Resonance • There are so many organic molecules (neutral molecules, ions, free radicals) where all the observed properties i.e. physical & chemical properties can’t be satisfactorily explained with the help of a single structural formula i.e. Lewis structure or valence bond structure. • In order to satisfactorily explain all the properties of such molecules we represent the molecule by two or more structures just by shifting of p orbital electrons or delocalization of electrons without changing relative positions of the atomic nuclei. These structures are known as resonating structure, canonical structure, contributing structure or resonance unperturbed structures and phenomenon is called as resonance. • The resonating structures are only the hypothetical structures and the real structure lies in between them. The resonating structures contribute to the real structure which is called as Resonance hybrid. The resonance hybrid is more stable than any of the resonating structures. • Compound showing resonance is not a mixture of some molecules having the structures shown by one canonical form and some having structures shown by another but the compound showing resonance have the same structure all the time www.OrganicChemistry.co.in
General cases of resonance 1. A=B-C=D 3. A=B-C¯ 5. A=B − C˚˚ 7. A ¯ − B 9. A ≡ B − C ≡ D 11. A ≡ B − C ¯ 13. A ≡ B − C ˚˚
2. 4. 6. 8. 10. 12.
A=B − C+ A=B − C˚ A=B − C≡D A ˚˚ − B A≡B−C+ A≡B−C˚
Capsules: 1. A=B −C+/ ¯/ ˚/ ˚˚/=/ ≡ 2. A ¯/ ˚˚ − B 3. A ≡ B − C+/ ¯/ ˚/ ˚˚/ ≡ www.OrganicChemistry.co.in
Points to draw resonating structures 1. Resonating structures differs only in placement of their pi electrons or non bonding electrons i.e. sigma bond skeleton remains same in all resonating structures. 2. The position of each atom in all resonating structures must be same. 3. All the resonating structures must be a proper Lewis structure i.e. elements of 2nd period (C, N, O, F) cannot expand their octet in any of the resonating structures. 4. All the resonating structures must have same numbers of paired electrons if they have pi bonds, positive, negative or lone pair of electrons and same numbers of unpaired electron if they are free radical. 5. If any conjugate position has more than one lone pairs of electrons then only one lone pair of electron will take part in conjugation. www.OrganicChemistry.co.in
Points to draw resonating structures 6. If any conjugate position has more than one pi bonds then only one pi bond will take part in conjugation. 7. If any conjugate position has pi bonds along with positive charge, negative charge, odd electron or lone pair of electrons then only pi bond will take part in conjugation. 8. Electron of negative charge or lone pair of electrons behaves as two pi electrons if it is in conjugation with pi bonds. www.OrganicChemistry.co.in
Points to check stability of resonating structures 1. Neutral molecules are generally more stable than charge separated structures. 2. Greater the number of pi bonds in resonating structure, more stable the molecule will be. 3. Resonating structures with complete octet for each atom is more stable no matter if electronegative atom has positive charge or electropositive atom has negative charge. 4. Resonating structures with electronegative atom having negative charge and electropositive atom having positive charge are more stable than those in which electronegative atom having positive charge and electropositive atom having negative charge when number of pi bond is same. www.OrganicChemistry.co.in
Points to check stability of resonating structures 5. Resonating structures with similar charge on adjacent atoms are insignificant due to electronic repulsion and result into instability in molecule. Structures with two fully positive charge or partial positive charge on adjacent atom; two fully negative charge or partial negative charge on adjacent atom; or negative charge and lone pair on adjacent atom are insignificant. 6. Resonance stabilization is more when at least two equivalent resonating structures are possible for a molecule. 7. As conjugation increases in the molecules, stability increases. 8. If conjugation is same in aliphatic and aromatic compounds then aromatic compound is more stable. 9. Structures with linear conjugation is more stable than those structures which have cross conjugation (if two groups are in conjugation with a particular group but not in conjugation with each other then the system have cross conjugation) www.OrganicChemistry.co.in
Points to check stability of resonating structures 10.Dissimilar canonical structures vary widely in their energy contents (stability) i.e. those canonical structures having higher energy (less stability) contribute less to the resonance hybrid and canonical forms having lower energy (more stability) contribute more to the resonance hybrid. Equivalent resonating structures contribute equally to the resonance hybrid. 11. Resonance is distance independent and delocalization of electrons occurs from one place to another till the conjugation is present. 12. Resonance energy – Potential energy difference between the most stable resonating structure and the real structure (resonance hybrid) is called as resonance energy. It has a fix value for a molecule. 13. Resonance stabilizes the molecule i.e. it lowers the energy content which can be revealed by lower heat of combustion & lower heat of hydrogenation values. www.OrganicChemistry.co.i
