Video Transcript
What chemist is often credited with
proposing and publishing the correct structure of benzene?
German chemist Friedrich August
Kekulé was the first to propose a chemical structure of benzene. Kekulé initially hypothesized that
benzene was a six-membered hydrocarbon ring with an alternating pattern of single
and double bonds between the carbon atoms. According to legend, Kekulé came up
with the idea of this cyclical structure of benzene after having a dream about a
snake eating its own tail. Later, Kekulé modified his
hypothesis about the structure of benzene. In 1872, he claimed that the
structure of benzene was not static. He proposed that the carbon–carbon
double bonds and single bonds swapped their locations on the ring rapidly and that
these two structures were in a dynamic equilibrium with one another.
In Kekulé’s simple model of
benzene, electrons are only located between atoms in the single and double bonds
that are present. Scientists later changed Kekulé’s
model to better explain the physical and chemical properties of benzene. A more accurate model of benzene
includes delocalized electrons. Instead of being found only between
atoms, delocalized electrons can spread out over part or all of an entire
molecule. In the case of benzene, there is a
ring of delocalized electron density. Each carbon atom in the benzene
ring is sp2-hybridized. With its three sp2 hybrid orbitals,
each carbon atom can form three 𝜎 bonds. Three of the valence electrons of
each carbon atom are found in sp2 hybrid orbitals. Therefore, each carbon atom can
form two 𝜎 bonds with neighboring carbon atoms and a third 𝜎 bond with the
hydrogen atom.
It’s important to note that, here,
we’re only showing the valence electrons and hybrid orbitals of the carbon atoms,
not the hydrogen atoms. The valence electrons involved in
the 𝜎 bonds are pretty much located between the atoms. Now, what’s very interesting is
that the fourth valence electron of each carbon atom is found in an unhybridized P
orbital. The valence electrons found within
these unhybridized P orbitals interact and form 𝜋 bonds that spread out over the
entire ring structure.
Although we now know that the
delocalized electron model of benzene is more accurate than the model that Kekulé
originally proposed, he is still credited with proposing and publishing the correct
structure of benzene.
