ChemistryMediumClass 12

Resonance — Rules, stability of resonance structures

GOC (General Organic Chemistry)

JEE Qs

Hard

min

Always systematically apply all resonance stability rules in the given priority order to accurately rank the stability of canonical forms.

🧮 Key Formulas

Rules for drawing resonance structures:

1. Only pi (π) electrons and lone pairs can be delocalized; sigma (σ) bonds remain intact.

2. Atoms must maintain their positions.

3. Total number of electrons and net charge must be conserved.

4. Second-row elements (C, N, O, F) cannot exceed an octet.

5. Formal charges must be correctly assigned to all atoms.

Rules for stability of resonance structures (priority order):

1. Structures with more covalent bonds (and thus fewer formal charges) are more stable.

2. Structures where all atoms (especially 2nd-row elements) have complete octets are more stable.

3. Structures with charge separation are less stable than those without.

4. Structures with negative charge on a more electronegative atom and positive charge on a less electronegative atom are more stable.

5. Structures with like charges on adjacent atoms are highly unstable.

1Resonance is a theoretical concept; the actual molecule is a single resonance hybrid, not an oscillating mixture of structures.
2Resonance structures are hypothetical contributing forms that collectively describe the electron delocalization in a molecule or ion.
3The greater the number of stable contributing resonance structures, the greater the resonance stabilization of the molecule.
4Resonance energy is the difference in energy between the resonance hybrid and the most stable canonical (contributing) structure.
5Electron delocalization leads to increased stability, equalization of bond lengths, and reduction of localized charge density.

✕Moving atoms instead of just pi electrons or lone pairs, leading to tautomers or different compounds.
✕Violating the octet rule for second-row elements (C, N, O, F), especially for carbon having more than 8 valence electrons.
✕Incorrectly prioritizing stability rules; for example, favoring charge separation over a complete octet.
✕Drawing structures where the net charge of the molecule changes between different resonance forms.

Class 11 Chemistry Part II Ch 12: Organic Chemistry - Some Basic Principles and Techniques (Electronic Displacements in a Covalent Bond)