Aromatic Hydrocarbon
The term aromatic derived from Greek word aroma means fragrant.
It was originally used in organic chemistry. They have least hydrogen to carbon
ratio in their molecular formula. They have characteristics odour. Most of the
organic compounds have six carbon rings in their molecule. Benzene is considered
as the parent member of class of aromatic compound.
Benzene and
those compound resembles the benzene in their chemical behavior are called Aromatic Hydrocarbons.
Structure
of benzene
The Molecular formula of benzene C6H6
determined by the elemental analysis and molecular weight determination. The
molecular formula of benzene is C6H6 . Benzene molecule
is short of eight hydrogen as compared to a saturated hydrocarbon with same
number of carbon atoms. It is considered to be highly unsaturated molecule. It
must therefore have four double bonds, two triple bonds two double bond and one
triple bond, a ring with three double bends or any other combination. Other
structures with different positions of double and triple bond could be
included.
The
kekule’s Structure
In 1865, Kekule proposed a ring structure for
benzene which is
A
cyclic planar structure of six carbons having alternate double and
single bonds. Each of the six carbons was attached to one hydrogen.
The
Dewar Structure
Sir James Dewar suggested a structure of benzene
which is given:
A
hexagonal ring of six carbon atoms with two opposite atoms joined by a long
single bond across the ring and with two double C-C bonds, one on each side of
the hexagon.
Dewar structures contribute to the resonance hybrid
of benzene.
Resonance
Hybrid Structure:
Structure of benzene is a resonance hybrid of the
two canonical structures.
In quantum mechanical terms, the
blending effect of resonance in the Lewis approach to bonding is the
superposition of wave functions for each contributing canonical structure.
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