Geometric isomerism or Cis / Trans isomerism is a form of steroisomerism describing the orientation of functional groups within a molecule. In general, such isomers contain double bonds, which cannot rotate but they can also arise from ring structures where in the rotation of bonds is greatly restricted. Cis and trans isomers occurs in organic molecules and inorganic coordination complexes. The term cis and trans are from Latin, in which cis means ‘on the same side’ and trans means ‘on the opposite side’. The term geometric isomerism is considered as synonym of Cis / Trans isomerism by IUPAC.
Examples: 1,2-dichloroethene and 2-Butene
These two isomers are not same; here the C-C bond rotation is restricted due to double bond.
Rules for Geometric Isomerism Nomenclature
- There must be presence of double bond in the molecule.
- If two similar groups are bonded to the carbons of the double bond are on the same side of the bond, the alkene is the Cis-isomer.
- If similar groups are on opposite sides of the bond, the alkene is Trans-isomer.
- Not all akenes are capable of showing geometric isomerism.
- If either carbon of the double bond holds two identical groups, the molecule do not show geometric isomerism.
Physical Properties of Geometric Isomerism
Geometric isomers have different physical properties. Some of the properties are discussed below:
1. Dipole Moment
The dipole moment of cis and trans isomers are different and they can be differentiated on the basis of dipole moment.
In cis-isomer, the vector sum of the two dipole moments is directed perpendicular to double bond and there is some value of dipole moment.
In trans-isomer, the two dipole moments lead to cancel out and net dipole moment will be zero.
2. Boiling Points
Trans-isomers are less polar and more symmetrical, have lower boiling points. There is no dipole moment, due to this forces of attraction are also very weak.
Cis-isomers are generally polar and less symmetrical and show higher boiling points. There is presence of dipole moment, due to this there are intermolecular dipole-dipole forces which add to London dispersion forces and raise the boiling point. Polarity is key in determining relative boiling points as it caused increased intermolecular forces.
3. Melting Points
Trans-isomers have higher melting points than cis-isomers due to the symmetry. Trans-isomers are more symmetrical and due to this they have proper packing in solid state as compared to cis-isomers.
Trans-isomers are most stable because bulky groups are far away from each other so there is less steric hindrance. Cis-isomers are less stable due greater steric hindrance, bulky are very near to each other and there atoms electrons repel each other, so they cause instability.
Geometric isomerism in Cycloalkanes
Cycloalkanes are less flexible than open chain structures. The bond rotation is restricted in the ring structures. In this type of isomerism, cis and trans-isomers can be assigned on the basis of groups orientation. If two same groups are on wedge or dotted bond then it will be assigned as cis-isomer. If two same groups of different bonds, one on wedge and other on dotted bond then it will be assigned as trans-isomer.