CH 106 - Lesson 2
Home Up Nomenclature Structure Isomers Reactions

 

Isomers

Just as we found with alkanes, alkenes can form isomers. In fact, they can form two distinctly different kinds of isomers.

Structural Isomers

By having side groups placed at different positions on the main chain, structural isomers can be formed. These two compounds are structural isomers.
CH2=C-CH2-CH3
|   
CH3  
2-methyl-1-butene		 CH2=CH-CH-CH3
  |
    CH3
3-methyl-1-butene

Geometric Isomers

An important structural feature of double bonds is that they cannot rotate. The carbon atoms cannot rotate on a double bond like they can on a single bond. This not only fixes the carbon atoms in position with respect to one another, but also the two atoms that are bonded to each of those carbon atoms are fixed in position.

This results in a special kind of isomerism that is shown in the following diagrams. It is called cis-trans-isomerism. It is also called geometric isomerism. The focus in geometric isomerism is on how the molecule extends out from the two carbon atoms that are double-bonded.

Both of these compounds are 2-butene. In the first of these compounds, the one called cis-2-butene, both of the extensions of the carbon chain are on the same side of the double bond. They are right next to each other. In the compound called trans-2-butene, the extensions of the carbon chain are on opposite sides. They are diagonal from one another.
   CH3   CH3
\   /
C=C
/   \
H     H
cis-2-butene		    CH3  H   
\   /
C=C
/   \
   H    CH3
trans-2-butene

Geometric isomerism and the cis-trans designation can also be applied to compounds with halogen atoms and other side groups attached to the two double-bonded carbon atoms.

Both of these compounds are 1,2-dibromoethene. Each has a bromine on the first and second carbon atoms. The compound on the left is called cis-1,2-dibromoethene. In it, both of the bromine atoms are on the same side of the double bond. They are right next to each other. In the compound on the right, the one called trans, they are on opposite sides. They are diagonal from one another.

62mod09.JPG (1500 bytes)

The prefixes cis- and trans- are used in cases like this to indicate the relative positions of atoms with respect to the double bond and one another. In this course, we will only occasionally deal with cis-trans isomerism. Sometimes geometric isomerism is pointed out and emphasized, but other times is overlooked.

The condensed and skeletal structural formulas shown here (and in part a of Example 6 in your workbook) let you know that there is a double bond in the compound but make it impossible to figure out whether it has cis- or trans- geometry.

CH3CHCHCH3

CH3-CH=CH-CH3

C-C=C-C
These structural formulas (part b of Example 6) do show cis- and trans- arrangements, but they are not emphasized. The top two are cis- and the bottom one is trans-.
H     H
|     |
H-C-C=C-C-H
| | | |
H H H H
H H H H
| | | |
H-C-C=C-C-H
|     |
H     H
H   H H
|   | |
H-C-C=C-C-H
| |   |
H H   H
In these diagrams (also shown in part c of Example 6 in your workbook), the point of showing the structural formulas in this way is to emphasize the geometry of the molecules and show whether they are in the cis- or trans- arrangement. Again, cis- on top and trans- on the bottom.
H  H   H  H
\ /    \ /
C      C
 / \    / \
H   C = C   H
/    \
H     H		 H       H
\     /
H   C = C   H
\ /    \ /
C      C
/ \    / \
H  H   H  H
H  H       
\ /      
H - C      H    
\     /
C = C
/     \
   H       C - H
        / \
        H  H

When we get to biochemistry you will see that these terms also apply to the cis- and trans- fatty acids that I am sure you have heard about.

 

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Clackamas Community College
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