Cis-Trans Isomer Mr.Wang

1. Cis-Trans IsomerMr.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. (http://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism) 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 CH3CH2 CH2CH3 Example 1 C C H H • Count how many carbons (C) the parent • Chain (the longest carbon chain) has. • ---- 6 “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 (-) CH3CH2 CH2CH3 C C H H

8. CH3CH2 CH2CH3 C C • 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 H H

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! http://www.chemguide.co.uk/basicorg/isomerism/geometric.html

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! http://www.chemguide.co.uk/basicorg/isomerism/geometric.html

15. Why single bonds can’t make isomers A simple test! 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. http://www.chemguide.co.uk/basicorg/isomerism/geometric.html

17. Boiling Point • Why is the boiling point of cis isomers higher? ---Stronger intermolecular forces between the molecules of the cis isomers Why? The difference between the two is that the cis isomer is a polar molecule whereas the trans isomer is non-polar.  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.

18. Melting Point • 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 http://wps.pearsoncustom.com/pcp_timberlake_chemistry_9_1491/36/9303/2381590.cw/index.html

29. 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: http://www.chemguide.co.uk/basicorg/isomerism/geometric.html • http://en.wikipedia.org/wiki/Cis%E2%80%93trans_isomerism • Picture: • http://cn.bing.com/images/search?q=cis-trans+isomer&FORM=HDRSC2#view=detail&id=24052F661AC01F88053EB8839F851B900012F2B6&selectedIndex=20 • http://www.chemguide.co.uk/basicorg/isomerism/geometric.html