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

CCl H

CCl H

CisTrans

How to name cis- isomersCH3 CH2

C C

CH2 CH3

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”

Example 1

• 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

• 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 2Trans-4-decene Draw actual shape

How to name trans- isomers

Example 2Trans-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

1,2-dichloroethane

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

Through free rotation, it is easy to get from one to another just by twisting around the carbon-carbon single bond.

However

In a double bond!

Why single bonds can’t make isomers

In a double bond!

1,2-dichloroethene

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

The carbon-carbon double bond won’t rotate. To get from the left to the right, you have to take the models to pieces.

So!

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!

Why single bonds can’t make isomers

Melting Point/Boiling Point

the trans isomer has the higher melting point;

the cis isomer has the higher boiling point.

http://www.chemguide.co.uk/basicorg/isomerism/geometric.htm

l

1,2-dichloroethene

but-2-ene

Boiling Point• Why is the boiling point of cis isomers higher?

---Stronger intermolecular forces between the molecules of the cis isomers

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.

Why?

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.

The "U" shape of the cis isomer doesn't pack as

well as the straighter shape of the trans isomer.

Why?

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. YesB. 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

HintsAnswers

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

1 more bonus!

Can they form cis-trans isomers?1. 3-ethyl-3-hexene2. 2,5-dimethyloctane

A. Both can form cis-trans isomersB. Neither can form cis-trans isomersC. 1 can but 2 can’tD. 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