4. Pyrrole:
INTRODUCTION:
PYRROLE is a heterocyclic aromatic
organic compound, a five-membered ring
with the formula ( C4H4NH ). It is a
colourless volatile liquid that darkens
readily upon exposure to air. Substituted
derivatives are also called pyrroles.
5. Resonance structure :
The lone pair on nitrogen is in the p orbital so
it is involved in the 6 pi-electron aromatic
system. Hence pyrrole is not very nucleophilic
and is only weakly basic at nitrogen. Looking at
the HOMO of pyrrole the lobes are much
bigger at the 2- and 5- positions, this indicates
that the reactions are most likely to take place
at these positions
6. The resonance contributors of pyrrole provide
insight to the reactivity of the compound.
Like furan and thiophene, pyrrole is more reactive
than benzene towards electrophilic aromatic
substitution because it is able to stabilize the
positive charge of the intermediate carbocation.
7. SYNTHESIS OF PYRROLE:
Pyrrole could be obtained through the
following reaction: It could be achieved by
treating Furan with Ammonia with the
exsitence of solid acid catalysts.
9. Acidic property of pyrrole:
Due to participation of N’s lone pair in
resonance/aromaticity pyrrole has
exceptional strong acidic property.
(that is,it can loss the hydrogen attached
with NITROGEN when reacting with any
base)
10. Acidic property of pyrrole:
PYRROLE can react with
Strong base,
Grignard reagent,
potassium metal etc to give salt like
compounds.
11. Sensitivity towards strong acids:
Pyrrole is sensitive towards strong acids.
This is due to protonation occur at one of the
carbon and the resulting protonated molecule
will add to another unprotonated pyrrole
molecule this continues until a pyrrole trimer is
formed.
The reaction is considered as electrophilic
addition of pyrrole.
12. ELECTROPHILIC SUBSITUTION
IN PYRROLE:
Pyrrole is reactive towards electrophilic substitution
reaction.
It is more reactive than benzene because of the
resonances that pushes away the electron density from
nitrogen towards carbons ,thus making the ring electron
rich.
The substitution is easier and mild reagents can be used.
14. Preferable position is C-2, the carbon
next to the heteroatom.
If there is an already substitution on C-2
then C-3.
The first substitution is on C-2 because it
has more stable intermediates(it stabilizes
three resonances structure)
15. The intermediate resulted from C-3
attack is stabilizes by two resonance
structure.
18. Introduction:
Pyridine is a basic heterocyclic organic
compound with the chemical formula C5H5N.
It is structurally related to benzene, with one
methine group (=CH-) replaced by a nitrogen
atom. Pyridine have 5 carbon atoms and one
nitrogen atom. All are sp² hybridized.The p-
orbital of nitrogen and all carbon atoms lie in
the same plane. Overlapping of p-orbitals result
in delocalization of six π-electrons in the cyclic
ring, following Hückel rule, imparts aromatic
character to pyridine
19. History
Impure pyridine was undoubtedly prepared by
early alchemists by heating animal bones and
other organic matter, but the earliest
documented reference is attributed to the
Scottish scientistThomas Anderson. In 1849,
Anderson examined the contents of the oil
obtained through high-temperature heating of
animal bones.Among other substances, he
separated from the oil a colorless liquid with
unpleasant odor, from which he isolated pure
pyridine two years later. He described it as highly
soluble in water, readily soluble in concentrated
acids and salts upon heating, and only slightly
soluble in oils.
20. Occurrance
Pyridine is not abundant in nature. In daily life,
trace amounts of pyridine are components of
the volatile organic compounds that are
produced in roasting and canning processes,
roasted coffee, potato chips, sunflower honey
etc
21. SYNTHESIS
Hantzsch pyridine synthesis
It is a multi-component organic reaction between an
aldehyde such as formaldehyde, 2 equivalents of a β-
keto ester such as ethyl acetoacetate and a nitrogen
donor such as ammonium acetate or ammonia.The
initial reaction product is a dihydropyridine which can
be oxidized in a subsequent step to a pyridine.The
driving force for this second reaction step is
aromatization.
22. Chemical properties
Pyridine is miscible with water and
virtually all organic solvents
It is weakly basic
pyridine behaves both as a tertiary amine
23. Resonance structure:
Electron density is on nitrogen as its lone
pair is not taking part in resonance
Positive charge occur on ortho and para
carbons.
24. Basic property:
The nitrogen center of pyridine features a
basic lone pair of electrons. Because this
lone pair is not part of the aromatic ring,
pyridine is a base, having chemical
properties similar to those of tertiary
amines.
Pyridine can act as Lewis base, donating
its pair of electron to a Lewis acid as in
the sulfur trioxide pyridine complex.
25. Pyridine as nucleophile:
Pyridine is a nucleophile as NITROGEN
because its lone pair is not delocalized,
26. The ring as nucleophile :
Electronegative atom like nitrogen lowers
the energy of the ring, this means a less
reactive nucleophile
.
But LUMO means a more reactive ELECTROPHILE
The ring as ELECTROPHILE:
27. BenzeneVS pyridine:
It is less reactive than benzene in
electrophile substitution.
But nucleophile substitution which is hard
for benzene comes easy for pyridine.
28. Electrophile substitution:
Many electrophilic substitutions on
pyridine either do not proceed or
proceed only partially they lead only to
the addition at the nitrogen atom.
