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Heterocyclic compounds, [A] Five Membered Heterocyclic Compounds, Introduction:, Hetero means other in the Greek language.The cyclic, compounds contains at least one hetero atom other than carbons atom, such compounds are known as heterocyclic compounds., Generally in heterocyclic compounds, ,oxygen,atoms are present as hetero atom., , nitrogen, , ,sulphur, , The heterocyclic compounds are relatively stable, aromatic, compounds. They may be five or six membered, containing, conjugated double bonds& obey Huckel rule (4n+2)π electrons. But, ethylene oxide and lactones (ϒ-butyrolactone) are unstable therefore, they doesn’t consider to be heterocyclic compounds., O, , O, O, , Ethylene oxide, R, , ϒ -Lactone, Nomenclature:, The international union of pure and applied chemistry (IUPAC), gives rules for the systematic naming to the heterocyclic compounds., Beside the IUPAC system many common (Trivial) names are also, used which are widely accepted in world. The following rules are, used for the IUPAC nomenclature of heterocyclic compounds., 1) The following prefixes are used according to the hetero atoms &, suffixes are used according to the size of heterocyclic ring.( the

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4) When the ring containing more than one, different hetero atom,, the numbering starts from that hetero atom which is in the, highest group number in the periodic table., 5) If the two different hetero atoms belong to same group, then the, numbering starts from those, which is having low atomic, number, and numbering to the hetero atoms around the ring, should be minimum., 1, , 1, , S, , 2, , 2, 5, , 3N, , N, , S, , N, , O, , 2, 5, , 3N, , 4, , 1, , 1, 2, , 6, , 3N, , 5, , 5, 3N, , 4, , 4, , 4, , 1,3-Thiazole, Oxathiazine, , 1,2,3-Thiadiazole, , 1,3-Oxazole, , 1,4,2-, , Classification:, They are classified according to size of heterocyclic ring, containing one or more hetero atom& condensed system i.e. two or, more ring gets fused ., 1) Five member heterocyclic compounds containing one hetero, atom., H, N, , S, , O, , Trivial Name Furan, IUPAC Name Oxole, , Thiophene Pyrrole, Thiole, Azole, , 2) Five membered heterocyclic compounds containing two hetero, atoms., H, N, , S, , O, , O, N, , N, , N, , N

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Oxygen atom containtwo electron in one SP2 hybridized orbital,, &unhybridized 2Pz2 of oxygen atom, which having lone pair of, electron which is conjugated with two π bonds and completes the, aromatic sextet by obeying he Huckel rule (4n+2) π e-., Molecular orbital picture of Furan:, Molecular orbital structure of Furan is represented in orbital, picture diagram as follows., unhybridised Pz orbital, , , , , , H, , H, , , , , , SP2- S overlap, [ C - H , bond], , SP2- SP2 overlap, [ C - C , bond], , , , , , H, , H, , , , , SP2- SP2 overlap, [ C -O, bond], , ·, , , , , lone pair of eFig. : Molecular Orbital Diagram of Furan, , Resonance structure of Furan:Structure of Furan can be represented, as resonance hybrid by following resonating structure.

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Resonance structure of Thiophene:, Structure of Thiophene can be represented as resonance hybrid by, following resonating structure., , S, , S, , S, , S, , S, , S, , Molecular orbital picture of Thiophene:, Molecular orbital structure of Thiophene represented in orbital, picture is as follows., unhybridised Pz orbital, , , , , H, , H, , , , SP2- S overlap, [ C - H , bond], , SP2- SP2 overlap, [ C - C , bond], , , , , , , H, , H, , SP2- SP2 overlap, [ C - S, bond], , , , , , , , , lone pair of eFig. : Molecular Orbital Diagram of Thiophene

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unhybridised Pz orbital, , , , , , H, , H, , , , SP2- SP2 overlap, [ C - C , bond], , , , , , , H, , H, , SP2- S overlap, [ C - H , bond], , , , , , SP2- SP2 overlap, [ C - N , bond], , -, , lone pair of e, , , , SP2- S overlap, [ C - N, bond], , H, , Fig. : Molecular Orbital Diagram of Pyrrol, , Resonance structure of Pyrrole:, Structure of pyrrole can be represented as resonance hybrid by, following resonating structure., , N, H, , N, H, , N, H, , N, H, , N, H, , N, H, , Stability of furan, Thiophene and pyrrole, The resonance hybrid structure for thiophene can be represented, as follows.

