Resonance Effect
- The effect by which the properties of a compound are explained (which otherwise cannot be explained by a single structure) by considering the actual structure to be a hybrid of several hypothetical structures is called Resonance Effect.
- It is due to delocalization of sigma and pie-electrons.
- It arises out of the wave nature of electrons.
- Since wave nature of an electron in a structure cannot be drawn, we represent hypothetical canonical forms which exhibit particle nature and whose hybrid the actual structure can be assumed to be.
- The actual structure of the molecule/ion is the weighted mean of the canonical or parent structures. But the energy of the molecule is lower than all the canonical forms.
- Molecules showing resonance must be conjugated and the orbitals appropriately oriented.
- When resonance effect releases or withdraws electrons it is called Mesomeric Effect.
- Resonance effect is a permanent effect.
- Resonance effect is a strong effect, generally stronger than Inductive Effect.
- Resonance is a relative effect.
- It is a need based effect.
Rules for Drawing Resonating Structures:
- While drawing resonating structures the position of atoms do not change. Only the position of electrons change.
- While drawing resonating structures the sum of lone pair and bond pair should be constant.
- Overall charge of a system must remain constant.
- Flow of electrons must be from high electron density to low electron density as the purpose of resonance is to balance the charge throughout the system.
- Second period elements cannot expand their octet while third period elemements can.
Basic Movement of Electrons:
A lone pair can either move to the adjacent bond or one electron may be transferred to the
other bonded atom
Different Cases:
1. Empty p-orbital conjugated with pie-bond:
2. pie-Bond Conjugated with pie-Bond:
5. Empty p-orbital Conjugated with Lone Pair:
6. Unpaired Electron Conjugated with Lone Pair:
Other Points About Stability of Resonating Structure:
- Non-polar structures with complete octet are more stable than polar structures with complete octet.
- Structures with complete octet are more stable than those with incomplete octet.
- Structures with more number of covalent bonds are more stable than those with less number of covalent bonds.
- Among structures with equal number of covalent bonds but different number of sigma & pie- bonds, the structure with more number of sigma-bonds is more stable.
- Structures with greater number of benzene rings (benzenoid structures) are more stable than those with less number of benzene rings ( ).
Differences Between Inductive & Resonace Effect
