.
Cl
d.
Br
5. a. What is meant by the term specific rotation of a compound?
b. Menthol has an optical rotation of + 2.46o when the measurement is made with a sodium lamp at
20oC on a solution containing 5 g of menthol in 100 mL of ethanol solution, using a sample tube 10 cm
long. What is the specific rotation of menthol?
6. a. Briefly explain what is meant by the Cahn-Ingold-Prelog rules?
b. Assign the following structures as R or S configuration:
H
H
i.
Cl
ii.
OH
H
iii.
Cl
H3C
iv.
H
H3C
HO
CH2OH
7. Draw structures for the following compounds:
a.
b.
c.
d.
(S)-2-butanol
(R)-1-bromo-1-chloroethane
(S)-1-bromo-3-chloro-2-methylpropane
(R)-2-bromoheptane
8. i. What is a meso compound?
ii. Which of the following compounds are chiral and which ones are achiral?
CH 3
Br
a.
b.
H
c.
H
Br
CH 3
d.
f.
e.
Br
CO 2H
H
OH
H
OH
g.
h.
i.
CO 2H
9. a. What are diastereomers? How are diastereomers and enantiomers different?
b. Draw and name all stereoisomers that are possible for 3-bromo2-butanol. Identify the enantiomers
and diastereomers. Make sure to assign configurations to all stereocenters.
10. Assign the configuration of the compounds represented by these Fischer projections as R or S :
CO 2H
CH 2OH
a.
H3C
b.
Cl
H
CH 3
CHO
H
CHO
c.
H 2C
CH
NH2
d.
CH2CH 3
H
OH
CH 2OH
CH 3
11. Draw all the stereoisomers of this compound and explain their relative stabililities.
OH
CH 3
12. Draw a stereoisomer of this compound that is chiral, and draw two that are not chiral.
HO
OH
13. Is the following compound chiral? Explain.
H3C
CH
C
CH
CH3
14. Indicate whether each of the following structures is (R)–2-chlorobutane or (S)-2-chlorobutane (use
models if necessary!!).
CH 3
Cl
a.
H3C
H
H
H
H
Cl
b.
H
CH 3
CH 3
H
Cl
CH 3
c.
H
Cl
CH 2CH3
d.
H
CH3
CH 2CH3
Chemsitry230-Answers to Assignment #4
1. a.
CH 2CH3
CH 2CH3
H3C
H3C
cis-1-ethyl-3-methylcyclobutane
trans-1-ethyl-3-methylcyclobutane
b.
cis-3,4-dimethyl-3-heptene
trans-3,4-dimethyl-3-heptene
c.
Cl
Cl
Br
cis-1-bromo-4-chlorocyclohexane
Cl
Br
trans-1-bromo-4-chlorocyclohexane
Cl
Br
Br
d.
Br
Br
Br
Br
cis-1,3-dibromocyclobutane
trans-1,3-dibromocyclobutane
2. a. The structure provided is the E diastereomer
H 3C
2
ethyl group has a higher priority
than the methyl group
1
CH 3CH 2
CH 2Cl
1
CH 3
2
chloromethyl group has a higher
priority than the methyl group
b. The structure provided is the E diastereomer
c. The structure provided is the Z diastereomer
d. The structure provided is the Z diastereomer
3. Draw both diastereomers first, then look for any steric interactions.
H
H
CH3
C
H 3C
H
C
H
C
H
H
C
C
trans-2-butene
H
H
C
H
H
cis-2-butene
In cis-2-butene, the two methyls are on the same side of the double bond. In this position, there is steric
interaction between the two groups. The resulting steric strain makes the cis-diastereomer less stable
than the trans-diastereomer, where such interactions are absent.
4. Look for a carbon with multiple substituents. If there are four different substituents on a carbon
atom, the structure has a stereocenter.
a. No stereocenters in the structure.
b. No stereocenters in the structure.
c. No stereocenters in the structure.
d. The structure has one stereocenter
CH3
Br
CH3
not a stereocenter
H
H2N
a stereocenter
5. a. Specific rotation of a compound: A physical constant (the optical rotation) that is characteristic
of a given optically active compound, and is calculated from the observed rotation of a compound
using the following equation:
 20D 

c  l 
Where α is the observed rotation using the D-Line (589.6 nm) of a sodium lamp at 20o C, the
concentration, C, of the solution is measured in grams of solute per millilitres of solution, and l is the
sample pathlength measured in decimetres (dm).
