classroom
activity
17
Stereoisomers
Show You Know!
1. For each of the following molecules provide an IUPAC name including absolute configuration for
stereogenic centers and E/Z for double bonds.
a
H3C
Cl
CH3
HO
b
H
H
(1S,2R)-2-methylcyclohexanol
c.
(2E)-4,4-dimethylhex-2-ene
d
H
Br
c
OH
a.
CH3
CH3
2. Provide a skeletal structure for each of the following names:
a.
(3R)-3-methylcyclohexene
(3R)-3-methylcyclohexene
c.
(3R,4E)-3-fluorohexa-1,4-diene
(3R,4E)-3-fluorohexa-1,4-diene
F
( Z)
(E)
(R)
( R)
CH3
3. Consider the following molecules:
F
CH3
HO
CH3
(3R,4R)-3,4-dimethylcyclopentanol
OH
OH
(2S,4S)-3-fluoropentane-2,4-diol
a. Why isn’t the absolute configuration provided for the indicated carbons in the molecules above?
It is not possible to assign R/S because the two alkyl groups are identical (for both).
b. Are these chiral molecules?
Yes. The indicated atoms above are simply not stereogenic centers.
4. For each of the following molecules draw its enantiomer and a diasteromer:
Molecule
b.
Enantiomer
CH3
Diastereomer
H
H
Br
Br
H
H
H
CH3
CH3
H
Br
5. For each of the pairs of molecules shown here, identify their relationship as: constitutional isomers,
enantiomers, diastereomers, conformational isomers, or unrelated.
H
a.
CH3
HO
OH
(S)
HO
Diastereomers: opposite configuration at one
stereogenic center.
H
H
CH2CH3
(S)
OH
redrawn
H
OH
c.
OH
CH3
H
H
H
H
(R)
(S)
HO
Redraw Newman as skeletal then assign R/S
to find that they are enantiomers
H
OH
OH
CH2CH3
CH3
Br
e.
H
H
H
H
Br
Enantiomers. All stereocenters are switched.
CH3
CH3
CH3
g.
CH3
CH3
Constitutional Isomers: Atoms connected in
different order
17 Stereoisomers
7. Draw all the possible stereoisomers of 2,3,4-pentanetriol in Fisher projections. The first structure
is drawn for you as an example. Decide which pairs of structures are enantiomers and which are
diastereomers.
green: identical not meso
purple: identical not meso
blue: meso identical
red: meso identical
Here are the realities: only
4 different molecules.
There should be 2n (23 = 8)
stereoisomers, but since
there is a point of symmetry,
there are really (22 = 4)
stereoisomers.
a. How many different stereoisomers does this molecule have?
4
b. How many of the Fisher projections you drew are meso compounds? 4