Geometrical or cis-trans isomerism and properties of geometrical isomerism.

What is geometrical isomerism?

Many compounds that contain one double bond exist in two forms which differ in most of their physical and in many of their chemical properties. 

There is no free rotation about a double bond for such type of compounds due to maximum overlap of the two pi orbitals which cause resistance to the rotation about the double bond. 

But two spatial arrangements are possible for these type of molecule. They are called geometrical or cis-trans isomers and this phenomenon is called geometrical isomerism.

                                 

      Geometrical isomers of cyclopropane-1,2-dicarboxylic acid

So, we can define, geometrical isomerism is a form of stereo- isomerism which arises due to spatial arrangement of two groups which are attached with two double bonded carbon atom in olefinic compounds.

Geometrical  isomerism is also called cis-trans or E-Z isomerism. Geometrical  isomerism is characterized by the existence of different configurations for a given structure which may or may not be optically active.

Geometrical isomerism is also possible in cyclic compounds, the ring structure being comparable to the double bond in olefinic compounds.

For example, geometrical isomerism for cyclohexane system, e.g, hexahydro terephthalic acid.

Geometrical isomerism for cyclobutane and cyclopentane systems are as follows,

Properties of geometrical  or cis-trans isomers.

Comparison of the properties of cis and trans-isomers of known configurations shows certain regularities such as, the melting and the stability of the cis-are lower than those of the trans-isomers.

Again, the density, refractive index, solubility, dipole moment, heat of combustion and the dissociation constant of the cis-are greater than those of the trans-isomer.

Why  the dipole moment of cis-2-butene is greater than trans -2-butene?

In trans-2-butene, the two methyl groups are  present  in opposite direction.So the group moment of two methyl groups are cancelled to each other. 

Hence, the dipole moment of trans-2-butene is ultimately zero. But in cis-2-butene, the two methyl groups are present in the same side. 

So the dipole moment of cis-2-butene is the summation of group moment of two methyl group.

Consequently, the dipole moment of cis-2-butene is greater than trans -2-butene.

Why trans-2-butene is more stable than cis-2-butene ?

The stability of cis and trans 2-butene may be explained in terms of steric effects. 

In the but-2-ene, the two methyl groups in the cis-isomer, being closer together than in trans isomer, experience greater steric repulsion.
consequently the cis-form is under greater strain than the trans. 

Thus steric repulsion destabilizes a molecule. Hence,  trans -2-butene is more stable than cis-2-butene.

There are few carbon-nitrogen compounds containing rigid C=N double bond such as oximes, semicarbazides, hydrazone etc exhibit geometrical or cis-trans isomerism like C=C double bonded compound. 

Again, there are some others organic compounds containing rigid N=N double bond such as azo-compounds, diazo-compounds etc exhibit geometrical or cis-trans isomerism like C=C double bonded compound.
For example, benzaldoxime (C=N) and azo-benzene( N=N) etc.

In oxime chemistry, the terms cis and trans are replaced by syn and anti respectively . 

In azo-compounds, the terms cis and trans are replaced by Z and E respectively.  

Summary




What is geometrical or cis-trans isomerism ?
Why trans-2-butene is more stable than cis-2-butene ?
Why dipole moment of trans-2-butene is zero ?

plane of symmetry and simple or rotational axis of symmetry