Resonance:
The phenomenon in which two or more structures can be written for a substance which involves identical positions of atoms is called Resonance. The actual structure of the molecule is said to be Resonance Hybrid of various possible alternative structures. The alternative structures are referred to as the Resonance Structures. A double headed arrow (↔) between the resonance structures is used to represent the resonance hybrid. Thus in the case of benzene, Kekule’s structures (1) and (2) represent the resonance structures. Actual structure of the molecule may be represented as hybrid of these two resonance structures, or by the single structural formula (3).
It should be clearly understood that the resonance structures (1) and (2) are not the actual structures of the benzene molecule. They exist only in theory. None of these structures adequately represents the molecule. In resonance theory, we view the benzene molecule (which of course is a real entity) as being hybrid of these two hypothetical resonance structures.
Look at the structures 1 and 2 carefully. All single bonds in structure (1) are double bonds in structure (2), that is consider a hybrid of them, then the carbon-carbon bonds in benzene are neither single bonds nor double bonds. Rather, they are something halfway between. This is exactly what we find experimentally. Spectroscopic measurements show that benzene is planar and that all of its carbon- carbon bonds are of equal length, I .40ْ A. This value lies in between the carbon-carbon single bond length (1.54ْ A) and the carbon-carbon double bond length (1.34ْ A).
Aromaticity:
The aromatic compounds apparently contain alternate double and single bonds in a cyclic structure, and resemble benzene in chemical behaviour. They undergo substitution rather than addition reactions. This characteristic behaviour is called Aromatic Character or Aromaticity. Aromaticity is, in fact, a property of the sp2 -hybridized planar rings in which the p-orbitals (one on each atom) allow cyclic delocalization of π electrons.
Criteria for Aromaticity:
On the basis of the above considerations, can be laid down criteria which help us in knowing whether a particular compound is aromatic or non-aromatic.
- An aromatic compound is cyclic and planar.
- Each atom in an aromatic ring has a p-orbital. These p-orbitals must be parallel so that a continuous overlap is possible around the ring.
- The cyclic π molecular orbital (electron cloud) formed by overlap of p orbital, must contain (4n+2) π electrons, where n = integer 0, 1, 2, 3 etc. This is known as Huckel Rule.
Let us apply these rules to the following examples:
