Monday, January 28, 2008

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

CH3HC=CHCH3 exhibits geometrical isomerism.

As CH3 and H are different groups

Maleic acid and Fumaric acid are geometrical isomers.

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

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

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