Benzofuran (coumarone)

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.
2
Chemical reaction Oxidation Benzofuran is much
less easily opened than that of furan , but it
can be spilt by the two routes outlined .
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 .
4
Treatment of benzofuran with halogens result in
2,3 addition product with initial electrophilic
attacke taking place at c -2
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 .
5
Reduction Benzofuran is easily reduced by
hydrogen over Palladium or sodium and alcohol the
2,3 dihydroderivative ,which is also known as
coumaran
coumaran -2-one is the lacton of 2
hydroxyphenyl acetic acid
6
Synthesis of Benzofuran 1- A useful laboratory
synthesis starts with coumaran ,and is generally
applicable
7
2- An internal claisen reaction may be used to
bring about ring closure .
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
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 .
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
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 .

12
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 .
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 s bonds . The third sp2 orbitals on each carbon
atom overlap with an S orbital of hydrogen to
form the C H s 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 s bonds.
14
The latter overlap between two p orbitals
produced a delocalized p molecular orbital
containing six electrons (Aromatic character )
one half of this p molecular orbital lies above
and the other half below the plane of s bonds .
Pyridine show some aromatic properties because
the resulting molecular orbitals satisfies the
Huckel's rule ( n 1 in 4n2) . 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
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