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chemistry Unit 14 learning aim B
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chemistry Unit 14 learning aim B
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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:
What is benzene?
The organic chemical compound benzene, commonly known as cyclohexatriene, has the molecular
formula C6H6. Benzene is a planar molecule made up of six carbon atoms grouped in a hexagonal ring,
that are each connected to a hydrogen atom. The carbon-carbon bonds are of the same length, and
because they only contain carbon and hydrogen atoms, benzene is categorized as a hydrocarbon.
Benzene is a colorless liquid that is produced naturally by forest fires and volcanoes, as well as a major
industrial chemical derived from oil and coal.
The chemical properties of benzene are:
Melting point 5.5 degrees Celsius
Boiling point 80 degrees Celsius
It is a clear colorless liquid
Less dense than water and soluble in water
The structure
It has a six-carbon ring that is represented by a hexagon, three double bonds, six hydrogen atoms, and
six carbon atoms with an average mass of 78 g/mol. All the atoms in the benzene structure are
hydrogens, and the structure's double bond is split by a single bond. The 6 pi electrons in the hexagon
are represented by a circle. Based on its structure and formula, benzene is classified as an aromatic
hydrocarbon. There are 6 carbons in benzene and 6 sigma bonds with 3 pi bonds. In bonding electron
,pairs, each carbon atom contains four electrons. Three of them are involved in sigma bonds, while the
fourth is involved in a pi bond.
The Hybridisation of benzene
In the benzene structure, each carbon atom will have one electron in the atomic p orbital. This is
because carbon atoms employ three of their electrons to form three sigma orbital links with the atoms
around them. As indicated in the diagram, negative charges will build across each plane. A delocalized
pi-electron system is produced as a result. The electrons are not evenly distributed throughout the
carbon atoms, unlike in kekule's structure. When the p electrons combine, three C-C double bonds are
created, including a sigma bond and a pi bond.
Delocalisation of benzene
Delocalization is the impermanence of electrons and bonds. Alternatively put, when there aren't any
double or single bonds. A pi bond is produced when the orbitals of two pairs of electrons cross.
Therefore, a delocalized pi bond is a bond that can take on various shapes. The six-member ring of
benzene has three double bonds. If the benzene's six carbons and hydrogens are immovable, the three
double bonds might be in two different places. As a result, it is referred to as being delocalized and is
thought to be in all conformations. The delocalized electrons allow the molecule to be stabilized.
Kekule’s structure
In 1865, August Kekule visualized the benzene ring structure in his dream. Because there had previously
been no structure for benzene, Kekule's structure was the first reasonable and sensible structure.
, He claimed that every other atom in the ring has a double bond in his structure. This ensures that each
carbon atom has four bonds. Between carbon 1 and 2, carbon 3 and 4, and carbon 5 and 6, there is a
double. Due to benzene's three double bonds, we would anticipate it reacting similarly to ethene:
ethene experiences an addition process in which one of the two bonds connecting the carbon atoms
breaks, and the electrons are used to link with other atoms. Benzene, on the other hand, rarely does
this. Substitution reactions in which one of the hydrogen atoms is replaced by something new are
common in benzene.
The lengths are also a concern, because all carbon-carbon bonds are supposed to be the same length.
Benzene should have a carbon-carbon double bond and a carbon-carbon single bond, with the C-C bond
length being 147 pm and the C=C bond length being 135 pm, according to Kekule's structure.
Because the carbon-carbon bond length in benzene is 140 pm, this demonstrates Kekule was wrong.
Furthermore, the pi-electron system is constant and equally distributed, and all carbon-carbon bonds
are the same length. This is not reflected in the structure of Kekule. The hexagonal structure of benzene
is perfect.
Thermochemical analysis
For one mole of gaseous benzene with Kekulé's structure, the predicted enthalpy of formation from its
component elements, carbon and hydrogen, in their standard states, is +252KJmol-1. it was only
+82KJmol-1 when the actual enthalpy of formation was tested. This is a far low value which suggests
that the benzene's structure is more reliable compared to Kekule’s model.
x-rays
The x-rays show that the actual bond length measurements show that there are six carbons. All carbon
bonds are equal in length. The electron density mag has shown that a symmetrical distribution of the
electron clouds that fitted a symmetrical hexagonal planar ring of six carbon atoms. X-ray also showed
all the bond angles that are 120 which are C-C-C- or C-C-H. This is because of 3 groups of electrons bond
in a trigonal planar arranged around each carbon atom of the benzene. So conclusively the benzene is a
planar molecule that gives the benzene and aromatic ring a symmetrical hexagonal shape.
A chemist named Kathleen Lonsdale in 1920s, from x-ray diffraction proved
that all the six internal bonds angles of hexachlorobenzene and
hexamethylbenzene were precisely 120, since the molecule was derived from
benzene, and it seems illogical to not assume that benzene had the same
perfect hexagonal ring of carbon atoms.
Infrared
Another method that links to the bonds inside a chemical is infrared. Each bond in a compound
has/stores energy, although to varying degrees. The energy levels of each form of electromagnetic wave
varied as well. The bonds absorb the energy as the waves strike them as they travel through the sample
and encounter them. The sample now vibrates because its energy level has increased. When most of the
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