α = +2.46o , c = 5.0x10-2 g/mL
, l = 1dm
 2.46
= +49.2o
2
5.0 x10  1
Therefore, the specific rotation of menthol is +49.2o
 20D 


6. a. Cahn-Ingold-Prelog rules: a system of priorities that is used to assign R, S and E, Z configurations
in stereoisomers. You may want to look up at your notes/textbook to review the priority rules.
b. When assigning the R and S designations, the group of least priority must point away from you.
H
H
H
i.
Cl
Cl
iii.
H3C
ii.
OH
iv.
HO
(R)-
(R)-
H
H3C
(R)-
CH2OH
(S)-
7.
CH2CH3
a.
CH3
b.
H
OH
H3 C
CH2Cl
c.
H3 C
H
Br
Cl
CH3
d.
H
CH2Br
H
Br
C5 H11
8. i. A meso compound: An optically inactive compound whose molecules are achiral by virtue of a
symmetry plane even though they contain tetrahedral atoms with four different attached groups.
plane of symmetry
ii. a. Achiral
b. Chiral
c. Chiral
d. Achiral
f. Chiral
g. Achiral
h. Achiral
i. Chiral
e. Achiral
9. a. Diastereomers are stereoisomers that are NOT mirror images of each other, whereas enantiomers
are stereoisomers that ARE mirror images of each other (in other words, diastereomers are
stereoisomers that are NOT enantiomers).
b. The easiest representations are probably the Fischer projections.
1 CH 3
H
2 S
OH
HO
2 R
H
H
2 S
OH
H
3 R
Br
Br
3 S
H
Br
3 S
H
4 CH 3
A
A
B
C
D
1 CH 3
1 CH 3
1 CH 3
(2S,3R)-3-bromo-2-butanol
(2R,3S)-3-bromo-2-butanol
(2S,3S)-3-bromo-2-butanol
(2R,3R)-3-bromo-2-butanol
4 CH 3
B
4 CH 3
C
HO
2 R
H
H
3 R
Br
4 CH 3
D
A
C
A
A
B
B
&
&
&
&
&
&
B
D
C
D
C
D
are enantiomers
are enantiomers
are diastereomers
are diastereomers
are diastereomers
are diastereomers
10. Remember, the group of least priority must point away from you.
CO 2H
CH 2OH
a.
H 3C
b.
Cl
H
CH 3
H
(R)-
(S)-
CHO
CHO
c.
H2C
NH 2
CH
d.
CH 2CH3
H
OH
CH 2OH
CH 3
(R)-
(S)-
11. This is a disubstituted cyclohexane ring. Draw the chair conformations of the various stereoisomers
and comment on their relative stabilities.
OH
HO
OH
HO
CH3
H 3C
CH3
H 3C
OH
OH
CH3
HO
OH
CH3 H3C
H3C
A
B
C
D
The most stable chair conformations are drawn above. Since in conformations C and D both
substituents are equatorial, we can conclude that enantiomers C and D are more stable than enantiomers
A and B. (Another way of putting it: diastereomer A is less stable than diastereomers C or D.
Diastereomer B is also less stable than diastereomers C or D).
12. In achiral stereoisomers, there must be a plane of symmetry. The molecule is chiral if there is no
plane of symmetry.
CH 3
CH 3
OH
HO
CH 3
OH
HO
achiral
achiral
CH 3
OH
HO
OH
HO
chiral
chiral
13. Yes!! The compound is chiral since we can draw two stereoisomers that are nonsuperimposable
mirror images of each other.
H
H 3C
H
H
•
•
CH 3
H3C
CH 3
H
These compounds are called allenes (they are a special type of alkadienes). The planes of the π-bonds
of allenes are perpendicular to each other. This geometry of the π-bonds causes the groups attached to
the end carbon atoms to lie in perpendicular planes and, because of this, allenes with different
substituents on the end carbon atoms are chiral (Note that this is not cis-trans isomerism. The two
structures drawn above are enantiomers).
14. Remember to have the group of least priority (which in this case is H) point away from you before
you do the R or S assignment. If the group of least priority does not point away from you, then try
making a model or try two consecutive group switches using the structure provided.
Cl
H 3C
a.
H
(R)-2-chlorobutane
H
CH3
H
CH3
H
H
b.
(S)-2-chlorobutane
H
Cl
CH3
CH 3
c.
H
Cl
(S)-2-chlorobutane
CH 2CH 3
Cl
d.
H
CH 3
CH 2CH 3
(R )-2-chlorobutane