1. Do Now What is the definition of a structural isomer?
What are the three structural isomers you know?
Give an example for each

2. Isomers Alkenes with more than 3C can form different types of isomer
All end in –ene
Chain isomers
Position isomers
Geometrical isomers

3. 1. Chain Isomers (Branching Isomers)
The hydrocarbon chain is arranged differently:
pent-1-ene and 2-methylbut-1-ene
EXT 1: Draw hex-1-ene and 3-methylpent-1-ene
EXT2: Draw and name your own alkene chain isomers

4. 2. Position Isomers
Isomers with the double bond in a different position:
but-2-ene and but-1-ene
EXT 1: Draw hex-3-ene and hex-2-ene
EXT2: Draw and name your own alkene position isomers
The longer the chain, the more possibilities there will be, so greater number of isomers

5. p-orbitals of unbonded C atoms containing one electron each.
C 1s2 2s2 2p2
Bonding in Ethene ↑↓ ↑↑
Pi bond prevents free rotation about the C-C bond
2C and 4H all in same plane
sigma (σ) bonds
p-orbitals overlap, above and below the CH plane to form Pi (∏) bond
C-C sigma bond +
C-C Pi bond =
C=C double bond

6. 3. Geometric (E-Z) Isomerism
E-Z isomerism tells us about the position of substituents at either side of a carbon-carbon double bond.
Two substituents may either be on the same side of the bond (Z) or on opposite sides (E).
Substituents groups joined by a single bond can rotate around the single bond, so there are no isomers but there is no rotation around a double bond.
Z- and E- isomers are separate compounds and are not easily converted from one to the other.

7. Z-but-2-ene and E- but-2-ene
Geometrical Isomers
Form of stereoisomerism:
Same structural formula but the bonds are arranged differently in space
Occurs ONLY around the C=C double bond
Z-but-2-ene and E- but-2-ene
EXT 1: Draw Z-hex-3-ene and E-hex-3-ene
EXT2: Draw and name your own alkene geometrical isomers
Often called E-Z isoomerism
Cis and trans in old language!

8. Why don’t we use cis and trans?
Cis (Z): cis-but-2-ene and Z-but-2-ene
Trans (E): trans-but-2-ene and E-but-2-ene
Cis-trans DOES NOT WORK when more than 2 different substituents around a double bond ...
E-Z notation based on atomic numbers:
Look at the atoms attached to each of the carbons in the double bond
When to 2 atoms of each pair of HIGHER atomic number are on the SAME side of the C=C ... Z isomer
When to 2 atoms of each pair of HIGHER atomic number are on the SAME side of the C=C ... E isomer
Give examples p 200 of textbook

9. Examples of Geometric Isomerism

10. More examples of Geometric Isomerism

11. Are these geometric isomers?

12. 1-bromo-2-chloro-1-fluoroethene

13. Task
Draw the following compounds and decide whether they show E-Z isomerism:
methylpropene
1-chloropropene
2-methylpent-2-ene
2-ethylpent-1-ene
2-bromo-3-chlorobut-2-ene

14. NO
methylpropene
YES E Z
1-chloropropene

15. NO
2-methylpent-2-ene NO
2-ethylpent-1-ene

16. YES Z E
2-bromo-3-chlorobut-2-ene