1. • GEOMETRICAL ISOMERISM • OPTICAL ISOMERISM
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

2. (Z) but-2-ene Or Cis-but-2-ene (E) but-2-ene Or Trans-but-2-ene
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
TRANS (E)
Groups/atoms are on OPPOSITE SIDES across the double bond
(Z) but-2-ene
Or Cis-but-2-ene
(E) but-2-ene
Or Trans-but-2-ene
E entgegen (opposite)
Z zusammen (together)

3. 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

4. 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.

5. 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)
Z 2-bromo-3-fluorobut-2-ene
E 3-methylpent-2-ene

6. Rule For Naming with E and Z
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.

7. 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.
All names should remain as shown.

8. GEOMETRICAL ISOMERISM IN ALKENES
Answer
Priority
1 I
2 Br
3 Cl
4 H
I and Br have the higher atomic numbers
So I and Br have highest priority to determine POSITION.
Then just go alphabetically
E isomer
E- 1-bromo-2-chloro - 2-iodoethene
All formuale are correct
Z isomer
Z- 1-bromo -2-chloro-2-iodoethene

9. GEOMETRICAL ISOMERISM IN ALKENES
trans but-2-ene
(E) but-2-ene
cis but-2-ene
(Z) but-2-ene
ATOMS OF C=C MUST HAVE
TWO DIFFERENT GROUPS

10. TASK : Is this a Geometric Isomer?NO
methylpropene
YES E Z
1-chloropropene

11. TASK : Is this a Geometric Isomer?NO
2-methylpent-2-ene NO
2-ethylpent-1-ene

12. 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

13.     GEOMETRICAL ISOMERISM IN ALKENES
How to tell if it exists
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
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