Cis-Trans Isomerism

Cis-Trans isomerism in organic chemistry is commonly termed as configurational isomerism or geometrical isomerisms. It is a type of stereoisomerism. Here the atoms comprising of isomers have different spatial arrangement even though they are made up to join in the same order. Cis-Trans isomerism is one of the forms of stereoisomerism.

Isomers are molecules that possess different arrangement in space and have the same molecular formula. Isomers are classified into Structural isomerism and Stereoisomerism. Structural isomerism is a type of isomerism where the functional group and atoms are joined together in a different way. They include functional group isomerism, chain isomerism, and position isomerism. One examples of an isomer includes Propadiene and Propyne comprising of different bond types.

Occurrence of Cis-Trans Isomerism

Cis-Trans Isomerism occurs when there is a restricted rotation within the molecule. In organic chemistry substituent groups are aligned in the same direction. The acyclic alkene, But-2-ene is one example of a substance displaying geometric isomerism.

Cis-Trans Isomerism - But-2-ene

The figure below illustrates the possible configurations of 1,2 dichloroethane. The molecules illustrated in the below diagram are not isomers. There exists a restriction in the rotation of a carbon-carbon bond. When there is no restriction in the rotation of a carbon-carbon bond then the two molecules are not the same molecules. There would be no rotation in the carbon-carbon bond.

Cis-Trans Isomerism

Physical Properties

There are 2 methods to name isomers that are Cis and Trans. They both possess different physical properties. The difference between the isomers can occur when there is a difference in a dipole moment and shape of a molecule. They can make a large difference when polar bonds are present. In cis isomer there exist an intermolecular dipole-dipole moment as 2 polar carbon – chloride bond dipole moment combine to provide total molecular dipole.

The consequence of double bonds exists in the case of Cis-Trans Isomers. The dipoles of the substituents will be deleted in case of Trans isomers. The substituents of the dipole will be added to the total molecular dipole in the case of cis isomer. They can even occur in inorganic chemistry mostly in diazines.

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Practise This Question

A Light of wavelength λ shines on a metal surface with intensity X, and the metal emits Y electrons per second of average energy Z. What will happen to Y and Z If X is doubled?