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4 position because carbocation intermediate is stabilized by, accommodation of the +ve charge on the hetero atom., The general mechanism of electrophilic substitution reaction, with furan, pyrrole and thiophene is as follow., Electrophilic attack, attack at, 2 or 5 position, H, , H, X, , X, , E, , H, X, , E, , -H, X, , E, , E, I, II, III, Resonance stabilization of carbocation intermediate, More stable, , X, , X = O , S , NH, H, , attack at, 3 or 4 position, , H, , E, , E, X, , X, , I, , II, , -H, X, , E, , Resonance stabilization of carbocation, intermediate Less stable, , Rectivity of Furan, Pyrrole and Thiophene:, Nitrogen atom in pyrrol easily form positive charge than oxygen, atom in furan. Therefore pyrrol is more reactive than furan, The +M effect of sulphur is smaller than oxygen, because the, overlapping of different sized p-orbital of carbon-sulphur is less than, the overlapping of carbon and oxygen. Hence thiophene is less, reactive than furan., The aromatic substitution reaction is carried out in mild, conditiondue to the more reactivity., The reactivity order is as follows:, , E

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dry ether, , n-C4H9Li, , CO2, , -n-C4H10, , S, , Li, , S, , HCl, , S, , Thiophene, , B), , COOH, , Thiophene 2-Carboxylic acid, , Reduction:, , Thiophene on reduction with sodium in liquid ammonia (Birch, reduction) gives a mixture of 2,3-dihydrothiophene (B) compounds., Whereas the hydrogenation in presence of large amount of catalyst,, (palladium) produces tetrahydrothiophene (C) as product., Pd, , Na in liq.NH3, , S, , S, , Thiophene, , (C), , C), , S, , S, , (A), , (B), , Pyrrole/Azole:, , Pyrrole is five member heterocyclic compound contain a, nitrogen and fix carbon atoms in its cyclic structure., , N, H, , Pyrrol, , Synthesis of Pyrrole:, 1) From Acetylene:, When a mixture of acetylene and ammonia is passed through a, red hot tube it gives pyrrole., , 2 CH, , CH, , NH3, , -N2, , N, H, , Pyrrol, , 2) From Furan:

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Pyrrole is prepared commercially by fraction distillation of bone, oil. It is manufactured by passing a mixture of furan, ammonia and, steam over a catalyst, NH3, , H 2O, N, H, , O, , Pyrrol, , Furan, , 3) From succinamide:, Succinamide on distillation with Zn dust produce pyrrole., O, , OH, , Zn dust, NH, , NH, , Keto form O, , enol form OH, , NH, , Pyrrol, , Succinamide, , Physical properties of Pyrrole:, 1) It is colorless liquid having B.P.=1310C (404 0K), 2) It is soluble in organic solvent (alcohol , ether ), but sparingly, soluble in water., 3) It produces red colour on pine splinter moistened with HCl., 4) It is basic in nature., 5) It gives substitution reaction., Chemical properties of Pyrrole:, 1) Nitration:, If the Pyrrole is treated with conc. Of HNO3 or nitrating, mixture, it undergoes polymerization. Hence, the nitration of, pyrrole is carried out by the treatment of pyrrole with conc. HNO3, in presence of acetic anhydride to yield 2-Nitropyrrole.

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But each carbon & nitrogen atom contain unhybridizedPz orbital, with one e- get sidewise overlapped to form new three π bonds get, delocalized over all atoms in the ring and possess the aromaticity by, obeying Huckel rule (4n+2)πe-., The lone pair of electron present on nitrogen is easily accept the, acidic protons to form a salt, hence the pyridine is basic in nature and, more basic than pyrrole& furan because of easily available of pair of, e- for protonation., Resonance structure of Pyridine:, Structure of pyridine can be represented as resonance hybrid by, following resonation structure., , N, , N, , N, , N, , N, , Molecular orbital picture of Pyridine:, Molecular orbital structure of Pyridine can be represented as, follows.

