22723_lec7.ppt

benzofuran

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
O
2
C -R
O
OH
Mixture
R = H , OHC 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
NO
2
+
O
HNO3
CH3COOH
ONO
2
ONO
2
59 % 14%

4. lec6 4
Treatment of benzofuran with halogens result in 2,3 –addition product with initial
electrophilic attacke taking place at c -2O
X2
O
x
x
2,3 - dihalobenzofuran
O
(CH3CO)2O
BF3
O
C - CH
3
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 CH
2
- 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
HBr
Br
H
HO- K+
K / OH
-HBr
+ -
OH
CH
C - CO2K
Br
+
H
-HBr O
COOH
Distile , CaO
-CO2 O

7. lec6 7COR
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
CH
2
- 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+SS
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 12N
CH
3
N
CH
3
N
CH
3
CH
3
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