POLARITY of a MOLECULE
Organic solvents are often classified as polar and non-polar according to their electrostatic properties.
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Any two dissimilar atoms connected by a covalent bond will have unequal sharing of the bonding electrons. (For solvent polarity we will only consider the σ−bonds.) In general only atoms on the right of carbon in the Periodic Table will form covalent bonds. The further the atom (Z) is to the right of carbon the more electronegative it would be; and the more polar the σ−bond formed.
The polarity of the bond is measured by the electric dipole moment;
| μ | = | δ x distance between the two atoms. |
| = | 1 e.s.u. x 1 angstrom = 1 Debye |
The charge of one electron is equivalent to 1.602 x 10-19 coulomb; and one angstrom, A = 2.08 x 10-11 meter. So 1 Debye is;
1 D = (1.602 x 10-19 C) x (2.08 x 10-11) = 3.332 x 10-30 C m.
However the polarity of the molecule is the vector sum of all such bonds, so we would also have to consider the configuration of the molecule. Molecule with a centre of symmetry will show no polarity even though there may be dipoles for each of its bonds. A good example is carbon tetrachloride. It is a tetrahedral molecule with the carbon at the centre and the chloride atoms radiating from the carbon. What we get is a positively charge interior and a negatively charged surface.
For larger molecule we would also have to consider the conformation of a molecule. The perpetual motion of a molecule chain makes it difficult to measure the dipole moment of the molecule.
DIELECTRIC CONSTANT, ε
Another parameter, the dielectric constant, is often used to compare the solvating power of molecule. Let the voltage between two opposing charges of equal magnitude q, separated by a distance in vacuum, be V0. Now we repeat the measurement but instead of a vacuum we introduce the compound. Let the voltage be V. Then the dielectric constant is given by the ratio V0/V (no unit).
Dielectric constant measures the polarity of the molecule along with its ability to induce dipole moment on the neighbouring molecules. This is because once there is a dipole moment in a molecule it acts as a magnet and so the next molecule will align itself in a positive-negative-positive-negative- order reinforcing the permittivity of the compound. So atoms with valence electrons situated far away from the nucleus, like the halides, will be more easily manipulated.
| Solvent | μ / D | ε | B.Pt. / �C |
|---|---|---|---|
| Dimethyl Sulfoxide Acetonitrile Dimethylforamide Acetone Water Methanol Ethanol |
3.96 3.92 3.82 2.88 1.85 1.70 1.69 |
49.0 36.2 36.7 20.7 78.5 32.6 24.3 |
189 82 153 57 100 65 78 |
HOW DOES POLAR MOLECULES OPERATE?
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Let us represent the molecule as an "egg" shape object. The polar molecule will have a slight positive (δ+) and negative (δ−) poles of equal magnitude at opposing ends. (This is just a conceptual representation. In a real situation it is not so neatly arranged as it is a dynamic system, and molecules are moving all over in three dimensional space.)
For a molecule to dissolve in a polar solvent it must also be polar. Non-polar molecules will not be invited to join the club, as it will disturb the electrostatic stability. So it is common to hear chemist speak of "like dissolve like". What it means is that polar compounds are more soluble in polar solvent, and non-polar compounds in non-polar solvents.
HOW DOES POLAR MOLECULES HELP IN CHEMICAL REACTIONS?
Chemical reactions involve breaking and making of bonds. Let us take the simple case of a SN1 reaction for an alkyl chloride. The most difficult elementary step in the reaction is the dissociation of the halide from the alkyl. In slow motion it would be;
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| Energy profile for the formation of carbonium ion |
The transition intermediate is more polarised than the initial alkyl chloride as the electrons are now in the process of leaving with the chloride. So it benefits more from the stabilisation by polar solvent; this would lower its energy. Consequently the activation energy barrier for this reaction would be lowered; making this reaction easier as compared to conducting the reaction in a non-polar solvent.
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The transition state during the SN2 hydroxylation of alkyl chloride is similarly stabilised by polar solvent making the reaction easier.
OTHER CONSIDERATIONS
In choosing a solvent the boiling point is equally important. If the reaction requires a high temperature, a low boiling point solvent will not be suitable. Also if you intend to isolate the product of the reaction by distillation you might want to choose a solvent with a boiling point very different from that of the product.
Needless to say the solvent must not itself take part in the reaction to give by-products.
