Heterocyclic compound is one wich possess a cyclic structure with at least two different kinds of at

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Introduction Heterocyclic compound is one wich
possess a cyclic structure with at least two
different kinds of atoms in the rings .The most
common type, contain largely carbon atoms
Nitrogen , Oxygen and Sulphur are the most common
hetero atoms. Heterocyclic compounds are very
widely distributed in nature , and essential to
life in various ways .Most of sugar , and their
derivatives including vatamine c , Vitamin B ,
Vitamin B6 , alkaloide and antibiotic Heterocyclic
compounds can be aliphatic or Aromatic in
character depending upon their electronic
constituents. In general , the aliphatic
heterocyclic compounds are very similar to their
open chain aliphatic analognes for e.g.
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Tetrahydrofuran (1) has many properties
characteristic to diethyl ether (11) In a
similar way the aromatic heterocyclic have many
properties resembling their aromatic carbo cyclic
analogues for example pyridine (III) and benzene
(IV)
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Classification of Heterocyclic compounds A One
ring 1 Three Membered Mono heterocycles
2 Four membered Mono heterocycles
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3 Five membered Mono heterocycles 4
Five membered di heterocycles
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5 Five membered triheterocycles 6
Six membered monoheterocycles
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7 Six memberd diheterocycles 8 Six
membered triheterocycles
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B Fused rings
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Nomenclature A Heterocyclic compounds 1
Number ring start from the hetero atom 2 If
there is more than one hetero atom , so number
ring will be in the order O gt S gt N 1.e if there
is oxygen and Nitrogen in the heterocyclic
compound , number ring will be from the oxygen
(So oxygen will take No 1 and Nitrogen No 2 ) 3-
If the compound is five membered heterocyclic
will add the prefix Ole at the end
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Thiophene Thiole
2 Bromo 3 methylthiophene 2 Bromo 3
methylthiole
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Thiazole 1,3-Thiazole
Isothiazole 1,2 Thiazole
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4 If the compound is six membered heterocyclic
we add the prefix ant at the end
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B Heterocyclic compound with fused rings
1-2 a , 2-3 b , 3 4 c, 4 5 d , 5 6
e , 6 1 f
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Heterocyclic analogenes of cyclopentadiene With
one heteroatom or Five membered monoheterocyclic
The simplest of five membered heterocyclic
compound are pyrrole , furan and thiophene .We
might expect each of these compound to have
properties of conjugated diene of an amine , an
ether or sulphide .Except for certain tendency to
undergo the oxidation typical of sulphide for
example pyrrole doesn't possess the basic
properties typical of amine. Instead these
heterocyclic undergo electrophilic reaction
subistitution reaction nitration ,sulphonation,
halogenation ,Friedel-craft Acylation even The
Reimer Tieman reaction and coupling with
diazonium salts .
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Heat of combustion indicate resonance
stabilization to the extent 22 28 kcal/mol less
than the resonance energy of benzene
(36kcal/mol)but much greater than of most
conjugated diene (about 3 kcal/mol).On this basis
pyrrole , furan and thiophene must be considered
aromatic The orbital picture for example for
pyrrole each atom of the ring carbon or nitrogen
is held by s bonds the atom uses three sp2
orbitals each carbon has left one electrons
occupy p orbitals overlap of the p orbitals give
rise to p clouds one above and one below the
plane of the ring .The p cloud contain a total of
six electron the aromatic sextet extra pair on
nitrogen which response for the basisty involve
in the p cloud and its not available for sharing
with acid in contrast to most amine , there for
,pyrrole is a weak base it thus pyrrole is better
represented by
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In which the broken circle represent the aromatic
sextet pyrrole can be considered as a hybrid of
the following structure
Furan and Thiophene have structure analogous to
the structure of pyrrole
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Pyrrole
Preaperation 1- from succinimide
2 From 1,4 Dicarbonyl compound
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3 From a-aminocarbonyl compounds knorr
synthesis condensation of an a amino ketone
with a ketone possessing a reactive methylene
group a- amino ketone prepared by reducing the
corresponding oximino compound with zinc
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Chemical properties Pyrrole is a weaker base than
pyrrolideine
This is because of 1 Resonance of p
electrons 2 Type of hybrid orbitals of nitrogen
atom The pair of electron in pyrrole occupies an
sp2 orbital while in pyrrolideine occupies
sp3orbital and as we know the electron occupies
an sp2orbital is held more tightly and is less
available for sharing with acid than the pair of
electrons which occupies sp3orbital.
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Electrophilic subistitution - The reaction
takes place predominantly at the a position we
could account for this orientation of the
following basis the controlling step is the
attachment of the electrophilic reagent which
takes place in such a way as to yield the most
stable intermediate carbonium ion Attack at
position 3 yields carbonium ion that is a hybrid
of structure I and II . Attack at position 2
yields a carbonium ion that is a hybrid not only
of structure III and IV ( analogous ) to (I and
II ) but also of structure V the extra
stabilization conferred by V makes this ion the
more stable one
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Furan Synthesis 1 Feist Benary synthesis
Treating a chloroketone with ethyl
acetoacetate
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2 From 1,4 dicarbonyl compound
3 -
4
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5 - Reaction
1 Sulphonation
Furan decomposed by the usual strong reagent but
the pyridine sulphur trioxide compound under mild
condition react with furan to give 2 sulphonic
acid
2 Halogenation
Furan react vigorously with chlorine and bromine
at room temperature
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3 Acylation
Carboxylic anhydrides or carbonyl halides
normally react only in presence of Friedel
Crafts or orthophosphoric acid all electrophilic
subistitution in furan take placte at position 2
and 3 but 2 is more stable for the same reason as
pyrrole .