What is Polarizability according to Organic Chemistry?

Polarizability refers to the ability of an atom/molecule to distort the electron cloud of neighboring species towards itself and the process of distortion of electron cloud is known as polarization.

Polarizability

The atom/molecule polarizing its neighbouring chemical species is usually more electronegative than its surroundings.  When polarization takes place in an environment, a dipole moment is induced. Since polarity is induced, the species become charged. The species which is getting polarized attains a positive charge and the species which are polarizing attains a negative charge (implying that it is more electronegative).

An example is given below:

Before polarization, the electron density around an atom/molecule remains evenly distributed and is not distorted. But however, this electron distribution changes rapidly upon polarization owing to the induced dipole moment.

Delving into Polarizability          

When polarization is easily achievable, then the species are known as ‘soft’ but if polarization for some requires a lot of effort and energy, the species are known as ‘hard’.

Upon polarization, the species attains a dipole (implying that it possesses two poles- negative and positive ends) - so the molecule or bond will get ${\text{δ}}^{+}$ and a ${\text{δ}}^{-}$respectively.

The symbol ${\text{δ}}^{+}$is indicative of a region / bond/ molecule which is electron deficient which is developed as a result of polarization or inductive effects, etc. whereas, the symbol ${\text{δ}}^{-}$is indicative of a region / bond/ molecule that has an electron density in excess.

In organic chemistry, when polarization of a bond/molecule takes place, the covalent bonds become polar and this results in a polar molecule.

Polar molecules are generally more reactive than regular molecules because they have charges (mostly partial charges which are indicated by the symbols ${\text{δ}}^{+}$ and${\text{δ}}^{-}$) and we are aware that charged species are unstable.

The magnitude of the degree of polarization determines what type of bond is going to be formed between two charges species-if the degree of polarization is small; an ionic bond is formed resulting in an ionic molecule. If the degree of polarization is large, results in the formation of a covalent molecule.

The ability of a cation to alter or distort an anion is called its polarization power and the tendency of the anion to get polarized by the cation is known as its polarizability. The same concept can be extended to organic chemistry but slightly modified because we do not really have the concept of cations and anions in organic compounds.

A polar covalent bond looks like this:

 (Where B is more electronegative than A and hence it bears a partial negative charge)

We can predict the properties (like dipole moment) and we can predict the order of reactivity of molecules by knowing the degree of polarization and the direction of the bond polarization.

When two different elements form covalent bonds, there is always some amount of bond polarization happening. The magnitude of this bond polarization can be determined by the distance of separation of the two elements in the periodic table (how far apart they are in the periodic table). The atom placed closer to fluorine will bear partial negative charge and the atom which is far from fluorine will bear a partial positive charge. 

In organic chemistry, we mostly deal with bonds involving carbon-

Polarization is seen as an important phenomenon in organic chemistry because of its application in stereochemistry (molecules that are polarized exhibit optical activity and stereochemistry deals with optical activities of such organic molecules).

Molecule with a dipole moment will tend to orient itself in the presence of an applied magnetic field. This dipole moment in molecules can either be induced or made permanent by subjecting the molecule to an external applied electric field.

Factors affecting Polarizability

There are three factors that have an influence on polarizability. They are:

Orientation of the molecule

When the molecules are oriented in the same direction as that of the applied electric field, the dipole moments of individual molecules will add up and the net dipole moment will be greater ad this will result in a greater magnitude of polarization.

 Atomic radii

When atomic radii increases, the number of electrons in the outer shell are greater implying that the electron cloud and the nucleus are placed far apart. As a result of this, easier polarization takes place. Therefore, larger the atomic radius, greater is the polarizability.

Electron density

Greater electron density implies greater polarizability simply due to the increase in the number of electrons in the valence shell.

Polarization has an effect on the Dispersion Forces of a Molecule

Dispersion forces (also called London forces) arise as a result of temporary dipole- induced dipole attractive forces between two species.

• The magnitude of these dispersion forces becomes stronger when polarizability is greater. This implies that the attractive forces between molecules are stronger as a result of which boiling points and melting points will increase with molecular mass.
• Shape of the molecules allows polarizability to affect dispersion forces. In elongated molecules, electrons can move around easily and increase polarizability which strengthens the dispersion forces. However, in the case of smaller, symmetrical molecules, polarizability is lesser and the dispersion forces are weaker.

Applications

• As discussed in the above mentioned section, polarizability influences the intermolecular forces of attraction between two species and a number of properties like melting point, boiling point, chemical reactivity, etc. are affected.
• Another special mention goes to applications of polarizability in optical chemistry and stereochemistry part of organic chemistry.

 Context and Applications

This topic is significant in the professional exams for both undergraduate and graduate courses, especially for Chemistry.