geometric isomermedical dictionary

<chemistry> Geometric or also called cis-trans isomers are stereoisomers in molecules with restricted rotation about a bond.

Cycloalkanes and alkenes form cis-trans isomers due to the restriction of rotation about the double bond or due to the restriction in a ring. In order for an alkene to freely rotate, the pi bond must be broken.

This process has a high activation energy and does not occur at room temperature. Cis isomers have the two substituents on each of the carbons of the double bond on the same side, whereas in the trans isomer they are on opposite sides. The expression cis and trans only applies to alkenes or cycloalkanes if one of the substituents on each of the carbons are the same. If there are three or four different substituents, E,Z or R,S nomenclature must be used.

(09 Jan 1998)