Resonance is the shifting of π electrons or lone pair with in the compound. Resonating structures are hypothetical and it is intramolecular phenomenon. The combination of resonating structures are called hybrid structure.
How to draw resonance structures
Resonance structures can be drawn by using the shifting electrons by using doubly headed arrows. In resonance structures, atoms remains at the same positions but only electron shifts from one place to another.
Conditions for Resonance Structures
Resonance may be possible in the following conjugation systems:
- π → σ → + / vacant p-orbital
- π → σ → lone pair / (– ) charge
- π → σ → ( . ) free radical
- π → σ → π
- Lone pair → σ → + / vacant p-orbital
1. π → σ → + / vacant p-orbital
In this type of cojugation, there must be presence of pi bond – sigma bond and (+) charge in any molecule.
2. π → σ → lone pair / (– ) charge
The resonance structures will be formed when pi bond – sigma bond and lone pair or –ve charge are present together and connected in the sequence.
Note: The total charge on all the resonating structures must be equal. Also the position of the atom must not change.
3. π → σ → ( . ) free radical
When pi bond – sigma bond – free radical are present together in the organic compound, they will form the resonance structures.
4. π → σ → π
The resonating structures are formed when pi bond – sigma bond – pi bond are present together in any compound.
5. Lone pair → σ → + / vacant p-orbital
In this type, Lone pair – sigma bond and (+) charge are present in the compound.
Stability of Resonance Structures
Rule#1: The resonating structure which has more covalent bonds, will be more stable.
Rule#2: Neutral resonating structure will be more stable than charged resonating structure.
a ˃ b
No. of covalent in both the resonating structures are three.
So the neutral specie will be more stable than charged resonating structure.
Rule#3: In charged species, negative charge on more electronegative atom will stabilize it. On the other hand, if +ve charge is present on more electronegative atom, it will give more instability to it.
a ˃ b
No. of covalent bonds in both the resonating structures are equal.
Both are charged, negative charge on more electronegative is present in structure (a)
Rule#4: The maximum number of covalent bonds that an atom may have is one for hydrogen, four for carbon, four for nitrogen, three for oxygen and two for halogens.
Structure (III) has five covalent bonds for the nitrogen atom and therefore, is not allowed.
Look at three structure of acetate ion:
Structure (III) is not a contributor because the carboxylic carbon atom has five covalent bonds, which is not allowed.
When three bonds are written for an oxygen atom, it has a unit positive charge on it, e.g.,
The structure (III) is not allowed because the hydrogen atom has two covalent bonds.
Rule#5: Two like charges on adjacent atoms destabilize the resonating structure whereas when two unlike charges present on adjacent atoms, resonating structure will be more stable.
Structure (I) and (II) are the important canonical forms of CH3N3 but structure (III) is negligible contributor. Structure (III) has two negative charges on the same nitrogen atom and two positive charges on two adjacent atoms.
Rule#6: To check the inductive effect on the resonating structures.
No. of covalent bonds same
Both charges are on Carbon
No Like charges present
(a) is more stable due to +I effect of methyl towards carbocation.