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Nomenclature and Isomerism

In IUPAC system the name of alkene is derived by replacing the suffix -ane of the corresponding alkane by -ene.

In common system, the name of alkene is derived by replacing the suffix -ane of the corresponding alkane by -ylene.

ISOMERISM IN ALKENES

The simplest acyclic alkene which can exhibit isomerism is butene, C4H8. The possible isomers of C4H8 are:

The three isomers shown above have different arrangement of atoms and hence are structural isomers. But-1-ene and but-2-ene are position isomers whereas But- 1-ene and 2-methylpropene are chain isomers. However, experimentally it is found that there are four isomers, having different physical properties. Careful study of the above isomers reveals that for but-2-ene, two isomers having different orientation of groups about doubly bonded carbon atoms are possible as shown in Fig. 41.2.

These two isomers are stereoisomers.

 

The stereoisomerism exhibited by alkenes due to the difference in the spatial arrangement of groups about the double bonded carbon atoms is called geometrical isomerism.

Structures I and II are not interconvertible because rotation about carbon-carbon double bond is hindered. Therefore, they represent two different isomers of but-2-ene. Such isomers are called geometrical isomers. The geometric isomers which have similar groups on same side are called cis (structure I) whereas the geometric isomers which have similar groups on opposite sides are called trans (structure II).

 

Conditions for Geometric Isomerism in Open Chain Compounds

l. There should be a double bond in the molecule.

2. The two atoms or groups attached to each doubly bonded carbon atom should beĀ· different. Thus, alkenes of the type abC = Cab or abC = Ccd exhibit geometric or cis-trans isomerism. If one of the two doubly bonded carbon atoms carries two identical groups then the molecule does not exhibit geometric isomerism. Thus, compounds of the type aaC = Cab or aaC = Cbb do not show geometric isomerism.