Revision Alkenes Isomerism

Isomerism in alkenes

1. Structural isomerism: Alkenes show chain isomerism and position isomerism.

a. Chain isomerism: C-4H-8 exists as two chain isomers, n-Butene and Isobutene.
b. Position isomerism: The isomers differ in the position of the double bonds. C-4H-8 can have the double bond as the terminal bond or it can be in the middle. The one with the terminal bond is n-Butene and the one with the double bond in the middle is but-2-ene. Thus butane with the structural formula C-4H-8 has three structural isomers.

2. Geometrical isomerism: Alkenes exhibit geometrical isomerism.

Molecules of the type C₂A₂B₂ are available as geometrical isomers. The two atoms attached to the same carbon atom are different. In this symbols, A and B are different.

The molecule in which similar atons or groups lie on the same side of the double bond is called cis-isomer. Both As are attached to the carbon on one side of the double bond. Simiarly two Bs.


If they are attached on the opposite sides, it is a trans-isomer

Examples
CH₃HC=CHCH₃ exhibits geometrical isomerism.

As CH₃ and H are different groups

Maleic acid and Fumaric acid are geometrical isomers.
HOOCHC=CHCOOH

Cis isomer is maleic acid and trans isomer is fumaric acid.

If they are attached on the opposite sides, it is a trans-isomer