1. Cis-Trans Isomer Mr.Wang

2. Introduction and goal Determine the cis-trans isomers
Name the cis-trans isomers
Different isomers have physical properties in molecular state

3. What is Cis-Trans Isomer?
In organic chemistry, cis/trans isomerism (also known as geometric isomerism) is a form of stereoisomerism describing the relative orientation of functional groups within a molecule. (
Confused? Let me explain in a easier way.

4. Cis-Trans Isomer
Compounds that have same chemical formula and bond but a different atom arrangement.

5. Cis-Trans Isomer Cl H C
Cis
Trans C Cl H

6. How to name cis- isomers
CH CH2
C C
CH CH3
H H
Example 1
Count how many carbons (C) the parent
Chain (the longest carbon chain) has.
“C”s  begin with “hex”
See if it is double bond or triple bond.
---- double bond  end with “ene”

7. Determine first carbon who has the double bond (or triple bond), write the number of the carbon in front of the parent chain. Separate by dash (-)
CH CH2
C C
CH CH3
H H

8. CH CH2
C C
CH CH3
H H
Look at 2 “H”s are on which side
---- same side  cis
Put the “cis-” in front of the parent chain.
put the branch in front of everything!
the answer is cis-3-hexene

9. How to name trans- isomers
Example 2
Trans-4-decene
Draw actual shape
Same procedure as naming example 1
“dec”  10 carbons
“trans”  opposite sides of “H”
“ene”  double bond
“4”  the fourth carbon that has double bond.

10. How to name trans- isomers
Example 2
Trans-4-decene
Draw actual shape

11. Exercise 4-methyl-cis-2-pentene Draw the actual shape of this
molecule using condensed structure

12. Extra Information
Ms.Zhou

13. Why single bonds can’t make isomers
Through free rotation, it is easy to get from one to another just by twisting around the carbon-carbon single bond.
However
1,2-dichloroethane
In a double bond!

14. Why single bonds can’t make isomers
In a double bond!
The carbon-carbon double bond won’t rotate. To get from the left to the right, you have to take the models to pieces.
1,2-dichloroethene
So!

15. A simple test! Why single bonds can’t make isomers
If you have to take a model to pieces to convert it into another one, then you've got isomers. If you merely have to twist it a bit, then you haven't!

16. Melting Point/Boiling Point
1,2-dichloroethene
the trans isomer has the higher melting point;
but-2-ene
the cis isomer has the higher boiling point.

17. Why is the boiling point of cis isomers higher?
---Stronger intermolecular forces between the molecules of the cis isomers
Why?
 In the cis isomer bonds are both on the same side of the molecule, which means that one side of the molecule will have a slight negative charge while the other is slightly positive. The molecule is therefore polar.
The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.

18. Melting Point Why? Why is the melting point of cis isomers lower?
the intermolecular forces aren't as effective as they
should be and so less energy is needed to melt the molecule.
Why?
The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.

19. Question time!
Ms.Liu

20. Questions time!!
If you are the first one who answers the questions correctly. You will get a candy
After I say: “1,2,3,go!” Raise your hand to answer the question
No incorrect answer penalty
The correct bonus answers will bring you 2 candies

21. Level 1Round1: basic understanding
Q: Are they cis-trans isomers?
A. Yes
B. No
NO!!

22. Explanation
There must be two different groups on the left-hand end and two different groups on the right-hand one.
You won't have geometric isomers if there are two same groups on one end of the bond

23. Further exploration Do atoms at the end of bond have to be the same?
NO!
And a more complicated one

24. Level 1Round 2 Cis- vs. Trans-Isomer
Answers
Hints

25. Level 2 Round1: Further Understanding
Q: Why is the melting point of the cis-isomers lower?
Hint: It has something to do with its structure

26. Explanation
The "U" shape of the cis isomer doesn't pack as well as the straighter shape of the trans isomer.
Cis-isomers don’t pack as efficiently as the trans-isomers in the solids.
The intermolecular forces in the cis-isomers don’t work well
It takes less energy to melt the cis-isomers
Lower melting point

27. Another way to explain this question
Polar vs. non-polar
Different intermolecular forces

28. Level 2 Final Round! & Bonus

29. B 1 more bonus! Can they form cis-trans isomers? 3-ethyl-3-hexene
2,5-dimethyloctane
Both can form cis-trans isomers
Neither can form cis-trans isomers
1 can but 2 can’t
1 can’t but 2 can B

30. Skinny summary& key points
Cis-trans isomers have the same chemical formula but different arrangements
cis- means atoms on the same side
trans- means atoms on the opposite side
Different isomers have different chemical properties and physical properties (ie. melting point & boiling point) due to different chemical structures, intermolecular forces
name cis-trans isomers: basic naming rules of organic molecules

31. Bibliography
Questions:
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