1.3. Aromaticity of heterocyclic compounds

Aromaticity

1. AROMATICITY K. RAVIKUMAR M. Pharm.(Ph.D) Associate professor Department of Pharmaceutical Chemistry School of Pharmacy Nalla Narasimha Reddy Education society’s Group of institutions Hyderabad

2. Objectives Students able to Define Aromaticity Explain the attributions of Aromaticity Define Huckle’rule Analyze the Aromaticity of compounds by Huckle’rule. NNRG, Hyderabad 01

3. Why benzene derivatives show different properties as compared to Aliphatic compounds Why benzene resist addition and oxidation reaction and undergo substitution reaction Why benzene shows unusual stability Why substituent's such as Cl , OH, NH 2 directly attached to ring behaves differently NNRG, Hyderabad 02

4. Criteria for Aromaticity To be Aromatic, there are four conditions that a molecule should satisfy Cyclic Planar Complete conjugation Huckel's rule NNRG, Hyderabad 03

5. The Criteria for Aromaticity Four structural criteria must be satisfied for a compound to be aromatic. 1. A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms . NNRG, Hyderabad 04

6. The Criteria for Aromaticity Four structural criteria must be satisfied for a compound to be aromatic . 1. A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms . NNRG, Hyderabad 04

7. The Criteria for Aromaticity Four structural criteria must be satisfied for a compound to be aromatic. 1. A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms . NNRG, Hyderabad 04

8. The Criteria for Aromaticity Four structural criteria must be satisfied for a compound to be aromatic. 1. A molecule must be cyclic. To be aromatic, each p orbital must overlap with p orbitals on adjacent atoms . Not Aromatic NNRG, Hyderabad 04

9. [ 2 . A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized . The p orbitals must overlap continuously around the ring Aeromatic compound (landscape) 05

10. [ 2 . A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized . The p orbitals must overlap continuously around the ring 05 NNRG, Hyderabad

11. [ 2 . A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized . The p orbitals must overlap continuously around the ring Aeromatic compound (landscape) 05 NNRG, Hyderabad

12. [ 2 . A molecule must be planar. All adjacent p orbitals must be aligned so that the electron density can be delocalized . The p orbitals must overlap continuously around the ring Aeromatic compound (landscape) 05 NNRG, Hyderabad

13. 3. A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. NNRG, Hyderabad 06

14. 3. A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. NNRG, Hyderabad 06

15. 3. A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. NNRG, Hyderabad 06

16. 3. A molecule must be completely conjugated. Aromatic compounds must have a p orbital on every atom. NNRG, Hyderabad 06

17. 4. A molecule must satisfy Hückel’s rule, and contain a particular number of π electrons. NNRG, Hyderabad 07

18. 4 . A molecule must satisfy Hückel’s rule, and contain a particular number of π electrons. H u ckel's rule : “An aromatic compound must contain 4n+2 π electrons (n = 0,1, 2, and so forth i.e., integers)” NNRG, Hyderabad 07

19. 4 . A molecule must satisfy Hückel’s rule, and contain a particular number of π electrons. Benzene is aromatic and especially stable because it contains 6 π electrons. H u ckel's rule : “An aromatic compound must contain 4n+2 π electrons (n = 0,1, 2, and so forth i.e., integers)” NNRG, Hyderabad 07

20. 4 . A molecule must satisfy Hückel’s rule, and contain a particular number of π electrons. Benzene is aromatic and especially stable because it contains 6 π electrons. H u ckel's rule : “An aromatic compound must contain 4n+2 π electrons (n = 0,1, 2, and so forth i.e., integers)” 4n+ 2 = π electrons 4n+ 2 = 6 π electrons 4n = 6 - 2 n = 4 / 4 = 1 (Integer) NNRG, Hyderabad 07

21. NNRG, Hyderabad 08

22. Cyclobutadiene is anti-aromatic and especially unstable because it contains 4 π electrons. NNRG, Hyderabad 08

23. Cyclobutadiene is anti-aromatic and especially unstable because it contains 4 π electrons. 4n + 2 = π electrons 4n + 2 = 4 π electrons 4n = 4 - 2 n = 2/4 = 0.5 (fraction) NNRG, Hyderabad 08

24. Cyclobutadiene is anti-aromatic and especially unstable because it contains 4 π electrons. 4n + 2 = π electrons 4n + 2 = 4 π electrons 4n = 4 - 2 n = 2/4 = 0.5 (fraction) “Cyclic, planar, and completely conjugated compounds that contain 4n π electrons are especially unstable , and are said to be Anti-aromatic ” NNRG, Hyderabad 08

25. Polycyclic Aromatic Hydrocarbons(PAH) NNRG, Hyderabad 09

26. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons NNRG, Hyderabad 09

27. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons 4n + 2 = π electrons 4n + 2 = 10 π electrons 4n = 10 - 2 n = 8/4 = 2 (Integer ) NNRG, Hyderabad 09

28. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons 4n + 2 = π electrons 4n + 2 = 10 π electrons 4n = 10 - 2 n = 8/4 = 2 (Integer ) Aromatic NNRG, Hyderabad 09

29. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons 4n + 2 = π electrons 4n + 2 = 10 π electrons 4n = 10 - 2 n = 8/4 = 2 (Integer ) Aromatic NNRG, Hyderabad 09

30. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons Anthracene Contains 14 π electrons 4n + 2 = π electrons 4n + 2 = 10 π electrons 4n = 10 - 2 n = 8/4 = 2 (Integer ) Aromatic NNRG, Hyderabad 09

31. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons 4n + 2 = π electrons 4n + 2 = 14 π electrons 4n = 14 - 2 n = 12/4 = 3 (Integer ) Anthracene Contains 14 π electrons 4n + 2 = π electrons 4n + 2 = 10 π electrons 4n = 10 - 2 n = 8/4 = 2 (Integer ) Aromatic NNRG, Hyderabad 09

32. Polycyclic Aromatic Hydrocarbons(PAH) Naphthalene Contains 10 π electrons 4n + 2 = π electrons 4n + 2 = 14 π electrons 4n = 14 - 2 n = 12/4 = 3 (Integer ) Anthracene Contains 14 π electrons 4n + 2 = π electrons 4n + 2 = 10 π electrons 4n = 10 - 2 n = 8/4 = 2 (Integer ) Aromatic Aromatic NNRG, Hyderabad 09

33. If we have the total number of π electrons are 6, 10, 14, 18 Nos – AROMATIC NNRG, Hyderabad 10

34. If we have the total number of π electrons are 6, 10, 14, 18 Nos – AROMATIC Follows Huckel's rule and gives n = integers such as 1, 2, 3, 4 respectively NNRG, Hyderabad 10

35. If we have the total number of π electrons are 6, 10, 14, 18 Nos – AROMATIC Follows Huckel's rule and gives n = integers such as 1, 2, 3, 4 respectively If we observe, the total number of π electrons are 4, 8 , 12, 16 Nos – ANTI / NON AROMATIC NNRG, Hyderabad 10

36. If we have the total number of π electrons are 6, 10, 14, 18 Nos – AROMATIC Follows Huckel's rule and gives n = integers such as 1, 2, 3, 4 respectively If we observe, the total number of π electrons are 4, 8 , 12, 16 Nos – ANTI / NON AROMATIC Follows Huckel's rule and gives n = fractions such as 0.5, 1.5 , 2.5, 3.5 respectively NNRG, Hyderabad 10

37. Considering aromaticity, a compound can be classified in one of three ways: NNRG, Hyderabad 11

38. A heterocyclic compound has hetero atom in the ring system other than carbon The heteroatom donates either one or two electrons to the π system Aromatic Heterocyclic Compounds All the compounds above have 6 π electrons NNRG, Hyderabad 12

39. A six-membered heterocycle with a nitrogen atom in its ring π electron structure resembles benzene (6 electrons) The nitrogen lone pair electrons are not part of the aromatic system (perpendicular o rbital ) Is it aromatic, anti-aromatic or non-aromatic? NNRG, Hyderabad 13

40. A five-membered heterocycle with one nitrogen π electron system similar to that of cyclopentadienyl anion Nitrogen atom is sp 2 -hybridized, and lone pair of electrons occupies a p orbital (6 π electrons) Is it aromatic, anti-aromatic or non-aromatic? NNRG, Hyderabad 14

41. Furan structure Furan and Thiphene structure are similar Are these aromatic, anti-aromatic or non-aromatic ? NNRG, Hyderabad 15

42. NNRG, Hyderabad 16

43. Hydrocarbons containing a single ring with alternating double and single bonds are c a l l e d annulenes . Completely conjugated rings larger than benzene are also aromatic if they are planar and have 4 n + 2 π electrons. To name an annulene, indicate the number of atoms in the ring in brackets and add the word annulene. NNRG, Hyderabad 17

44. [10]-Annulene has 10 π electrons, which satisfies Hückel's rule, but a planar molecule would place the two H atoms inside the ring too close to each other. Thus, the ring puckers to relieve this strain. Since [10]-annulene is not planar, the 10 π electrons can’t delocalize over the entire ring and it is not aromatic. NNRG, Hyderabad 18

45. Summary Defined Aromaticity Criteria for Aromaticity Defined Aromatic, Anti-aromatic and Non-aromatic compounds Huckel’s Rule Annulenes NNRG, Hyderabad 19

46. Reference 1- Organic chemistry by Bahl and Bahl 2- Organic chemistry by Morrison Boyd 3- Organic chemistry by I. L. Finar Vol -I NNRG, Hyderabad 20