Properties of many organic compounds cannot be predicted with the help of single Lewis dot structure. For example benzene. If we try and predict the structure of benzene we come up with a prediction of it having three C-C bonds and three C=C bonds but the actual property deviates from this prediction. Thus, we define resonance structures for defining properties of these compounds. The resonance structures (canonical structures) are hypothetical which do not represent any real molecule individually. They contribute to the actual structure in proportion to their stability. The energy of actual structure of the molecule (the resonance hybrid) is lower than that of any of the canonical structures.The resonance energy increases with the number of important contributing structures. The resonance structures have the same number of unpaired electrons and same positions of nuclei. The stability of resonance increases with:
- Number of covalent bonds
- Number of atoms with octet of electrons (except hydrogen which has a duplex)
- Separation of opposite charges,
- Dispersal of charge
- A negative charge if any on a more electronegative atom, a positive charge if any on the more electropositive atom, increases the stability of the atom.
The phenomena in which polarity is produced in the molecule, either by the interaction of two π-bonds or between a π-bond and lone pair of electrons present on an adjacent atom is termed as resonance effect.This effect mainly takes place due to the delocalization of π-electrons. The resonance effect (R effect) is mainly classified into two categories,
- Positive Resonance Effect (+R effect)
When the transfer of electrons is away from an atom or substituent group attached to the conjugated system (presence of alternate single and double bonds in an open-chain or cyclic system) due to resonance, we call this effect as positive resonance effect. Some of the substituent groups which attribute to positive resonance effect are – COOH, –CHO, >C=O, – CN, –NO2, etc.
- Negative Resonance Effect (- R effect)
When the transfer of electrons is towards the atom or substituent group attached to the conjugated system (presence of alternate single and double bonds in an open-chain or cyclic system) due to resonance, we call this effect as negative resonance effect. Some of the substituent groups which attribute to negative resonance effect are – COOH, –CHO, >C=O, – CN, –NO2, etc.
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