22723_lec7.ppt
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Benzofuran is a colorless liquid with an aromatic structure that can be treated as a resonance hybrid with major contribution from structure 1. It is more stable to chemical attack than furan due to its aromatic properties. Benzofuran undergoes electrophilic substitution at the 2-position and addition reactions at the 2,3-positions. It can be reduced to the 2,3-dihydroderivative coumaran. Several syntheses of benzofuran and coumaran are described starting from bromo derivatives.
22723_lec7.ppt
- 1. lec6 1 Benzofuran (coumarone) General properties Benzo furan (1) some times called coumarone , is a colourless liquid b.p.137cº which may be isoleted from coal tar .Its structure has not been investigated in detail ,but it has aromatic properties and is best treated as a resonance hybrid to which 1 is the major contributor ; minor contribution are made by charged structure such as (1a) and (1b). Benzofuran is much more stable to chemical attack than furan. 6 5 1 O 1 7 4 3 2 O + - 1a 1b O - +
- 2. lec6 2 Chemical reaction Oxidation Benzofuran is much less easily opened than that of furan , but it can be spilt by the two routes outlined . O O3 O O O2 C -R O OH Mixture R = H , OH C CH ONa Na
- 3. lec6 3 2 – Electrophilic subistitution . Benzofuran display a lesser tendency for 3 – subistitution for- mylation of benzofuran reportedly gives only the 2-formly derivative and nitric acid nitration produced 2 – nitrobenzofuran ;alater study using dinitrogen tetroxide give 3 – nitrobenzofurane as amajor product togother with smaller percentage of the 2 – isomer . O N2O4 PhH 10c0 O NO2 + O HNO3 CH3COOH O NO2 O NO2 59 % 14%
- 4. lec6 4 Treatment of benzofuran with halogens result in 2,3 – addition product with initial electrophilic attacke taking place at c -2 O X2 O x x 2,3 - dihalobenzofuran O (CH3CO)2O BF3 O C - CH3 O This is in very marked contrast to indole and thianphthene , where attack takes place predominantly at position 3 , and has been attributed to the much greater electronegative of oxygen than that of nitrogen or sulphur . Some benzofurans even form 2,3 – epoxides with perbenzoic acid . O O O epoxide C6H5COOH O
- 5. lec6 5 Reduction Benzofuran is easily reduced by hydrogen over Palladium or sodium and alcohol the 2,3 – dihydroderivative ,which is also known as coumaran O O 2,3 - dihydroderiv (coumaran) red H2/Pd -H2 coumaran -2-one is the lacton of 2 – hydroxyphenyl acetic acid CH2 - COOH OH O O coumaran - 2 - one (lactone ) -H2O
- 6. lec6 6 Synthesis of Benzofuran 1- A useful laboratory synthesis starts with coumaran ,and is generally applicable O O Br2 O O H Br Br H HO- K+ K / OH -HBr + - OH CH C - CO2K Br + H -HBr O COOH Distile , CaO -CO2 O
- 7. lec6 7 COR O Na C =O O R CH2CO2Et Br CH2CO2Et -NaBr NaOEt -H2O O COOEt R Decarboxylation +H2O O R + CO2 + EtOH 2- An internal claisen reaction may be used to bring about ring closure .
- 8. lec6 8 3- When the sodium derivative of etheyl salicylate is used the product is ethyl coumaran -3-one -2-carboxylate which can be decarboxylated giving the best synthesis of coumaran -3-one C =O ONa OEt Br CH2CO2Et -NaBr NaOEt -EtOH C = O CH - COOEt O Decarboxylation H2O C =O O OEt CH2- CO2Et O O Coumaran-3-one-2- carboxylate Coumaran-3-one
- 9. lec6 9 Heterocyclic Analogues of benzen with one hetero atom The most important ring system of this class , that of pyridine , can be derived from benzene by the replacement of a carbon and hydrogen atom by one of nitrogen .This change leaves the aromatic structure of the rings essentially unaltered .A similar replacement of carbon and halogen atoms by one of oxygen cannot be envistigated because of valency consideration unless Oxygen atom subsequently bears appositive charge . N O O O Pyridine Pyrylium cation 1,2 - Pyran 1,4 - Pyran S 1 - R - Thia (IV) benzene R + S S Thiapyrylium cation 1,4 - Thiapyuran S 1,2-Thiapyuran +
- 10. lec6 10 This is the case with the pyrylium cation , which has aromatic properties , Reduction of the pyrylium cation could lead to the non – aromatic 1,2 and 1,4 – pyrans and derivatives of both are known .Sulphur analognes of the pyrylium salt and pyrans which are called thiapyrulium salt and thiapyrans, respectively, and some unstable tetravalent sulphur containing compound , the thia (IV) benzenes , have been prepared recently .
- 11. lec6 11 Pyridine Pyridine was discover by Anderson , he isolated this base from bone oil an he obtained pure picoline (methyl pyridine ) and Lutidine (dimethylpyridine ) from the same source . Compound containing the pyridine ring are widely distribute in nature , examples being Vatamine B6 and the nicotinamide, adenine , dinucleotide phosphate are of great biochemical importance , it also feature in the structure of many drugs , dyes andalkaloide . Pyridine is an aromatic compound similar to benzene .The asymmetry of the ring , greatly increase the number of structural isomers in comparison with the benzene . N .. + - N .. .. + - N .. .. - N .. .. N
- 12. lec6 12 N CH3 N CH3 N CH3 CH3 There is only one monomethylbenzene ,but there are three monomethylpyridine . Pyridine has 19 possible methyl substitution while benzene gives rise to only 12 methylsub . benzene . The position of subistituent around the pyridine ring are numbered (1) or (2).The cation is called pyridinum and hexa hydropyridine is known as piperidine . N H N 2 N 1 2 3 4 5 6 1 Pyridinum + N H Piperidine
- 13. lec6 13 Structure of pyridine In pyridine all ring atoms (five carbon and one nitrogen ) , are sp2hybriization .Two of the sp2 orbitals in each atom overlap with each other to from C – C an C – N σ bonds . The third sp2 orbitals on each carbon atom overlap with an S orbital of hydrogen to form the C – H σ bonds; the third sp2 orbital on nitrogen is occupied by two electron (lone pair electrone ) i.e .sp2 orbital containing Nitrogen lone pair .All the six bond in pyridine lie in one plane and the bond angles are approximately equal to 120 cº .Also each ring atom in pyridine possesses an unhybridized p orbital containing one electron and these are perpendicular to the plane containing the σ bonds.
- 14. lec6 14 The latter overlap between two p orbitals produced a delocalized π molecular orbital containing six electrons (Aromatic character ) one half of this π molecular orbital lies above and the other half below the plane of σ bonds . Pyridine show some aromatic properties because the resulting molecular orbitals satisfies the Huckel's rule ( n = 1 in 4n+2) . According to the resonance theory ,pyridine is considered to be hybrid of five structure as shown before . Measurement of bond length by x – ray confirms the hybrid nature of the pyridine molecule .The C – C and C – N bond lengths of 1.39 Aº an 1.57 Aº are intermediate between those corresponding to a single and double bond .
- 15. lec6 15 Physical properties Pyridine is a colour less liquid , b.p 115Cº . It has a very pungent dour , pyridine is miscible with water and most organic solvent it is very hygroscopic .Pyridine react with Na and so should be dried over solid potassium hydroxide or barium oxide .Almost all classes of organic compound are soluble in pyridine , even many of high melting solids which scarcely dissolve in solvents such as ethanol and benzene .It is consequently used as a solvent