7.2 bonding in benzene 7.2

1. Original slide
prepared for the
Benzene
Formula C
6
H
6
unsaturated

2. Original slide
prepared for the
Benzene
•Its molecular formula, C
6
H
6
, was established in 1834,
after the compound's discovery by Faraday.
•The formula of benzene (C
6
H
6
) caused a mystery for
many years after its discovery, as no explanation had
been found that could account for all the bonds —
carbon usually forms four single bonds and hydrogen
one.
•The chemist Friedrich August Kekulé was the
first to deduce the ring structure of benzene (1865).
•The cyclic nature of benzene was confirmed by the
crystallographer Kathleen Lonsdale.
•

3. Original slide
prepared for the
Benzene
•Using X-ray diffraction, researchers discovered that all
of the carbon-carbon bonds in benzene are of the same
length, and it is known that a single bond is longer than
a double bond.
•In addition, the bond length, the distance between the
two bonded atoms in benzene is greater than a double
bond, but shorter than a single bond.
•There seemed to be in effect, a bond and a half between
each carbon.

4. Original slide
prepared for the
Benzene
•Benzene is also a planar molecule (all the atoms lie
in one plane). Each carbon atom is bonded to its
neighbor with one electron from each atom. Since
each atom has two neighbors, this uses two electrons
from each atom.
•Another electron from each carbon is used to bond
the hydrogen attached to it.
•The remaining six electrons orbit the atomic nuclei at
right angles to the plane of the ring and also overlap
each other, consequently blurring their orbits both
above and below the ring ( 3 electrons in upper ring
and 3 electrons in down ring).

5. Original slide
prepared for the
Benzene
•As a result, the electrons are shared equally
between the carbon atoms and exist as two clouds,
one above and one below the plane of the carbon
ring.
•Since these latter six electrons are not confined to
specific carbon atoms, they are said to be
delocalised.
•This is usually represented in structural formula as
a hexagon with a circle in the centre to represent
the shared nature of the electrons

6. Original slide
prepared for the
Bonding in benzene
C C
C C
C C H
HH
H
HH
6 electrons in a
delocalised p bond
above and below
the plane of the
atoms

7. Original slide
prepared for the
References
Steve Lewis for the Royal Society of Chemistry
Benzene