dipole moment bond moment group moment and Influence of dipole moment

What is dipole moment ?

When  a covalent bondis formed between two dissimilar atoms ( C –Cl ), the two electrons forming the covalent bond are no longer symmetrically disposed between the two atoms .

 Because  each atom has a differentelectronegativity that is, attractions for electrons.

Thus the electrons forming the covalent bond are displaced toward the most electro negative atom(Cl )without any separation of the nuclei.

As  a  result the most electronegative  atoms will be slight negative charged and theless electronegative atoms will be slightly positive charged . This  phenomenon is called dipole moments.

For example , methyl chloride , hydrogen chloride etc.

methyl chloride

What is bond moment ?

When two atoms of different electronegativities are bonded to each other the bond becomes dipolar. It has a dipole moment which is called bond moment.

What is group moment ?

In case of poly atomic molecule the moment of individual group is determined by summing up the bond moment of the bond present in the group which  is called group moment.

For example, the dipole moment of nitro methane, methane etc.

Unit of dipole moment: 

The C.G.S unit of dipole moment is debye or esu-cm and S.I unit is Coulomb-meter.

 

Influence of dipole moment  on  physical and chemical properties  of  molecule .

The greater the value of the dipole moment the greater is the polarity of the bond.

The greater the dipole moment of the compound higher is the melting and boiling point. 

The greater the dipole moment value means more ionic character of the molecule and hence the molecule is more soluble in polar solvent.Dipole moment value can identify the structure of a molecule. 

For example, carbon dioxide molecule have two C = O group, yet the net dipole moment value of CO2 is zero. This means that carbon dioxide molecule have linear structure.

Any molecule with zero dipole moment value also indicate that the molecule is trans isomer.

For example , cis-1,2-dichloro ethene have diploe moment value 1.86 D whereas this value of trans isomer is zero. 

Why the dipole moment value of benzene and carbon tetra chloride are zero ?

Both benzene and carbon tetra chloride molecules have symmetrical hexagonal and tetrahedral structures respectively. So their resultant dipole moments are zero.

In other words, the center of gravity of positive and negative pole for both the molecule are a point that is distance between positive and negative pole is zero.

So dipole moment of benzene and carbon tetra chloride are zero.

  Compare the dipole moment of p-xylene , p-dichloro benzene and p-chloro toluene . 

In p-xylene, the +I effect of two methyl groups are operating in opposition to each other and hence their effects are cancelled.

Similar explanation in case of p-dichloro benzene, where  - I  effect of two chlorine atoms acts in opposite direction and the effect cancelled to each other.

Hence , both  p-xylene and p-dichloro benzene have zero dipole moment .

But in case of p-chloro toluene, +I effect of methyl group and - I effect of Cl atom acting in the same direction and hence has highest dipole moment. 

  

Why p-nitro toluene have a greater dipole moment ?

p-nitro toluene has a greater dipole moment  because the +I effect of methyl group and -I effect of  nitro group acts in same direction. So the net dipole moment is the sum up of two group moment .

Compare the dipole moment of carbon tetra chloride and chloroform .

The dipole moment of carbon tetra chloride is zero because in carbon tetra chloride each pair of the four C - Cl bonds equal and oppositely directed dipole moments.

Thus , their values are cancelled . Hence, carbon tetra chloride has zero dipole moment.

On the other hand methyl chloride has appreciable dipole moment and it is 1.87 D . Two C - H bond moments cancel to each other but one C - H  and one C - Cl bond dipole moments are added up.

Compare the dipole moment of ethyl chloride and vinyl chloride .

The dipole moment of ethyl chloride is greater than that of vinyl chloride. Because in vinyl chloride each carbon atom are sp2  hybridized and hence the electron are hardly bounded to the nucleus of the carbon atom.

Although the calculated value of dipole moment of vinyl chloride is slightly less than expected value ,due to resonance of lone pair on Cl atom with the pi electron.

The dipole moment of aromatic compound containing two different or same group can be calculated by the following empirical formula .

where , μ = dipole moment ,  μ1 ,μ2 group moment and θ = angle between the two group.
The calculated value of ortho, meta and para dichloro benzene are shown below.

What is dipole moment ?
What is bond moment ?
What is group moment ?
Compare the dipole moment of p-xylene , p-dichloro benzene and p-chloro toluene . 
Compare the dipole moment of carbon tetra chloride and chloroform .
Compare the dipole moment of ethyl chloride and vinyl chloride 

What is hyper-conjugation or Baker-Nathan effect