Coupon Accepted Successfully!


Dipole Moments

A dipole consists of a positive and an equal negative charge separated by a distance within a molecule. The degree of polarity of a bond is given by the dipole moment (μ), which is the product of charge (e) and the distance (d) between them: μ = d × e, where e is the electronic charge and d is the distance between the atomic centers.
1 d = 10–18 esu-cm = 3.336 × 10–30 C-m.

Applications of dipole moment measurements

  1. Diatomic molecules: Diatomic molecules such as H2, Cl2, and N2 have no dipole moment; so these molecules are symmetrical.
  2. Triatomic molecules: Some of these molecules possess zero dipole moment; so they have a symmetrical linear structure, e.g., CO2, CS2, and HgCl2. Others such as water and sulphur dioxide have definite dipole moments. They are said to have angular or bent structure or V-shaped structure.
  3. Tetratomic molecules: Some molecules such as BCl3 have zero dipole moment. They are said to possess a flat and symmetrical (triangular) structure. Other examples are BF3, BBr3, CO 3 2–, and NO 3. PCl3, AsCl3, NH3, PH3, AsH3, and H3O+ have appreciable dipole moments. They possess trigonal pyramidal structures.
    Methane and CCl4 have zero dipole moment. So they possess symmetrical tetrahedral structures with C atom at the center of the tetrahedron.
    Benzene has zero dipole moment. All the 6C and 6H atoms are assumed to be in the same plane (symmetrical hexagonal structure).
    Measurement of dipole moments will enable us to detect cis- and trans-isomers of organic compounds. The trans-isomer, which is symmetrical, has zero dipole moment while the cis-isomer has a definite dipole moment. For example, cis-dibromoethylene (μ = 1.4 D) and trans-dibromoethylene (μ = 0).
  4. Dipole moment in aromatic ring system: We know that when substituted benzene is treated with a reagent, different products (namely ortho, meta, and para) are formed. In order to find their dipole moment, we need to know about the nature of the groups linked to the benzene ring.
    Case (i): When X and Y both are electron pushing or electron withdrawing. Let the bond dipole of C  X bond be represented by μ1 and that of C –– Y bond by μ2. Now let us assume that the electron-pushing groups have positive bond moment and the electron-withdrawing groups have negative bond moment.
    Description: 14862.png
    Description: 14870.png
    Case (ii): When X is electron pushing and Y is electron withdrawing or vice versa. Let the bond moment of C–X dipole be μ1 and that of C–Y dipole be μ2.
    Description: 14877.png
    Description: 14885.png
    Description: 14892.png

Dipole moment and percentage ionic character

The percentage of ionic character in a compound having some covalent character can be calculated by the following equation:
% ionic character = Description: 14939.png
Theoretical dipole moment is confined to when we assume that the bond is 100% ionic, and it is broken into ions. Observed dipole moment is with respect to fractional charges on the atoms of the bond.

Test Your Skills Now!
Take a Quiz now
Reviewer Name