Assigning Stereochemistry VI

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© S. Lievens,
University of California, Davis
March 2012
For use in UCDavis Chemistry 8/118 Series
Assigning Stereochemistry VI


E and Z in Alkenes
Alkenes can have multiple geometric isomers (non-superimposable, nonmirror images)
If there are exactly two substituents and two hydrogens attached an
alkene the isomer may be labeled as cis- or trans-.
H
H
a trans alkene
H opposite
R'
R
vs.
H
R
R'
H
a cis alkene
H on same side

If there are two or more substituents attached to an alkene the isomer may
be labeled as E or Z
o All cis- molecules are Z, but not all Z molecules are cis-; all transmolecules are E, but not all E molecules are trans-. Cis- and transare accepted when appropriate, but E and Z work for all
asymmetrical alkenes.

Assigning Relative Configuration: E vs. Z
o For each sp2 carbon in the alkene identify the two substituents and
prioritize them 1 and 2 using the Cahn-Ingold-Prelog Rules.
 See Assigning Stereochemistry II
o If both high priority groups are on the same side of the alkene the
molecule is Z. (similar to a cis)
 This can be remembered by Z is on the Zame Zide.
o If the high priority groups are on opposite sides of the alkene the
molecule is E. (similar to trans)
o If the there is no difference between the groups then the molecule
is symmetrical and thus E/Z is unnecessary. (e.g. monosubstituted
alkenes)
an E alkene
1st priority opposite 2 Y
1
1
W
W
Z 2
vs.
2
X1
Y
X
Z 2
21
1
a Z alkene
1st priority on the same side
© S. Lievens,
University of California, Davis
March 2012
For use in UCDavis Chemistry 8/118 Series
Examples:
Compound A:
Br
assign priority
Br
H
2
2
O
1
1
High priority
H
H
(3Z)-4-bromo-3-pentenal
on the same side
O
Compound B:
2
assign priority
1
High priority
1
2
(3E)-3-methyl-3-hexene
on opposite sides
H
Compound C:
2
assign priority
Cl
High priority
(3E)-1-chloro-4-ethyl-5-methyl-3-hexene
Cl
on opposite sides
1
1
2
H
Compound D:
O
2H
assign priority
O
OH
High priority
2
1
OH
Br
Br 1
(2Z)-2-bromo-3-butenoic acid
on the same side
Compound E:
H B
1
assign priority
1
2
H
(2Z,4E)-2,4-heptadiene
or
cis,trans-2,4-heptadiene
A
A
2
H
H B
22
© S. Lievens,
University of California, Davis
March 2012
For use in UCDavis Chemistry 8/118 Series
Practice Examples:
Assign E or Z to each molecule.
Br
OCH3
OH
Cl
Br
Cl
A
CHO
B
COOH
Br
C
D
Br
SCH3CH2NH2
CN
Br
F
E
G
H
CN
Cl
HOOC
I
OH
OCH3
L
K
J
CN
CHO
N
M
O
P
OH
O
Br
OHC
Br
Cl
Cl
Cl
R
Q
CBr3
Cl
I
Cl
U
T
S
W
V
23
X
© S. Lievens,
University of California, Davis
March 2012
For use in UCDavis Chemistry 8/118 Series
Practice Examples Key:
Br
(Z)
OCH3
OH
(E)
(E)
Cl
(Z)
Br
Cl
A
CHO
B
COOH
(E)
Br
C
D
Br
SCH3CH2NH2
(E)
CN
Br
F
E
(Z)
(Z)
G
H
CN
(E)
(Z)
(Z)
Cl
HOOC
(E)
I
OH
OCH3
L
K
J
CN
(E)
CHO
(Z)
(Z)
(E)
N
(E)
M
O
OH
O
Br
(E)
OHC
P
Br
(Z)
Cl
Cl
Cl
(E)
(E)
R
Q
T
S
CBr3
Cl
(E)
(E)
(E)
Cl
U
W
V
24
I
(Z)
X
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