Resonance Effect/unit-12

Resonance:

There are many molecules whose behaviour cannot be explained by a single structure. To explain their behaviour two or more structures may be proposed and the actual molecule is said to be resonance hybrid structures. This phenomenon is called resonance. Thus if a molecule can be assigned two or more structures, none of which is capable of describing all the known properties of the compound, then the actual structure is intermediate or resonance hybrid of these structures. This phenomenon is called resonance. The various structures are called resonance structures.

The phenomenon of resonance can be explained with the help of CO₂ molecule. It has been represented by the structural formula:

Though this satisfies the conventional valency requirements, yet it cannot explain some experimental facts. The actual carbon-oxygen bond in CO₂ has been found to be 115 pm whereas the normal carbon-oxygen double bond is 122 pm and that for triple bond is 110 pm. This means that carbon oxygen bond in carbon dioxide is intermediate between a double and a triple bond. To account for this, two other structures were proposed. The actual molecule is said to be a resonance hybrid of these three structures:

Let us consider another example of benzene. It may be represented by a cyclic structure having alternate C—C single and C = C double bonds.

Therefore, we expect two different bond lengths of carbon-carbon bonds due to C—C single and C = C double bonds. However, benzene has a uniform C—C bond distance of 139 pm. This value is intermediate between the C—C single and C = C double bonds. Thus, the structure of benzene cannot be represented by a single structure. The benzene molecule may be regarded as resonance hybrid of the following two structures:

The resonating structures of carboxylic acid may be written as:

The resonance structures of acetate ion (CH₃COO^–), aniline (C₆H₅NH₂) and phenol (C₆H₅OH) are shown below:

It may be noted that the energy of the actual structure of the molecule i.e. resonance hybrid is lower than that of any of the resonating structures. The difference in energy between the actual structure and the most stable of the resonating structures is called resonance energy or resonance stabilisation energy. The different resonance structures contribute to the actual structure in proportion to their stability.

Rules for writing resonance structures: The following rules are applied for writing resonance structures:

1. The resonance structures should have same positions of nuclei.
2. All the resonance structures should have same number of unpaired electrons.
3. The contributing structures should have nearly same energy.
4. All the atoms should have octet of electrons (except hydrogen which has duplet). The resonance structures which violate octet rule should not be considered.