General Organic chemistry for JEE & NEET

Resonance and Resonance (Mesomeric) Effect Resonance: The phenomenon in which two or more structures can be assigned to a molecule, but none of them represents the molecule correctly, is referred to as resonance or mesomerism. The correct structure of the molecule is said to be a resonance hybrid of the various possible alternative structures which themselves are known as resonating structures or canonical structures. Characteristics of Resonance hybrid: (i) The resonance hybrid is more stable than any one of the various resonating structures. The difference in energy between the hybrid and the most stable canonical structure is called resonance energy. (ii) In a resonance hybrid the bond lengths are the intermediate of those in the contributing structures e.g. carboncarbon bond length (1.39 Å) in benzene is in between carbon-carbon single bond length (1.54 Å) and carbon=carbon double bond length (1.34 Å). Necessary conditions for resonance: (i) Arrangement of all the atoms should be same in all resonating structures. (ii) The number of paired and unpaired electrons must be same in all the resonating structures. (iii) Resonating structures must be planar (iv) The energies of all the resonating structures should be either same or comparable. Resonance (Mesomeric) Effect It may be defined as the permanent and complete shifting of electrons which may be either pi electrons or lone pair of electrons. This transfer may be from a multiple bond to an atom or lone pair from an atom to another atom via a multiple bond. In case the compound is having conjugated system of double bonds (or triple bonds), the resonance effect is transmitted through whole of the conjugated system and the effect is called conjugative effect. The resonance effect is also of two types like inductive effect. Positive resonance (or meromeric effect) effect (+R or +M): If the atom or group of atoms is giving electrons through resonance, its effect is called +R or +M effect e.g. Note that the delocalisation involves sigma and pi bond orbitals (or p orbitals in case of free radicals); thus it is also known as sigma and pi conjugation. This type of electron release due to the presence of the system H-C-C=C is known as hyperconjugation. More the number of H-C bonds attached to the unsaturated system more will be the probability of electron release by this mechanism. Thus the electron release by this mechanism will be greater in methyl (possessing three hyperconjugated H-C bonds), less in ethyl (having two such bonds) and least iso-propyl (one) and essentially zero in tert-butyl (no hyperconjugated H-C bond) group. For detail theory of general organic chemistry visit https://bit.ly/2SiXl7X For online test of general organic chemistry https://bit.ly/2SlxQTu