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BTEC applied science unit 14 b benzene
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BTEC applied science unit 14 b benzene . I received a distinction first time around
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Unit 14: Applications of Organic ChemistryLearning aim B: Understand the reactions and properties of aromatic compounds.
Assignment title Aromatic ring chemistry for designer chemicals
P2: Explain the structure of benzene using sigma and pi bonding, providing
evidence for the structure.
What is Benzene ? –
Benzene is primarily used as a feedstock, or raw material, to make other industrial chemicals,
such as ethylbenzene, cumene and cyclohexane. Benzene is also used as a solvent in the
chemical and pharmaceutical industries.
Most benzene exposure comes from the air from a number of sources, including forest fires,
auto exhaust and gasoline from fuelling stations. Benzene in cigarette smoke is a major
source of exposure. Very low levels of benzene have been detected in fruits, vegetables, nuts,
dairy products, eggs and fish. Most people are exposed to only very tiny amounts of benzene
from water and food.
Structure of Benzene:
For a quarter of a century following its discovery, benzene's structure continued to puzzle the
world's greatest scientists. It was known to have a molecular weight of 78 which was due to
the presence of six carbon atoms (6 x 12) plus six hydrogen atoms (6 x 1). August Kekule
was one of the organic chemists who was working on the structural elucidation of benzene.
He originally placed all six carbon atoms in a row but soon realised that this did not make
sense for where was he to put all the hydrogen atoms? The story goes that whilst trying to
solve the problem of benzene's Kekule had a daydream whilst dozing on a London bus.
The old saying goes that: "you
wait ages for one bus, then
three come along at once". In
Kekule's case, six came along.
Kekule visualized a snake with its tail in its mouth that was spinning around. That snake was
benzene, biting itself in the tail, which made sense if it possessed alternating single and
,double bonds. Kekule's dream is shown as an interesting animation. Kekule's dream of a
snake eating its own tail, is an ancient symbol called the Ouroboros which represents the
cyclicality of life. One should bear in mind that this 'story' first appeared in the
journal Berichte der Durstigen Chemischen Gesellschaft, which is a parody of the respected
journal Berichte der Deutschen Chemischen Gesellschaft. What is clear is that Kekule's
understanding of the tetravalent nature of carbon was built on the foundation of the often
overlooked work of Archibald Scott Couper. Every student who has ever drawn covalent
bonds as lines on paper joining atoms is following in Couper's footsteps. The former East
Germany (DDR) commemorated the work of Kekule on a stamp which shows his structure of
benzene.
Kekule drew what he believed to be two identical structures for benzene and now he needed
to find proof that his 'daydream' was correct. Rod Beavon of Westminster School has written
an online biography of Kekule.
Kekule structures for benzene
It is important to realise that benzene has a planar structure. The distance between adjacent
carbon atoms found by X-ray diffraction is 0.139 nm (139 picometres). This is a distance
which is intermediate between the longer single C-C bonds (147 pm) and the shorter double
C=C bonds (135 pm). The relative length of the C-C bonds in benzene can be explained in
terms of the delocalized electrons, which leads to the intermediate bond lengths. The cyclic
nature of benzene was finally confirmed by the eminent crystallographer Kathleen Lonsdale.
, Molymod of benzene.
In 1931 Linus Pauling proposed his resonance theory which describes delocalised
electrons and is able to account for benzene's known reactions. This theory explained the
stability of the delocalised electrons (lower energy) and the reason why benzene's reactions
are mainly electrophilic substitution reactions. Pauling's theory states 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.
Problems with Kekule's structure were first hinted at when it became apparent that the
enthalpy of hydrogenation of benzene (-208 kJ mol-1) was found not to be three times the
value found for cyclohexene (-121 kJ mol-1) with its one C=C bond. The 'missing' energy of
hydrogenation (155 kJ mol-1), is called resonance energy, and is a measure of benzene's
stability. The aromatic stability comes from the sideways overlap of electrons in the π-bond
above and below the six carbon atoms in the ring. The delocalised electrons are shown as a
circle in the hexagon. The reason substitution is preferred is that benzene and its derivatives
are more thermodynamically stable after a substitution reaction than if an addition reaction
took place. For those who realise the bond order in benzene is in fact 1.5 then another way to
represent the structure of benzene IV is as a hexagon with a dotted circle inside of the
hexagon. Like the spelling of sulfur, the drawing of benzene can also lead to debate amongst
chemists.
Problems with Kekule’s idea –
The structure proposed by Kekule was unable to explain the following facts.
The Kekule structure predicts that there should be two different 1,2-dibromobenzene.
But in practice, only one 1,2-dibromobenzene has ever been found.
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