5.1: Cis-Trans Isomers Result from Restricted Rotation
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Several organic compounds may have identical compositions but will have widely different physical and chemical properties because the arrangement of the atoms is different. Isomers and identical compounds both have the same number of each kind of element in a formula. A simple count will establish this fact.
As a consequence of the double bond, some alkene compounds exhibit a unique type of isomerism. Rotation around a single bond occurs readily, while rotation around a double bond is restricted. The pi bond prevents rotation because of the electron overlap both above and below the plane of the atoms.
A single bond is analogous to two boards nailed together with one nail. A double bond is analogous to two boards nailed together with two nails. In the first case you can twist the boards, while in the second case you cannot twist them.
Geometric Isomers are compounds with different spatial arrangements of groups attached to the carbons of a double bond. In alkenes, the carbon-carbon double bond is rigidly fixed. Even though the attachment of atoms is the same, the geometry (the way the atoms "see" each other) is different.
When looking for geometric isomers, a guiding principle is that there MUST BE TWO DIFFERENT "GROUPS" ON EACH CARBON OF THE DOUBLE BOND. A "group" can be hydrogen, alkyls, halogens, etc.
Identical compounds may appear to have different arrangements as written, but closer examination by rotation or turning will result in the molecules being superimposed. If they are super impossible or if they have identical names, then the two compounds are in fact identical.
Isomers of compounds have a different arrangement of the atoms. Isomer compounds will differ from identical compounds by the arrangement of the atoms. See example below.
Both compounds have the same number of atoms, C5H12. They are isomers because in the left molecule the root is 4 carbons with one branch. In the right molecule, the root is 3 carbons with 2 branches. They are isomers because they have the same number of atoms but different arrangements of those atoms.
Completely different compounds: If the number of each element is different, the two compounds are merely completely different. A simple count of the atoms will reveal them as different.
In the example on the left, the chlorine atoms can be opposite or across from each other in which case it is called the "trans" isomer. If the the chlorine atoms are next to or adjacent each other, the isomer is called " cis".
If one carbon of the double bond has two identical groups such as 2 H's or 2 Cl's or 2 CH3 etc. there cannot be any geometric isomers.
Consider the longest chain containing the double bond: If two groups (attached to the carbons of the double bond) are on the same side of the double bond, the isomer is a cis alkene. If the two groups lie on opposite sides of the double bond, the isomer is a trans alkene. One or more of the "groups" may or may not be part of the longest chain. In the case on the left, the "group" is a methyl - but is actually part of the longest chain.
A common mistake is to name this compound as 1,2-dimethylethene. Look at all carbons for the longest continuous chain - the root is 4 carbons - butene.
For the structures below:
a. Draw both cis/trans isomers, if any, of the structure based upon the name.
b. Look at the graphic and state whether the compound is cis, trans, or not cis/trans isomers.
- Charles Ophardt, Professor Emeritus, Elmhurst College; Virtual Chembook