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Unit 14 B
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Unit 14.2Benzene
So what is benzene?
Benzene is mostly known as raw material and used in industrial chemicals; a few examples
of this include ethylbenzene, cumene and cyclohexane. In industries such as the
pharmaceutical industry, it can use benzene as a solvent.
Benzene can come from a number of places. A few examples are volcanoes, forest fires and
crude oil. one way humans are exposed to benzene is cigarette smoke and there are very
high levels of benzene contained inside. water and food are among the items where there is
the least amount of benzene.
So what is the structure of benzene?
For many years benzene structure had confused many scientists and they did not quite
understand the structure and how it worked. the benzene structure was known to have 6
carbon atoms with 6 hydrogens attached to each one. one scientist who is famously known
and is named august Kekule was one of the chemists that were trying to figure out the
structure of benzene.at first, his initial thought was that all the carbons were in a row but
soon the idea faded away due to him realising that the hydrogens had no place to go. at this
point, he was puzzled and decided to take a nap he had a dream in this nap and had
visualised a snake spinning around clockwise with its tail in its mouth as if it was biting itself.
he imagined the snake being a benzene structure and gave him the idea of benzene being a
circular shape.
After this interesting dream, he straight away drew the structure of benzene that he believed
was the structure from his dream and two of them in this case all he had to do now was find
evidence to support his theory.
Here below is what he had drew
,Benzene has a complex structure and it is vital to understand that it has a planar
shape.in x-ray diffraction, the adjacent carbon atoms' distance is approximately
0.139nm. This distance is the intermediate between the extended or the longest c-c
bond and the smaller c=c.looking at the delocalised electrons allows us to find the
relative length of the C-C bond in benzene. This leads to the intermediate bond
lengths.
Another scientist named Linus Pauling had a different idea to Kekule's theory.his
theory includes the explanation of the stability of the delocalised electrons and
reasons explaining why the reactions of benzene are mostly electrophilic substitution
reactions.in his theory, he stated that “instead of the Kekule structures I and II shown
below we have a single structure III with the delocalised electrons shown on paper
as a circle in the middle of a hexagon.”
http://www.chm.bris.ac.uk/motm/benzene/benzeneh.htm quote taken from here
Scientists had found problems with Kekule's idea; it became clear that the enthalpy
of the hydrogenation of benzene was discovered that it was not the same value that
is found in cyclohexane with its C=C double bond. The energy that is missing from
, the hydrogenation is known as resonance energy. This energy is a measure of how
stable benzene is. The sideways overlap of electrons located in the π-bond which is
above and below the 6 carbon atoms in the ring will give the aromatic stability. the
hexagon has the electrons arranged in a circle. In benzene after a substitution
reaction has occurred benzene would be a lot more stable compared to if it had an
additional reaction this is the reason why benzene mostly undergoes substitution
reactions.
The problems with Kekule's idea
In his proposed theory of the structure of benzene, there were a number of factors
that his structure was unable to explain
The first thing his theory could not explain was that he had predicted that in his
structure there were two 1,2dibromobenzene and were both different too but in
practice, there was only one 1,2dibromobenzene to have ever been found.
The second point of his theory states that the benzene structure contains three
double bonds; it also states that the benzene's chemical properties should take after
those of alkenes. This is not the case in his theory. In reality, benzene is very stable
and is able to make substitution reactions very easily.
In benzene, all the bond lengths are of equal length however in Kekule's structure his
theory was that they were not all the same so for single bonds, he predicted that its
length would be 1.54A and for double bonds 1.34A which are both incorrect.
The bonding of benzene
The aromatic compounds have low hydrogen to carbon ratio the chemists working
on it had expected the structure to contain a large number of double or triple bonds.
Double bonds can be easily split apart by reagents that are oxidative; a few
examples include potassium or ozone and quickly adding bromine and chlorine.
These sorts of reactions were added to aromatic compounds. unexpectedly some
products had shown to have kept many of the double bonds and these were the
compounds that showed chemical stability that was high in comparison to
cycloalkenes and well-known alkenes. one treatment with permanganate solution
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