ASPIRANTSHUB.IN
Aromatic
Hydrocarbons:-
The
hydrocarbons which exhibit aromaticity are
called
aromatic
hydrocarbons. The compounds are classified into
two
types
namely, benzenoid and non-benzenoid
hydrocarbons.
(a) Benzenoid Hydrocarbons
Benzene is the simplest benzenoid hydrocarbon. All compounds which, contain benzene-like ring structures and have properties similar to benzene, are called benzenoid hydrocarbons.
(1)
Benzene (C6H6)
It has a planar hexagonal ring structure containing six carbon atoms. It has three alternate double bonds and single bonds. The 6 pi electrons are delocalized on the entire ring.It obeys
Huckles rule for the value of n = 1 i.e., 4n+ 2 = 6. Therefore it is aromatic.
It does not undergo the expected addition reactions with HCl ,HBr etc. Rather it gives substitution reactions in which
Molecular
Orbital Structure of Benzene:-
In benzene
molecule all the six carbon atoms are sp2
hybridized.Due to
this
a hexagonal ring
is formed with an angle of 1200
between C—C bonds. Each
carbon atom
forms a C—H sigma bond by sp2-s
overlap.
The
hexagonal ring
and all the
C—H bonds are in the same plane. Each
carbon atom has a pure p orbital,perpendicular to the plane of the ring,
having one electron each. The adjacent p orbitals overlap side by
side to form three pi bonds.This results
into two
circular
shaped bonds, above and below the ring. This
participation of p-orbital electrons in
several bonds is called
delocalization.
(2) Naphthalene (C10H8)
According
to
resonance theory. a molecule. ofnapthalene can
he considered to
be a
hybrid of
3 kekules structures ..
The
fusion of two benzene rings takes place here. The two carbon atom
common to both rings are called
the annular carbon atoms and carry no
H-atoms.There
are10
carbon atoms which lie at the corners of two fused
hexagons.
All
the carbon atoms are sp2 hybridised and each of them
bears one
electron in
p
orbital which is perpendicular to the molecular plane
on which C and H atoms lie. Thus 10 parallel
p-oribitals
overlap with each other to
form circular
delocalised pi
molecular orbitals above and below of both the hexagonal
rings.Therefore
it is aromatic.
(3)
Anthracene
(C14H10)
It is a
tricyclic hydrocarbon in which three benzene rings
are
fused together in linear
manner.
Anthracene has 14
pielectrons which are
delocalized
. For the
Huckel number 3, the number of pi
electrons
required as
per the Huckels rule is 4 x 3 + 2 =14.
Therefore
anthracene satisfies Huckels rule for n =
3, so it is aromatic.
It has four resonance structures.
All
14
carbon
are
sp2 hybridised. The sp2
orbitals of adjacent carbon atoms overlap with each other forming
C—C
sigma bonds.
Now, each of them bears one
electron in p-orbitals which lies
perpendicular to molecular
plane
on which C
and H atoms lie. Thus 14 p
orbitals
overlap with each other to give delocalised circular
pi electron cloud above and below the plane
of rings.
(4) Phenanthrene (C14H10)
It is a tricyclic benzenoid hydrocarbon in which the three benzene rings are fused in an angular manner forming a non-linear molecule.It is an isomer of anthracene. It has 14 pi electrons which are delocalized forming a closed ring. When n = 3, 4n + 2 = 14. Hence it satisfies Huckels rule for n = 3, which is a Huckel number. Hence phenanthrene is aromatic. It has five resonating structures.
It has 14 carbon atoms which are sp2 hybridized. They overlap to from C—C sigma bonds, resulting in three six membered rings. Now each carbon has a p orbital, with one electron, which is perpendicular to the plane of the rings. They overlap laterally on both sides ,forming circular electron clouds above and below the plane of the rings. Hence it is aromatic
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>>Aromaticity
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>>Huckels Rule, Resonance Energy
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>>Aromatic
Hydrocarbons
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>>Benzenoid Hydrocarbons ----------------------
>>Non Benzenoid Hydrocarbons
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>>Anti-Aromaticity
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>>Anti-Aromatic compounds
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