Resonance

Resonance by Dalpat Singh

1. resonance Presented by – Dalpat singh Pg-1 Lachoo memorial college , jodhpur

2. Resonance :- The transfer of electrons from one end of conjugation ( π -system) to the other through delocalization of electron is called resonance. When a compound can be represented by more than one Lewis structure and actual structure is hybrid of all these structure, the co m pound is said to possess resonance .

3. Resonance :- All the properties of a compound cannot be explained by single structure. Different arrangement of electrons within conjugated π- system but with identical positions of atoms is called resonating structure or canonical structure. Delocalisation of electrons leads to decrease in potential energy of molecule c a n o n i cal s t r u ctu r es o f benzene Resonance hybrid structure of benzene

4. Resonance :- T he actual structure of the molecule is considered as the hybrid (average of all of the contributing structures). Resonance structures are imaginary . Resonance hybrid is more stable than canonical structures . 7.The energy of resonance hybrid is always less than the energy of any one of the contributing structures

5. Rules for Drawing Resonance Structure - - All the resonating structure must be valid lewis structure which obey octet rule. They should posses same no. of electrons and equal net charge. Each atom should have a complete valence shell and be shown with correct formal charges. The positions of atoms should be same in all the resonance structures. ex:- the following structure are not the resonance structures since the positions of one H-atom is not same. In fact, they are different molecules which in dynamic equilibrium with each other. These are called tautomers.

6. Rules for Drawing Resonance Structure-- 4. The bond order of two connecting atoms may vary between two different resonating structures. 5. The resonating structure may or mayn’t be equivalent. 6. The atoms that are the part of delocalised system must be arranged in one plane or nearly so. The reason is to get maximum overlap between the orbitals. 7. All the canonical forms do not contribute equally i.e Benzene

7. Resonance Effect :- The increase in electron density at one position with corresponding decrease at another position is called resonance effect or mesomeric effect. Types of Resonance Effec t or mesomeric effect :- (1). Positive resonance or mesomeric effect{+R/+M} (2). Negative resonance or mesomeric effect { -R/-M} The electron releasing groups show +M/+R effect , when they release electron to the rest of the molecule by delocalization. The electron density on rest of the molecule increased. Examples:- -OH, -SH, -OR,-SR. The electron withdrawing groups leave –M/-R effect by attracting delocalised electrons from rest of the molecules as a result the electron density decreases on the rest of the molecule. Examples:-  -NO 2 , C=O, -COOH, -C≡N.

8. N H 2 + NH 2 – + NH 2 e t c . – +R effect of –NH 2 group -- O O O O O O – e t c. + N + + N + N – – – – + –R effect of –NO 2 group -- Example:-

9. thanks Submitted to Prof. M R K Sherwani