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H, , unhybridised Pz orbital, , , , , , , , , H, , H, , , , SP2- S overlap, [ C - H , bond], , , , SP2- SP2 overlap, [ C - C , bond], , , H, , H, , , , , SP2- SP2 overlap, [ C - N, bond], , N, lone pair of e-, , , , Fig. : Molecular Orbital Diagram of Pyridine, , Pyridine is more basic than Pyrrole, and less basic than, piperidine?, In pyrrole the lone pair of electron on nitrogen atom get, delocalized over the ring, hence it is not easily available for, protonation of acidic proton, and make it weak base.

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In pyridine, the nitrogen atom has unshared pair of electrons &, not involve in delocalization in the ring. Therefore they are easily, available for acidic proton., , , , N, H, , N, , N, H, , Piperidine, Kb =2 x 10-3, , Pyridine, Kb =2.3 x 10-9, , Pyrrol, Kb =2.5 x 10-14, , But as compareto pyridine, pipeidine is sec. amine, has more, electron density on nitrogen atom than in pyridine ring. The pair of, electron on nitrogen in piperidine is more easily available than, pyridine., Hence pyridine is more basic than pyrrole, and less basic than, piperidine., Synthesis of Pyridine:, 1) From Acetylene:, Pyridine can be prepared by passing a mixture of acetylene and, hydrogen cyanide through a red hot tube., , 2 CH, , CH, , HCN, N, , Pyridine, , 2) From β-picoline:, When β-picoline is oxidized to gives Nicotinic acid, further it is, distilled with soda lime (NaOH+CaO) to yield Pyridine as product.

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COOH, , NaOH / Cao, , KMnO4, N, , Na2CO3 + H2O, , N, , N, , Pyridine, , 3) From pentamethylenediamine Hydrochloride:, Pyridine can be synthesized by heating of hydrochloride salt of, pentamethylenediamine give piperidine, further oxidation of, piperidine with conc. H2SO4 at 3000C or by catalytic, hydrogenation with Pd-C, yields pyridine., , -, , Cl +H3N, , NH3+ Cl-, , Pd / C, N, H, , NH4 Cl, , HCl, , N, , Pyridine, , Physical properties of pyridine:, 1) It is a colorless liquid with boiling point=1150C., 2) It has characteristics unpleasant smell., 3) It is miscible with water and hygroscopic., 4) It is used good solvent for organic and inorganic compounds., 5) It is basic in nature., General electrophilic substitution reaction of Pyridine:, 1) If we observe the resonance structure of pyridine it shown that,, the positive charge appears at position 2,4,6 in pyridine, structure. It indicates that due to electron deficiency,, nucleophilic substitution reaction is possible at 2,4,6- position in, peridine., 2) But the electron density is more at 3,5- position in resonating, structure of pyridine. Hence the electrophilic substitution, reaction is possible at 3,5- position.

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N, , N, , N, , N, , N, , 3) If the elctrophilic substitution takes place at 3,5-position/ βposition, the resulting carbocation intermediate is stabilized by, resonance without appearing +ve charge on nitrogen atom., Therefore it gives more stable product at 3,5-postion/ β-potion, in pyridine structure., 4) If the electrophilic substitution reaction of pyridine takes place, at 2,4,6-postions (α,γ-position), the resulting carbocation, intermediate is not more stable. Because it is stabilized through, resonance with the appearing of +ve charge on nitrogen atom, which is unfavorable for electrophilic substitution reaction., Hence e- substitution of pyridine takes place at 3,5- position., Substitution at 3 or  position, H, , H, , E, , E, , E, , E, , N, , N, , N, , H, , E, , -H+, , N, , N, , Stable intermediate resonance structure, , Stable product, , Substitution at 4 or  position, , H, , E, , E, , H, , E, , H, , E, , E, , -H+, N, N, N, unstable intermediate resonance structure, , N, , N, unstable product, , Substitution at 2, 6 or  position, , N, , N, , N, , N, , +, , H -H, , H, , H, , E, , E, , E, , unstable intermediate resonance structure, , Chemical properties of pyridine:, , N, E, , E, unstable product