The orbital structure of benzene consists of six carbon atoms in a ring, with each carbon atom containing one unhybridized p orbital perpendicular to the plane of the ring. These p orbitals overlap side-to-side between adjacent carbon atoms, forming two delocalized pi molecular orbitals above and below the plane of the ring. This delocalization of the pi electrons imparts unique stability to the benzene molecule. The pi molecular orbital diagram of benzene contains six pi orbitals arising from the six carbon p orbitals, with the lowest energy orbitals having zero nodes and the highest energy orbital having three nodes dividing the ring into three sections.

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UNIT I: Benzene and its
derivatives
Orbital picture

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Each carbon in the benzene ring is
sp2 hybrized with a p orbital
perpendicular to the ring plane
Being planar and cyclic allows
benzene's p orbitals to undergo cyclic
overlap

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The orbital structure of
benzene:
All the carbon atoms in
benzene are sp hybridised.
The three sp hybridorbitals are
lying in one plane and oriented
at an angle of 120°.
There is six sigma C-C bonds
and six sigma C-H bonds.
There is still one un hybridized
2p orbital on each carbon atom.
This orbital consists of two
lobes, one lying above and the
other below the plane of the
ring.

4. The unhybridized 2p orbital on each carbon atom can overlap sidewise with the
2p orbital of the two adjacent carbon atoms in two different ways as shown below
giving rise to two sets of -bonds.
Since 2p orbital on any carbon atom can overlap sideways with the 2p orbital on
adjacent carbon atom on either side equally well, a continuous -molecular 3
orbitals will result which embraces all the six p-electrons as shown:

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 The net result is that there are two continuous
rings-like electron clouds, one above and the
other below the plane of atoms as shown. This
delocalisation of π-electrons imparts unique
stability to the benzene molecule.

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Pi Molecular Orbital Diagram For Benzene

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The number of pi molecular orbitals in the pi-system equals the number
of contributing atomic p orbitals. For butadiene (n=4) we saw that the
energy levels of the pi system stacked like a four-story apartment
building. Both hexatriene and benzene have six contributing p-orbitals (n = 6),
so we should expect six pi orbitals for each.
Building The Pi Molecular Orbital Diagram For Benzene
Six p orbital's in the pi systems of benzene will produce six pi
molecular orbitals.
The Lowest-Energy Molecular Orbital's Of Benzene Have Zero
Nodes
phases of all p-orbitals aligned the same way. It doesn’t matter if
you draw the “shaded” or “white” lobes up or down, so long as
they are all drawn the same way
Pi Molecular Orbital Diagram For Benzene

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What about benzene? This is where things get
interesting.
Benzene Has Nodal Planes. The Maximum Energy Level Has 3 Nodal
Planes
In the case of cyclic systems, the (n–1) rule fails
The highest energy level has (n–1) nodes
The Highest-Energy Molecular Orbital's Of Benzene
The Highest Energy molecular orbitals have p orbitals with completely
alternating phases.

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This orbital has zero overlap between adjacent p orbitals and therefore
electrons in this orbital have the minimum possible delocalization. They are
therefore the highest energy

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Where Do We Place The Nodes In The Intermediate Energy Levels Of
Benzene?
As we said above, the tricky thing in building pi molecular orbitals is knowing where
to put the nodes in the intermediate levels.
For hexatriene, the second floor (one node) is fairly straightfoward: we put the node
in the centre, like this:
It’s impossible to draw a cyclic pi system with one node, but we can draw a
system with one nodal plane. Here, for instance, we’ve drawn a nodal plane
that cuts through two of the single bonds:
a second way to do it.
We can also draw a
nodal plane through
the atoms
These two molecular orbitals (π2 and π3 ) have the same number of nodal planes,
and therefore have the same energy

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This is really the key
difference in the molecular
orbital picture of a cyclic
system versus an acyclic
system: two units can co-
exist on the same floor. For
benzene, that results in a
lowering of energy.

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Here’s the third, fourth, and
fifth “floors” for the hexatriene
pi system, which have two,
three, and four nodes,
respectively.
For benzene, the next level up has two nodal planes. Again, there’s two ways to do it:
cut through the bonds, or cut through the atoms. Again, these are of the same energy.

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