Cis-Trans Isomer Mr.Wang

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

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.

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

Cis-Trans Isomer Cl H C Cis Trans C Cl H

How to name cis- isomers CH3 CH2 C C CH2 CH3 H H Example 1 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”

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 (-) CH3 CH2 C C CH2 CH3 H H

CH3 CH2 C C CH2 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

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.

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

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

Extra Information Ms.Zhou

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

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

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!

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

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.

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.

Question time! Ms.Liu

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

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

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

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

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

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

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

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

Level 2 Final Round! & Bonus http://wps.pearsoncustom.com/pcp_timberlake_chemistry_9_1491/36/9303/2381590.cw/index.html

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

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

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