Isomers which posses the same molecular and structural formula but differ in arrangement of atoms or groups in space around the double bonds, are known as geometrical isomers and the phenomenon is known as geometrical isomerism. Geometrical isomerism are shown by the compounds having the structure.


(a) Cis – trans isomerism : When similar groups are on the same side it is cis and if same groups are on the opposite side it is trans isomerism. See fig.1.

(b) Syn - anti isomerism : See fig.2.

Syn anti isomerism is not possible in ketoxime since only one from is possible two `-CH_3` groups are at one `C`. See fig.3.

(c) In cyclic compounds : See fig.4.

Dipole moment : Usually dipole moment of cis is larger than the trans-isomer.

Melting point : The steric repulsion of the group (same) makes the cis isomer less stable than the trans isomers hence trans form has higher melting point than cis.

Different chemical properties : Syn-addition makes cis forms into meso and trans into `d` and `l`, anti addition makes cis into `d` and `l` and trans into meso. See fig.