CHEM 109A
CLAS
Stereochemistry Part 1 - KEY
1. Draw all possible isomers for….
a. 2,3-dichlorobutane
1
H
Cl
H
4
Cl
H
Cl
4
H 3C
3
Cl
H
CH 3
H
Cl
Cl
1
H
4
Cl
H
H 3C
3
CH3
3
3
H
Cl
1
4
1
threo enantiomers
(2S,3S)-2,3-dichlorobutane
(2R,3R)-2,3-dichlorobutane
4
1
H
Cl
H
H
H
Cl
4
Cl
Cl
1
H
4
Cl
3
CH 3
H 3C
3
3
1
Cl
H 3C
CH 3
3
H
Cl
Cl
H
4
1
internal
plane of
symmetry
H
internal
plane of
symmetry
erythro enantiomers
The (2R, 3S) is a meso compound which means that it is identical to the
(2S,3R) compound, so there are only 3 possible isomers.
(2R,3S)-2,3-dichlorobutane
(2S,3R)-2,3-dichlorobutane
b. 2-bromo-3-chlorobutane
1
H
Br
H
4
Cl
H
Br
4
H 3C
3
Br
H
3
CH 3
H
Cl
H
4
H
H3C
CH
3
3
3
Cl
Br
1
H
Cl
1
4
1
1
H
Br
H
H
H
Br
4
H 3C
3
3
Cl
H
threo enantiomers
(2S,3S)-2-bromo-3-chlorobutane
4
(2R,3R)-2-bromo-3-chlorobutane
Br
3
CH 3
Br
1
Cl
H 3C
CH 3
3
H
Cl
Cl
1
4
H
H
1
4
erythro enantiomers
Since the two stereocenters are NOT attached to the same groups (Br is
not the same as Cl) there is NO plane of symmetry in the erythro
enantiomers and they are not a meso compound, so there are 4 possible
isomers.
(2S,3R)-2-bromo-3-chlorobutane
(2R,3S)-2-bromo-3-chlorobutane
c. 2,4-dibromopentane
4
1
Br
4
H
3
H
4
1
H
3
(2R,4R)-2,4-dibromopentane
1
1
Br
Br
3
Br
4
H
3
(2S,4S)-2,4-dibromopentane
Page 1 of 5
CHEM 109A
CLAS
Stereochemistry Part 1 - KEY
1
1
Br
Br
4
Br
4
H
Br
H
H
H
3
3
internal
plane of
symmetry
(2R,4S)-2,4-dibromopentane
The (2R, 4S) is a meso compound which
means that it is identical to the (2S, 4R) compound, so there are only 3
possible isomers.
d. 1,3-dichlorocyclopentane
4
1
H
Cl
2
1 Cl
2
Cl
3
1
4 H
H
3
H
3
Cl
3
4
(1R, 3R)-1,3-dichlorocyclopentane
4
1
(1S, 3S)-1,3-dichlorocyclopentane
4
H
H
2
1 Cl
H
3
Cl
4
Cl
Cl
3
H
1
(1R, 3S)-1,3-dichlorocyclopentane
internal
plane of
symmetry
The (1R, 3S) is a meso compound which means that it is identical to the
(1S, 3R) compound, so there are only 3 possible isomers.
e. 1,4-dimethylcyclohexane
H
H
H
H
cis-1,4-dimethylcyclohexane
trans-1,4-dimethylcyclohexane
No asymmetric C/stereo centers (both sides of the ring are the same), so
can’t use R, S nomenclature. Look for other possible isomers – here
geometric/cis-trans isomers.
2. Name each of the following using R,S or E,Z designations where necessary.
(Similar to Ch 5 #61)
1
2
4
a.
H
3
(3S)-3-methylpent-1-ene
3-methylpent-1-ene
F
F
I
I
Cl
Cl
b.
Br
Br
1-bromo-2-chloro-2-fluoro-1-iodo-ethene (E)-1-bromo-2-chloro-2-fluoro-1-iodo-ethene
Page 2 of 5
CHEM 109A
CLAS
Stereochemistry Part 1 - KEY
E b/c more than 2 different groups/substituents on the double bond and
highest priority groups (based on atomic number) are opposite each other
across the double bond. [cis-trans more appropriate for compounds w/
only alkyl groups]
1
OH
CH 3
H
H
HO
H
Cl
H
H
OH
H
H
3
H
Cl
c.
can be redrawn as
4
1
1
3
Br
2
HO
d.
