The phenomenon of resonance is said to occur if for a molecule, we can write two or more than two Lewis structures which differ in the position of electrons and not in the relative position of atoms. The various Lewis structures are called canonical structures or the resonance structures. The real structure of the molecule is not represented by any one of the canonical structures but is a resonance hybrid of the various resonance structures. The various resonance structures are separated by a double headed arrow. This arrow should be clearly distinguished from that used for an equilibrium.
Resonance hybrid is always more stable than any of its canonical structures. Thus, the actual molecule of benzene is more stable than either of two Kekule structures. The magnitude of stability conferred for a molecule as a result of resonance is expressed in terms of resonance energy or delocalization energy. It is defined as the difference in internal energy of the resonance hybrid and that of the most stable canonical form.
Heat of hydrogenation :- Heat of hydrogenation is the amounts of heat evolved when one mole of an unsaturated compound is hydrogenrated.
Halogenation :- The halogenation of benzene and some others arenes is carried out at low temperatures, in the absence of sun light and in presence of Lewis acid such as anhydrous ferric or aluminium chloride as catalyst.
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