STEREOISOMERISM
Molecules have the SAME MOLECULAR FORMULA but the atoms are
joined to each other in a DIFFERENT SPACIAL ARRANGEMENT - they
occupy a different position in 3-dimensional space.
There are two types...
• GEOMETRICAL ISOMERISM
• OPTICAL ISOMERISM
GEOMETRICAL ISOMERISM IN ALKENES
Introduction
GEOMETRIC ISOMERS
•occurs due to the RESTRICTED ROTATION OF C=C bonds
• get two forms...
CIS (Z)
Groups/atoms are on the
SAME SIDE across the double bond
(E) but-2-ene
Or Trans-but-2-ene
E entgegen (opposite)
TRANS (E)
Groups/atoms are on OPPOSITE
SIDES across the double bond
(Z) but-2-ene
Or Cis-but-2-ene
Z zusammen (together)
GEOMETRICAL ISOMERISM IN ALKENES
RESTRICTED ROTATION OF C=C BONDS
Single covalent bonds can easily rotate. What appears to be a different structure is
REALLY the same.
ALL THESE STRUCTURES ARE THE SAME BECAUSE C-C BONDS HAVE ‘FREE’ ROTATION
GEOMETRICAL ISOMERISM IN ALKENES
RESTRICTED ROTATION OF C=C BONDS
C=C bonds have restricted rotation so the groups on either end of the bond are
‘frozen’ in one position; it isn’t easy to flip between the two.
Animation doesn’t
work in old
versions of
Powerpoint
This produces two possibilities. The two structures cannot interchange easily
so the atoms in the two molecules occupy different positions in space.
GEOMETRICAL ISOMERISM IN ALKENES
Cahn-Ingold-Prelog (CIG) Priority Rules
PRIORITY: H < CH3 < CH2CH3
Look at HIGHEST atomic numbers of atoms attached to C=C
Highest values opposite = E (like trans)
Highest values together = Z “zame zide” (like cis)
9
6
35
6
Z 2-bromo-3-fluorobut-2-ene
1
6
6
6
6 1
E 3-methylpent-2-ene
GEOMETRICAL ISOMERISM IN ALKENES
Rule
For Naming with E and Z
Compare the atomic number (Z) of the atoms directly
attached to the stereocentre (double bond); the group
having the atom of higher atomic number receives
higher priority.
GEOMETRICAL ISOMERISM IN ALKENES
Next
Let us Try
2 stereoisomers of:
1-bromo - 2-chloro - 2-iodoethene
and draw the structures of them, then name them.
GEOMETRICAL ISOMERISM IN ALKENES
Answer
•
•
•
•
•
Priority
1
I
2
Br
3
Cl
4
H
E isomer
E- 1-bromo-2-chloro - 2-iodoethene
I and Br have the higher atomic
numbers
So I and Br have highest priority to
determine POSITION.
Then just go alphabetically
Z isomer
Z- 1-bromo -2-chloro-2-iodoethene
GEOMETRICAL ISOMERISM IN ALKENES
CH3
H
C
CH3
C
H
CH3
C
CH3
C
H
H
trans but-2-ene
cis but-2-ene
(E) but-2-ene
(Z) but-2-ene
ATOMS OF C=C MUST HAVE
TWO DIFFERENT GROUPS
TASK : Is this a Geometric Isomer?
CH3
H
C
NO
C
CH3
H
methylpropene
H
H
C
CH3
H
C
C
Z
Cl
Cl
1-chloropropene
CH3
YES
C
E
H
TASK : Is this a Geometric Isomer?
CH2
CH3
C
CH3
NO
C
H
CH3
2-methylpent-2-ene
H
CH2
C
H
CH2
CH3
NO
C
CH2
2-ethylpent-1-ene
CH3
GEOMETRICAL ISOMERISM IN ALKENES
Isomerism in butene
Here are some structural isomers of C4H8 that are alkenes.
Of these which exhibits geometrical isomerism.
but-1-ene
cis but-2-ene
(Z) but-2-ene
trans but-2-ene
(E) but-2-ene
2-methylpropene
GEOMETRICAL ISOMERISM IN ALKENES
How to tell if it exists
Two different
groups
attached
Two different
groups
attached
Two similar
groups
attached to
same end
Two similar
groups
attached to
same end

Two similar
groups
attached to
Same end
Two different
groups
attached

Two different
groups
attached
Two different
groups
attached


GEOMETRICAL ISOMERISM
Once you get two
similar
atoms/groups
attached to one end
of a C=C, you
cannot have
geometrical
isomerism
GEOMETRICAL ISOMERISM