Cl
Br
Br
HO
4
3
(2R,3R)-3-chloro-pentan-2-ol
CH 3
3-chloro-pentan-2-ol
4
Br
1,2-dibromo-2-methyl-pentan-3-ol
H
4
1
(2S,3S)-1,2-dibromo-2-methyl-pentan-3-ol
OH
H3 C
1
OH
OH
4 H3 C
OH
OH
e.
2-methyl-pentane-1,2,5-triol
2
3
but group 4 is neither in front
OH
4
2
CH3
HO
OH
HO
3
1
or in back, so rotate so the methyl is back
Cl
f.
5-(2-chloroethyl)-2,4-dimethylnon-4-ene
(2R)-2-methyl-pentane-1,2,5-triol
Cl
(4E)-5-(2-chloroethyl)-2,4-dimethylnon-4-ene
Additional Information:
Types of Isomers – See Isomers Summary Table
Chiral vs. Achiral
Chiral means handed – chiral molecules rotate plane polarized light
Enantiomer – non-superimposable mirror images, same chemical properties (i.e. b.p.,
m.p., solubility; can NOT separate except w/ chiral chromatography) but diff. interaction
w/ plane polarized light – optically active
Rotates plane polarized light
Page 3 of 5
CHEM 109A
CLAS
Stereochemistry Part 1 - KEY
c.w. = dextrorotatory (+, d)
c.c.w. = levorotatory (-, l)
could be
R or S
Polarimeter
analyzer
Lt source
(Na D-line,
589 nm)
s
a
m
pl
e
Detector
(observed
rotation,
α)
ℓ
polarizers
specific rotation [α] – characteristic of compound (Tbl 5.1, pg 214)
[α]Tλ = α
ℓc
enantiomeric excess (ee) – how much excess of an enantiomer is in a mix
ee = α obs x 100 %
[α]
Vocabulary
Racemic mixture (a.k.a. racemate) – contains equal amounts of d and l enantiomers and
so does NOT rotate plane polarized light, use (±) to denote.
Diastereoisomer – stereoisomers that are NOT enantiomers, diff. physical and chemical
properties (can separate).
Meso compound – compound w/ 2+ asymm centers & plane of symmetry (NO
enantiomers), is achiral.
If a compound w/ 2 asymm centers has SAME 4 groups bonded to each, 1
stereoisomer (R,S or S,R) will be meso compound.
If a compound w/ 2 asymm centers is cyclic, the cis-isomer will be meso
compound.
Asymmetric center/chiral center – C bound to 4 diff atoms/groups
Stereocenter/stereogenic center – asymmetric center or sp2, sp3 Cs where cis-trans
occurs
R, S naming system (for naming enantiomers)
R – rotates right, clock-wise S – rotates left, counter-clock-wise
If more than 1 asymm center, determine R or S for each center.
1. Rank atoms/groups attached to asymm. center according to priority
1 (highest priority) to 4 (lowest priority)
a. inc. Z (atomic #) inc priority
b. if Z same (i.e. 2 Cs), look @ Zs of next atoms
Page 4 of 5
CHEM 109A
CLAS
Stereochemistry Part 1 - KEY
c. if atom double bonded, treat as if bound to 2 of those atoms; if triple bonded,
treat as if bound to 3 of those atoms
d. if atom has isotopes, use A (mass #) to determine relative priority
2. Orient molec so that 4 (group with lowest priority) is away from you (vertical bond in
Fischer Proj.) and draw arrow from 1 to 2.
If c.w./right then R
If c.c.w./left then S
EX.
3
H
HO
2 HO
Cl
H
4
Cl
Cl
1
1-chloroethanol
H
HO
(1R)-1-chloroethanol
3
2
OH
OH
(3R)-pent-1-en-3-ol
1
pent-1-en-3-ol
would be R if 4 was back, so...
H
OH
(3R)-pent-1-en-3-ol
N and P as asymmetric centers
Amine inversion – if lone pair on N, rapid interconversion of enantiomers (sp3 to
2
sp and back to sp3) @ RT and so is achiral. Phosphorous behaves similarly to nitrogen.
(Sulfur is different b/c it has access to d orbitals to make bonds)
N
R
R
R
R
N
R
R
R
N
R
R
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