1. Aromaticity of Benzenoid and Non-benzenoid compounds
PART-V By
Dr. Atul Prasad Sikdar
Associate Professor
Department of Chemistry
Mangaldai College : Assam

2. Ferrocene organometallic compound with the formula Fe(C5H5)2
Contains non-bezenoid aromatic CPD anion

3. Ferrocene

4. Ferrocene
Synthesis :
a) 2 C5H5MgBr + FeCl2 → Fe(C5H5)2 + MgCl2 + MgBr2
b) Fe + 2 C5H6(g) → Fe(C5H5)2 + H2(g)
c) Fe(CO)5 + 2 C5H6(g) → Fe(C5H5)2 + 5 CO(g) + H2(g)
d) 2 NaC5H5 + FeCl2 → Fe(C5H5)2 + 2 NaCl
e) FeCl2.4H2O + 2 C5H6 + 2 KOH → Fe(C5H5)2 + 2 KCl + 6 H2O
f) FeCl2 + Mn(C5H5)2 → MnCl2 + Fe(C5H5)2

5. Ferrocene
Reaction :
a) Electrophilic substitution

6. Ferrocene
b) Lithiation : Ferrocene reacts readily with butyl lithium to give 1,1'-dilithioferrocene, which in turn is a versatile nucleophile

7. Azulene Azulene is an organic compound and an isomer of naphthalene
Azulene is usually viewed as resulting from fusion of cyclopentadiene and cycloheptatriene rings

8. Azulene It exhibits aromatic properties
the peripheral bonds have similar lengths and it undergoes Friedel-Crafts-like substitutions
The stability gain from aromaticity is estimated to be half that of naphthalene.
Its dipole moment is 1.08 D
(Naphthalene- 0 D)

9. Azulene
May be viewed as the fusion of the aromatic 6 π-electron cyclopentadienyl anion and aromatic 6 π-electron tropylium cation
This explains polarity : one electron goes from seven membered ring to the five membered

10. Azulene
seven-membered ring is electrophilic and the five-membered ring is nucleophilic.
The dipolar nature of the ground state is reflected in its deep colour, which is unusual for small unsaturated aromatic compounds.
exhibits fluorescence from an upper-excited state (S2 → S0)

11. Azulene
The blue color of the mushroom Lactarius indigo is due to the azulene derivative (7-isopropenyl-4-methylazulen-1-yl)methyl stearate

12. Azulene
Synthesis

13. Mesoionic Compounds Sydnones
Mesoionic Compounds -Five membered heterocycle which cannot be satisfactorily represented by any covalent or dipolar structure, but only as hybrid of polar structure and they possess sextet of electron.
Widely studied example : Sydnones(Sydney+lactone)

14. Sydnones
Synthesis : From primary amine

15. Sydnones Synthetic utility :
1) As a synthon for preparation of other heterocycle :
-Though aromatic, sydnone ring is readily cleaved by hydrochloric acid, and as dipolarophile undergoes 1,3-dipolar cycloaddition reaction with unsaturated systems-----leads to other heterocycle

16. Sydnones Synthetic utility :
2) As a synthon for preparation of Hydrazines

17. Spectroscopy of Aromatic Compounds
Aromatic compounds can be identified by:
Infrared (IR) Spectroscopy
Ultraviolet (UV) Spectroscopy
Nuclear Magnetic Resonance (NMR) Spectroscopy

18. Infrared Spectroscopy
Aromatic rings have C–H stretching at 3030 cm1 and peaks in the range of 1450 to 1600 cm1
Substitution pattern of the aromatic ring:
Monosubstituted: cm-1
cm-1
o-Disubstituted: cm-1
m-Disubstituted: cm-1
cm-1
p-Disubstituted: cm-1

19. Example: Toluene (IR) 3030 cm1 Monosubstituted: 690-710 cm-1

20. Ultraviolet Spectroscopy
Aromatic rings have peaks near 205 nm and a less intense peak in nm range
Aromatic compounds are detectable by UV spectroscopy since they have a conjugated p electron system

21. Nuclear Magnetic Resonance Spectroscopy
1H NMR:
Aromatic H’s are strongly deshielded by ring and absorb between 6.5 and 8.0 
Peak pattern is characteristic positions of substituents

22. Ring Current is a property unique to aromatic rings
When aromatic ring is oriented perpendicular to a strong magnetic field, delocalized  electrons circulate producing a small local magnetic field
This opposes applied field in middle of ring but reinforces applied field outside of ring

