Resonance effect- applications

1. Resonance effect- applications D Suneetha Lecturer in Chemistry Govt. College (A) RAJAHMUNDRY

2. Learning outcomes This lesson will help • Understand the concept of resonance • Know the rules for drawing resonance contributors • Resonance hybrid • Resonance effect and its applications

3. O – O 1.49 A O=O 1.21Å Bond length = 1.28 A

5. Resonance or Mesomerism • The representation of structure of a molecule as a weighted average of two or more hypothetical structures, which only differ by the arrangement of electrons but with same positions for atoms is referred to as Resonance or Mesomerism or delocalization • The averaging of electron distribution over two or more hypothetical contributing structures (canonical forms) to produce a hybrid electronic structure is called resonance.

6. Resonance or Mesomerism • Resonance is a phenomenon that occurs within a species that has two or more valid Lewis structures. • Each valid structure is called a resonance structure or a resonance contributor or a canonical structure. • The resonance structures are only imaginary and the actual structure of the molecule is considered as the hybrid of all the valid resonance structures.

7. Resonance hybrid Resonance hybrid:The average of all the contributing structures is known as resonance hybrid. It is considered as the actual structure

8. Resonance contributors and resonance hybrid of Urea

9. Resonance structures SO2 HNO3 HCHO

10. 1,3- butadiene Resonance hybrid

11. Rules for drawing resonance contributors: • In all the resonance contributors, the positions of the atoms remain same • Only the Π electrons and non-bonding electrons can move between atoms • The total no. of electrons in the molecule does not change • Each of the resonance contributor for a particular compound must have the same net charge • A resonance contributor must be a valid Lewis structure if possible all atoms should have an octet • Greater the no. of covalent bonds, greater is the contribution to the resonance hybrid • The structure with less charge separation is more stable

12. Invalid resonance contributors

13. Invalid resonance contributors These are tautomers but not resonance contributors The positions of the atoms are changed

14. valid resonance contributors

15. valid resonance contributors The more electronegative atom should bear the –ve charge

16. Resonance effect or Mesomeric effect • The flow of electrons from one part of conjugated Π – system (species having alternate double bonds ) to the other, caused by phenomenon of resonance is called Resonance effect or Mesomeric effect • This is of two types : (i) + R or +M effect (ii) – R or –M effect

17. +R or +M effect + R or +M effect • If a group donates electrons to the conjugated - system through resonance, then it is said to have +R or +M effect ι ι

18. –R or –M effect – R or –M effect • Groups which withdraw electrons towards themselves through resonance are said to have – R or –M effect ι

19. – R or –M effect - R effect of – CHO group - R effect of – CN group

20. Acidity of carboxylic acids Carboxylic acids are named such because they can donate a hydrogen to produce a carboxylate ion

21. Acidity of carboxylic acids Carboxylic acids dissociate in water to form carboxylate ion and hydronium ion. The carboxylate ion formed is stabilized through resonance by effective delocalization of the negative charge.

22. Acidity of carboxylic acids (-)

23. Acidity of carboxylic acids  An electron withdrawing group (- R effect) ensures effective delocalization of negative charge on the conjugate base (carboxylate ion) through resonance or inductive effect hence increases its stability

24. Acidity of carboxylic acids  An electron donating group (+ R effect) destabilizes the conjugate base (carboxylate ion) formed and hence decreases the acidity of carboxylic acids

25. Ethanoic acid is stronger acid than Ethyl alcohol No resonance possible in ethoxide ion hence it is less stable

28. Acidic character of Phenols +R or +M effect Phenols are more acidic than alcohols because of the formation of phenoxide ion which is resonance stabilized

29. Phenols are more acidic than alcohols Alkoxide ion, no resonance Phenoxide ion , resonance stabilized

30. Nitro phenol more acidic than phenol - M effect of -NO2 Nitro group withdraws the electron density in phenoxide ion hence makes it more stable

31. Amides are weak bases than Amines + M effect of -NH2 Lone pair of electrons in amide are not available due to resonance

32. Relative basic strength of amines Resonance stabilized Weak base No lone pair No resonance possible, unstable Strong acid Strong base

33. Reactivity of Benzene • M effect of -NO2 • Activates nucleophilic substitution +M effect of -NH2 -activates electrophilic substitution

34. Reactivity of Benzene + M effect of -OH Phenol is more reactive towards electrophilic substitution reactions at ortho and para positions

35. Q & A • Which resonance structure is stable? structure II (-ve charge is on O atom) • Which is more stronger Formic acid or Acetic acid? Formic acid

36. Summary • Resonance • Resonance hybrid • Some valid and invalid resonance structures • Resonance effect • Effect on acidity of carboxylic acids • effect on basicity of amines • Effect on reactivity of benzene

37. WEBLINKS http://www.learnerstv.com/animation/animation.php?ani=187&cat=Chemistry writing resonance structures

38. Thank you