Isomerism in Double-Bonded Molecules

These features of a double bond can then be summarized in tbe following statements  

A double bond between two atoms creates a planar molecule. 

Rotation around the double bond by 90° breaks one of tbe bonds. Hence, the rotation around the double bond is not free. 

SCHEME 7.6 The two CH2 groups of H2C=CH2 are shown with their fourth connectivities oriented in the red plane that bisects the two H—C—H angles. In this orientation, these connectivities can click to form an additional bond between the C atoms, which forms H2C=CH2, with a double bond overall between the C atoms. However, twisting the CH2 groups to mutually perpendicular orientations breaks one of the components of the double bond and results in a single C—C bond (the two unpaired electrons on the C atoms in the two H—C—H units are no longer in the same plane, as shown by the two red patches). 

The interesting isomers are the ones that arise by replacement of H s on the different carbon atoms. One isomer is called cis ( on the same side, in Latin), where the two CTs are on the same side of the double bond. The other is called trans ( beyond one another, in Latin), where the two CTs are on opposite sides of the double bond. Since there is no free rotation around the double bond, cis and trans isomers coexist and are stable molecules having different properties (refer to Section 7.1 about van t Hoff and the isomerism of maleic and fumaric acids). 

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