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Un it 14 assignemnt B
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- Organic Chemistry
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- PEARSON (PEARSON)
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UNIT 14: APPLICATIONS OF ORGANIC CHEMISTRYAssignment B: Aromatic ring chemistry for designer chemicals
Explain the structure of benzene using sigma and pi bonding, providing evidence for the structure.
(P2)
As soon as Michael Faraday discovered benzene in 1825, he determined its empirical formula as CH. Benzene's
formula is C6H6 and its Mr is 78, according to German chemist Eilhard Mitscherlich in 1834. However, the
structure of benzene was not fully understood until 1931.
Benzene is an unsaturated compound based on its molecular formula. The difference between it and alkenes is
that it cannot readily undergo addition reactions. There is a fundamental difference between its structure and
bonding based on this observation.
A major industrial compound that is made from coal and oil, benzene is a naturally occurring by-product of
volcanic eruptions and forest fires.
There are several important aromatic compounds that are derived from benzene, making it one of the simplest
and most organic aromatic hydrocarbons. Despite its colourlessness, this compound has a distinctive smell.
Polystyrene is mainly produced with this compound. In addition to being highly toxic, it is also known as a
carcinogen, meaning that exposure to it may lead to leukaemia.
Structure of Benzene
As shown in the figure below, benzene has the structure of an aromatic hydrocarbon. It contains 6 hydrogens
atoms and 6-carbon atoms, therefore its chemical formula its C6H6 and its mass is 78. 112 more or less. It
contains three double bonds and a hexagon-shaped six-carbon ring. A corner representing carbon atoms is
bond to other atoms.
Benzene is classified as a hydrocarbon based on its chemical formula.
,Shape:
In comparison to the three molecules shown above, benzene has a completely different structure. The carbon
atoms of benzene are not even bonded together, instead forming a hexagon with one hydrogen atom and two
other carbon atoms. Benzene is symbolised as follows:
Angle:
There are two carbon-carbon bonds and one carbon-hydrogen bond between each carbon atom in benzene.
(C-C bond and C-H bond). By spreading themselves out as far as they can, they try to minimize the distance
between them. Consequently, each bond has a 120° angle. As a result, benzene forms a planar molecule of
trigonal shape.
Kekulé’s structure of benzene
An article on the structure of benzene was published by Friedrich August Kekulé in 1865. For years, scientists
had been puzzled by this mystery. The dream led him to conclude that benzene has a cyclic nature because he
associated it with snakes biting their own tails. The next diagram shows benzene's C-C single bond alternating
with its C=C double bond, as Kekulé proposed. Cyclohexa-1,3,5-triene is the systematic name for this molecule.
, Problems with the Kekulé’s structure:
Problems with the chemistry
It is reasonable to expect that benzene has the same reactions as ethene as a result of its three double bonds.
In addition reactions, Ethene, however, links additional atoms with electrons when the bond between the
carbon atoms breaks.
This is rarely the case for benzene. The hydrogen atoms are usually substituted by new ones in substitution
reactions.
The low reactivity of Benzene
As it has 3 double bonds, the Kekulé model cannot explain why benzene is less reactive than alkenes, as both
are capable of polarizing molecules and reacting with nucleophiles. There is also no reaction between the
double bonds and bromine water (the test for double bonds), so this test is not successful
Electrophilic addition reactions
The bromine water test is a common method for testing alkenes. As the bromine atoms attach to the
hydrocarbon in an electrophilic addition reaction if there is a C=C double bond, the water will become
decoloured. Benzene, which has three C=C double bonds, would react like this according to Kekulé's structure.
In contrast, it does not - the solution remains red-brown when it is mixed with bromine water. As a result,
double bonds cannot be found in benzene.
Isomeric products/ equilibrium model:
According to Kekule (the rapid equilibrium model), benzene can convert from two forms within a short period
of time (the rapid equilibrium model).
As the benzene structure was changing rapidly, he suggested that it was possible that this could explain
benzene's low reactivity.
Associate two carbon atoms with each other in benzene. As an example, consider substituting bromine for
hydrogen atoms. There's a double bond between the two affected carbons in one isomer of benzene, and a
single bond between the two isomers in the other. Below is an example.
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