23. Ring Current produces different effects inside and outside the ring
Outside 1H are deshielded and absorb at a lower field
Inside 1H are shielded and absorb at a higher field

24. Ring Current is characteristic of all Hückel aromatic compounds
Aryl 1H absorb between d
Benzylic 1H absorb between d downfield from other alkane 1H

25. Example: p-bromotoluene (1H NMR)
The 4 aryl protons: Two doublets at 7.02 and 7.45 d
The benzylic CH3 protons: a singlet at 2.29 d
Integration 2:2:3

26. 13C NMR Carbons in aromatic ring absorb between 110 to 140 
Shift is distinct from alkane carbons but is in same range as alkene carbons

28. Multiple Substituent Effects in Electrophilic Aromatic Substitution(EAS)1

29. all possible EAS sites may be equivalent
The Simplest Case
all possible EAS sites may be equivalent
CH3
CH3 O O
CH3COCCH3
CCH3
AlCl3 +
CH3
99% 2

30. Another Straightforward Case
CH3
NO2
CH3 Br
Br2 Fe
NO2
86-90%
directing effects of substituents reinforce each other; substitution takes place ortho to the methyl group and meta to the nitro group 2

31. regioselectivity is controlled by the most activating substituent
Generalization
regioselectivity is controlled by the most activating substituent 6

32. all possible EAS sites may be equivalent
The Simplest Case
all possible EAS sites may be equivalent
strongly activating
NHCH3 Cl
NHCH3 Cl Br
Br2
acetic acid
87% 2

33. When activating effects are similar...
CH3
C(CH3)3
CH3
HNO3
H2SO4
NO2
C(CH3)3
88%
substitution occurs ortho to the smaller group 5

34. position between two substituents is last position to be substituted
Steric effects control regioselectivity when electronic effects are similar
CH3
NO2
CH3
HNO3
H2SO4
98%
position between two substituents is last position to be substituted 5

35. Regioselective Synthesis of Disubstituted Aromatic Compounds
Factors to Consider
order of introduction of substituents to ensure correct orientation 7

36. Synthesis of m-Bromoacetophenone
Which substituent should be introduced first?
CCH3 O 8

37. Synthesis of m-Bromoacetophenone
para
If bromine is introduced first, p-bromoacetophenone is major product.
CCH3 O
meta 8

38. Synthesis of m-Bromoacetophenone
CCH3 O Br O
CH3COCCH3
Br2
AlCl3
CCH3 O
AlCl3 8

39. order of introduction of substituents to ensure correct orientation
Factors to Consider
order of introduction of substituents to ensure correct orientation
Friedel-Crafts reactions (alkylation, acylation) cannot be carried out on strongly deactivated aromatics 6

40. Synthesis of m-Nitroacetophenone
Which substituent should be introduced first?
CCH3 O 8

41. Synthesis of m-Nitroacetophenone
If NO2 is introduced first, the next step (Friedel-Crafts acylation) fails.
CCH3 O 8

42. Synthesis of m-Nitroacetophenone
CCH3 O O
CH3COCCH3
HNO3
H2SO4
CCH3 O
AlCl3 8

43. order of introduction of substituents to ensure correct orientation
Factors to Consider
order of introduction of substituents to ensure correct orientation
Friedel-Crafts reactions (alkylation, acylation) cannot be carried out on strongly deactivated aromatics
sometimes electrophilic aromatic substitution must be combined with a functional group transformation 6

44. Synthesis of p-Nitrobenzoic Acid from Toluene
CO2H
CH3
Which first? (oxidation of methyl group or nitration of ring)
NO2
CH3 8

45. Synthesis of p-Nitrobenzoic Acid from Toluene
CO2H
nitration gives m-nitrobenzoic acid
CH3
NO2
CH3
oxidation gives p-nitrobenzoic acid 8

46. Synthesis of p-Nitrobenzoic Acid from Toluene
NO2
CO2H
CH3
NO2
CH3
HNO3
Na2Cr2O7, H2O
H2SO4, heat
H2SO4 8

47. Reduction of aromatic compounds: The Birch reduction

48. Birch Reduction
Benzene can be reduced to 1,4-cyclohexadiene by treating it with an alkali metal(sodium (Na), lithium (Li), or potassium (K)) in a mixture of liquid ammonia and alcohol.

49. Birch Reduction

50. Birch Reduction
Electron donating group containing double bonds are not reduced
Electron withdrawing group containing double bonds are reduced

51. Thanking You