Geometric ISOMERS of Alkenes
Chapter 12 Section 3
ALKENES
 The CARBON – CARBON double bond is rigid (no free
rotation) because of the shapes of the orbitals involved in its
formation
Pi bond
2 p-orbital electrons
lie above and below
the 2 carbons
Sigma bond
the electrons lie
in a line between
the 2 carbon nuclei
Rotation around the double bond is restricted
because the pi bond would have to be broken to
allow for rotation. THUS, THE DOUBLE BOND IS
RIGID
Geometric Isomers
In cycloalkanes, cis/trans applied to the
branches
trans = on OPPOSITE sides of the
ring
cis = on the SAME side of the ring
Geometric Isomers
In alkenes, cis/trans applies to the
location of the parent chain
around the double bond, NOT the
branches
Geometric Isomers
If one of the carbons of the
double bond has 2 identical
substituents there are NO
cis/trans isomers.
Example:
1,1-dichloroethene
no cis/trans because both
carbons have identical
substituents
1,2-dichloroethene
cis/trans isomers
DO exist in this
molecule
Which exist as geometric isomers?
1-pentene
3-ethyl-3-hexene
3-methyl-2-pentene
Draw these molecules.
Cis-3-octene
 Trans-5-chloro-2-hexene
 Trans-2,3-dichloro-2-butene
HOMEWORK
 3RD EDITION: P351 #19, 23, 24, 26
 4TH EDITION: P359 #21, 25, 26, 28