Can you please explain Resonance Structure and resonance effect with examples?
Resonance is a process of delocalisation of electrons. A molecule or ion with such delocalized electrons is represented by several resonance structures or canonical forms.
Several Lewis structures are used collectively to describe the actual molecular structure. The actual structure is an approximate intermediate between the canonical forms, but its overall energy is lower than each of the contributors. This intermediate form between different contributing structures is called a resonance hybrid. Contributing structures differ only in the position of electrons, not in the position of nuclei.
The classic example of the application of the theory of resonance is the formulation of the structure of benzene. The structure of benzene as a six-membered ring of carbon atoms was introduced by the German chemist F.A. Kekule in 1865. To make the structure compatible with the quadrivalence of carbon, he introduced alternating single and double bonds in the ring, and in 1872, in order to account for the fact that no isomers of benzene (no isomeric orthosubstituted benzenes differing in having single or double bonds between the substituted carbon atoms) had been observed, he introduced the idea of an oscillation between structures of the form:
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