The first reasonable structure for Benzene was theorised by German chemist
Kekulé in the mid-19th century. The structure he theorised was that the 6 C
atoms were bonded cyclically in a hexagonal shape with alternating double and
single bonds, with a single H atom attached to each C atom. Although accepted at
the time, the model had significant issues, which suggested a different structure.
Kekulé’s model of Benzene in Skeletal form
Firstly, the fact that Benzene possesses three double bonds would suggest that it
should react vigorously by electrophilic addition as other alkenes would; this
however is not the case. Instead Benzene reacts by nucleophilic substitution
losing its H atoms in exchange for new atoms from the heterolytic fission.
Secondly, the length of the bonds would all be different as σ and π/σ bonds have
slightly different lengths, which would have caused a slightly different irregular
hexagon shape. However, in reality the bonds are identical in length.
Thirdly, Kekulé’s model would theorise Benzene as being extremely unstable so
it would readily react, however Benzene is actually extremely stable.
Scientists have therefore revised the Kekulé model significantly using modern
science and knowledge to account for the issues with the original model. In order
to understand this model we must first consider how the C atoms actually bond
with other atoms covalently. Using a basic GCSE model we would understand
that Carbon has 4 free electrons available for bonding, however our AS model of
sub orbitals suggests that C only has two free electrons for bonding. This is
because due to Pauli’s exclusion principle electrons will fill orbitals of the same
type one electron at a time and so only the two P electrons are free for bonding.
Think of sub orbitals as bus seats and electrons as people, all the double seats
would be taken up and then people would fill the spare seats once no double
seats were free.
However, there must be a way to overcome this, as we know that C makes 4
bonds. The solution is simple; the energy gap between an S and P orbital is
relatively small so a small amount of energy is used to promote an S electron to a
P electron, which will occupy the empty 2Pz orbital. C is now described as being
in an excited state. This means four free electrons are available however bonding
cannot yet take place. This is because bonds of different lengths would be
produced, as the S electron would bond slightly differently compared to the
three P electrons, and we know that the C to C bonds in Benzene must be equal in
length so something must happen.
This key change is the hybridisation of 3 of the electron orbitals to form 3 sp2
orbitals and a single P orbital. Not all orbitals undergo hybridisation in this
instance as each Carbon is only bonded to three other atoms. The hybrid orbitals
position each other 120 degrees from each other in the same plane and the P
orbital remains perpendicular to them.
These hybrid orbitals are then used to make sigma bonds with 2 C atoms and a H
atom, whilst the P orbital constructs a Pi bond with a neighbouring C atom.
However the P orbital in one C, overlap with both neighbouring P orbitals to
form a continuous region of space above and below the nuclei in which the π
bonded electrons can be found. Consequently, as the electrons can now be found
anywhere within these regions and as they are no longer fixed to one pair of
carbons, we can consider the electrons to be delocalised. However, each pair of
electrons follow a molecular orbit within this region of space.
A diagram depicting the combining of P orbitals to form one of the three
delocalised molecular orbits.
Sourcehttp://chemwiki.ucdavis.edu/Theoretical_Chemistry/Chemical_Bonding/Valence
_Bond_Theory/Resonance/Delocalization_of_Electrons
Now our molecule of Benzene is bonded we can see if it accounts for the issues of
Kekulé’s original model.
Firstly, the Benzene of this model is a regular Hexagon due to all the atom to
atom bonds being identical and this matches the initial data that all of the bonds
were equal; therefore the issue of Benzenes shape is resolved.
Secondly, the delocalised electrons of this model give the molecule a much
higher stability as the electrons are more spread out. This resolves the original
models issue of Benzenes stability.
Thirdly, this model would mean it is very difficult for addition reactions to take
place as in order for new atoms to join the molecule some of the delocalised
electrons would need to be used. This would destabilise the molecule and as the
molecule is so stable to begin with it would be difficult to push it to this point.
Therefore, it would be much more likely that H would instead be replaced with a
foreign atom and undergo nucleophilic substitution. Therefore, the issue of
Benzenes unwillingness to undergo electrophilic addition is resolved.
In summary, the new model of Benzene possessing hybrid orbitals used in
bonding and delocalised electron orbits rather than Pi bonds resolves the
problems with Kekulé’s original model.
The new continuous ring model of Benzene in
skeletal form.