Download cis- trans - KSU Web Home

Chapter 12 Alkenes, Alkynes, and
Aromatic Compounds
12.2
Cis–Trans Isomers
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
1
Cis and Trans Isomers
In an alkene, cis and trans isomers
are possible because the double
bond
 is rigid
 cannot rotate
 has groups attached to the
carbons of the double bond that
are fixed relative to each other
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
2
Cis–Trans Isomers
Cis–trans isomers can be
modeled by
 making a “double bond”
with your fingers with
both thumbs on the
same side (cis isomer)
 or fingers with thumbs on
opposite sides from each
other (trans isomer)
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
3
Cis–Trans Isomers (continued)
Cis–trans isomers occur when
different groups are attached to
the double bond.
 In a cis isomer, groups are
attached on the same side of
the double bond.
 In the trans isomer, the
groups are attached on
opposite sides of the double
bond.
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
4
Naming Cis–Trans Isomers
 The prefixes cis- or trans- are placed in front of the
alkene name when there are cis–trans isomers.
cistrans-
Br
Br
C C
H
H
Br
C C
H
cis-1,2-Dibromoethene
General, Organic, and Biological Chemistry
H
Br
trans-1,2-Dibromoethene
Copyright © 2010 Pearson Education, Inc.
5
Cis–Trans Isomerism
 Cis–trans isomers do not occur if a carbon atom in the
double bond is attached to identical groups.
(not cis or trans)
General, Organic, and Biological Chemistry
(not cis or trans)
Copyright © 2010 Pearson Education, Inc.
6
Pheromones
A pheromone
 is a chemical
messenger emitted by
insects in tiny
quantities
 called bombykol is
emitted by the silkworm
moth to attract other
moths and has one cis
and one trans double
bond
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
7
Learning Check
Name each, using cis–trans prefixes when needed.
Br
Br
C C
A.
H
H
H
CH3
C C
B.
H
CH3
Cl
CH3
C.
C C
H
Cl
General, Organic, and Biological Chemistry
Copyright © 2010 Pearson Education, Inc.
8
Solution
Br
Br
cis-1,2-Dibromoethene
C C
A.
H
H
H
CH3
trans-2-Butene
C C
B.
H
CH3
Cl
CH3
C.
C C
H
Cl
General, Organic, and Biological Chemistry
1,1-Dichloropropene
Identical atoms; no cis or
trans
Copyright © 2010 Pearson Education, Inc.
9