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

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

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

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

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

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

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

At what position Es take place

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

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

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

Five Membered Heteroaromatic Rings Thiophene Synthesis Pyrrole

Five Membered Heteroaromatic Rings Synthesis Furan

Pyrrole Acetylation Reactions ESR Vilsmier Rex Reduction Sulfonation Nitration

Pyrrole Rex with carbene Halogenation

Furan Acetylation Vilsmier Rex Sulfonation Nitration

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

Thiophene Acetylation Sulfonation Nitration Halogenation

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]

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

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

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

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

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

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

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

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

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