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17 Stereoisomers
Show You Know!1. ForeachofthefollowingmoleculesprovideanIUPACnameincludingabsoluteconfigurationfor
stereogenic centers and E/Z for double bonds.
CH3
OH
ClOH
H
CH3
CH3
CH3
H
H
Bra
bc
d
a. (1S,2R)-2-methylcyclohexanol
c. (2E)-4,4-dimethylhex-2-ene
2. Provide a skeletal structure for each of the following names:
a. (3R)-3-methylcyclohexene c. (3R,4E)-3-fluorohexa-1,4-diene
(3R)-3-methylcyclohexene
(R)
(Z)
CH3
(3R,4E)-3-fluorohexa-1,4-diene
(R)(E)
F
3. Consider the following molecules:
CH3
CH3
OH
(3R,4R)-3,4-dimethylcyclopentanolOH OH
F
(2S,4S)-3-fluoropentane-2,4-diol
a. Whyisn’ttheabsoluteconfigurationprovidedfortheindicatedcarbonsinthemoleculesabove?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 Enantiomer Diastereomerb.
Br
CH3
HH
Br
H
CH3
H
H
Br
CH3
H
5. For each of the pairs of molecules shown here, identify their relationship as: constitutional isomers, enantiomers, diastereomers, conformational isomers, or unrelated.
a.
(S) (S)
OHH
HO H
CH2CH3
H OHCH3
HHO
Diastereomers:oppositeconfigurationatonestereogenic center.
(R)
(S)
OHH
HHO
redrawn
c.OH
OH
CH2CH3
HHCH3
H
Redraw Newman as skeletal then assign R/S tofindthattheyareenantiomers
OH
e. Br
CH3
HH Br
H
CH3
H Enantiomers. All stereocenters are switched.
g.
CH3
CH3
CH3
CH3
Constitutional Isomers: Atoms connected in different order
7. Drawall thepossiblestereoisomersof2,3,4-pentanetriolinFisherprojections.Thefirststructureis drawn for you as an example. Decide which pairs of structures are enantiomers and which are diastereomers.
green: identical not mesopurple: identical not mesoblue: meso identicalred: 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
17 Stereoisomers
