المحاضرة 2

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N.
.
Pyridine
Pyridine is aromatic as
there are six delocalized
electrons in the ring.
Six-membered heterocycles are more closely
related to benzene as they are aromatic on the
basis of their p-electron systems without the need
for delocalization of heteroatom lone pairs. The
empirical resonance energy for pyridine is about 28
Kcal/mol, only slightly lower than that for benzene.
SIX MEMBERED AROMATIC HETEROCYCLES
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Pyridine has divalent negatively charged N, which is
a stable condition for N. The positive charge is
dispersed to carbons around the ring, specifically to
C-2 and C-4.
Structure of Pyridine
The net effect is to reduce the p-electron density in
the ring relative to benzene, and as result pyridine
is electron deficient compared to benzene.
N 2
3
4
As a result, unlike benzene pyridine is polar
molecule due to the electronegative nitrogen.
N
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Six membered heterocycles
with an electronegative
heteroatom are generally
electron deficient compared
to benzene. Such compounds
are classified as p-deficient.
Electron-withdrawing heteroatoms decrease
the p-electron density at the carbon atoms
and are thus p-deficient relative to benzene.
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Structure of five membered heterocycles
The five-membered aromatic
heterocycle ring has a p-electron
excess (six on five atoms), while in
benzene, the p-electron density is
one on each ring atom.
Five membered heterocycles with an electronegative
heteroatom are generally electron rich compared to
benzene (six electrons for five carbons). Such
compounds are classified as p-excessive.
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36 kcal/mol
29 kcal/mol
22 kcal/mol
16 kcal/mol
The ease with which the lone pair
electron is released is directly
related to the electronegativity of
the heteroatom. Thus the lower
the electronegativity of the
heteroatom, the higher the
aromaticity.
The degree of aromatic character
in a five membered ring is
determined by the ease with
which the lone pair may be
released into the delocalized
system.
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Aromatic Heterocycles
p- Excessive p- Deficient
This classification is not trivial; there is a vast
difference between the properties of the two types
of aromatic compounds.
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Reactions of p-deficient
heterocyclic aromatic compounds
A hallmark of p-deficient heterocyclic systems
is their low reactivity with electrophilic
agents, slower than benzene. For example,
pyridine is less reactive than benzene by a
factor of 106 when subjected to conditions of
nitration. The reactivity is on the order of that
of nitrobenzene, which is well known to
require much more drastic conditions than
those for benzene itself.
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For example, 3-bromopyridine is formed when
pyridine is reacted with bromine in the presence of
oleum (sulfur trioxide in conc. sulfuric acid) at 130°C.
Conversely pyridines are susceptible to nucleophilic
attack.
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The reactivity is greater than that of benzene
and is in roughly the same range as found for
benzenes bearing electron releasing groups
such as in aniline. The greater electron density
in these rings accounts for this higher
reactivity.
A significant feature of the p-excessive ring
systems is that they undergo electrophilic
aromatic reactions faster than benzene.
Reactions of p-excessive heterocyclic
aromatic compounds
These heterocycles undergo electrophilic aromatic
substitution reactions much faster than benzene
under similar conditions. The reasons for this are:
I. The resonance energy of the heterocycles is less
than that of benzene, i.e. less aromatic than
benzene.
II. The five-membered aromatic heterocycle ring has
a p-electron excess (six on five atoms), while in
benzene, the p-electron density is one on each
ring atom.
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Reaction with bromine requires no Lewis acid and
leads to substitution at all four free positions.
no Lewis acid