(a) Resonance:
1. Certain organic compounds can be represented by more than one structure and they differ only in the position of bonding and lone pair of electrons. Such structures are called resonance structure and this phenomenon is called as resonance. This phenomenon is also called as mesomerism or mesomeric effect.
2. For example, the structure of aromatic compounds such as benzene and conjugated system like 1,3 butadiene cannot be represent by a single structure and their observed properties can be explained on the base of a resonance hybrid.
3. Resonance structure of benzene.
(I) and (II) are called as resonance hybrids of benzene.
4. For 1,3 butadiene:
(I), (II) and (III) are called as resonance hybrids of 1,3 butadiene.
(b) Hyper conjugation:
1. The denationalization of electrons of σ bond is called as hyper conjugation. it is a special stabilizing effect that results due to the interaction of electrons of a σ bond with the adjacent empty non-bonding Porbitais resulting in an extended molecular orbital.
2. Hyper conjugation is a permanent effect.
3. For example, in propane. the σ- electrons of C-H bond of methyl group can be delocalised into the it-orbital of doubly bonded carbon as represented below.
