1. Five Membered Heteroaromatic Rings
Their aromaticity derived from delocalization of lone pair of X
so X is electron deficient
ring is electron rich

2. Facts Five Membered Heteroaromatic Rings
1- aromaticity inversely proportion to electro negativity
Thiophene > Pyrrole > Furan
2- Electrons not available for protonation—hence not basic

3. Facts Five Membered Heteroaromatic Rings
3- 6  electrons over 5 ring atoms ….. Electron rich… so
more reactive than benzene towered electrophilic substitution
Pyrrole > Furan >Thiophene>benzene

4. Aromaticity Five Membered Heteroaromatic Rings 1-
6  electrons distributed over 5 ring atoms
each 2e make coupling …. So
is planner, all sp2

5. Aromaticity Five Membered Heteroaromatic Rings
2- Their protons show the same sort of chemical shift in NMR
as the protons of benzene with δ = 6.5 ppm-8 ppm

6. Five Membered Heteroaromatic Rings
3-Its weak acid not basic as the secondary amines
Only for pyrrole
Lone pair of N is involved in  cloud and not
Available for sharing with acids

7. 4-the patteren of reactivity
The pattern of reactivity with Electrophilic reagents
Cycloalkanes …… by addition reaction
Aromatic compounds ……. By substitution
addition followed by proton loss
[ onium intermediate]
Ordre of reactivity
Pyrrole > Furan >Thiophene>benzene

8. At what position Es take place

9. Why? 5-The order of aromaticity Thiophene > Pyrrole > Furan
In case of Thiophene
[S] donate & accept electrons…… so
delocalization as complete as benzene
Why?
In case of Furan
[O] electronegativity more …. Diene-like character
CH2=CH-CH=CH2

10. Five Membered Heteroaromatic Rings
6- The pattern of reactivity with Electrophilic reagents
Cycloalkanes …… by addition reaction
Aromatic compounds ……. By substitution
addition followed by proton loss
[ onium intermediate]
Ordre of reactivity
Pyrrole > Furan >Thiophene>benzene

11. Sources & Synthesis Five Membered Heteroaromatic Rings
Pyrrole & Thiophene …. Coal Tar
Pyrrole ring …. Prophyrin system…..
Chlorophyll & Hemoglobin
Furan ….. Decarbonylation of Furfuraldehyde …….
Oat hulls, corncobs or rice hulls

12. Five Membered Heteroaromatic Rings
Thiophene
Synthesis
Pyrrole

13. Five Membered Heteroaromatic Rings
Synthesis
Furan

14. Pyrrole Acetylation Reactions ESR Vilsmier Rex Reduction Sulfonation
Nitration

15. Pyrrole
Rex with carbene
Halogenation

16. Furan
Acetylation
Vilsmier Rex
Sulfonation
Nitration

17. Rex of furfural Cannizaro Rex Walf-Kishner reduction Benzoin-
condensation

18. Thiophene
Acetylation
Sulfonation
Nitration
Halogenation

19. Five Membered Heteroaromatic Rings
Containing 2X , at least one nitrogen
1,3-Azoles
1,2-Azoles
Thiazole [ 1,3-thiazole]
Oxazole [ 1,3-oxazole]
Imidazole [ 1,3-diazole]
Isothiazole [ 1,2-thiazole]
Isoxazole [ 1,2-oxazole]
Pyrazole [ 1,2-diazole]

20. Five Membered Heteroaromatic Rings
Containing 2X , at least one nitrogen
Aromaticity & Bascisity
Strong base, due to the greater electron releasing capacity
of the two nitrogen's

21. Five Membered Heteroaromatic Rings
Containing 2X , at least one nitrogen
Aromaticity & Bascisity

22. Five Membered Heteroaromatic Rings
Containing 2X , at least one nitrogen
Importance of the ring
Building blockes as Histidine and Histamine
It exist in two tautomeric forms ….. As base & weak acid

23. Fused Five Membered Heteroaromatic Rings
With one X
Found in coal tar & orange blossoms, humane feces
In amino acids as Tryptophan, alkaloid * pigments

24. Fused Five Membered Heteroaromatic Rings
With one X
Synthesis Fischer ndole Synthesis

25. Fused Five Membered Heteroaromatic Rings
With one X
Basicity of Indole
Not basic because lone pair is delocalized and
contributed to the Aromatic system

26. Fused Five Membered Heteroaromatic Rings With one X
It undrego Electrophilic Subistitution at
Position 3

27. Fused Five Membered Heteroaromatic Rings
With one X
Vilsmier Rex
Rex with carbene

28. Fused Five Membered Heteroaromatic Rings
With tow X
In nature as N-ribosyl-dimethyl benzimidazole …. In Vit B12
Commercially as an parasisticide