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Mohammed Salam Unit 14: LA B
Aromatic ring chemistry for designer chemicals
The accepted structure of benzene in terms of hybridization, sigma and pi bonding and delocalization
of electrons.
Hybridization:
Benzene is a hydrocarbon composed of six carbon atoms linked in a ring with one hydrogen atom
bonded to each. Benzene hybridization is of the sp2 type. In benzene, each carbon atom is connected to
two other comparable carbon atoms rather than just one, and each carbon atom creates sp2 hybrids
with two other carbons and one hydrogen atom. The carbon atoms in benzene are sp2 hybridised,
which means they have three hybrid orbitals and one unhybridized p orbital. The remaining cyclic array
of six p-orbitals (one on each carbon) overlap to form six molecular orbitals, three bonding and three
anti-bonding. The bond angle in benzene is 120 degrees, and the geometry is trigonal planar. The six
carbon-carbon bonds in benzene are of the same length, and the molecule has a planar hexagonal
structure.
Sigma and Pi bonding:
Benzene is a cyclic hydrocarbon with six carbon and six hydrogen atoms. Benzene's structure can be
described using sigma and pi bonding.
There are six sigma bonds between carbon atoms and six sigma bonds between carbon and hydrogen
atoms, for a total of 12 sigma bonds. Each carbon atom forms sigma bonds with two other carbon atoms
and one hydrogen atom by using the sp2 hybrid orbitals.
Pi bonds are: Because benzene has three alternating double bonds, it is extremely stable. Each carbon
atom possesses three electrons in bonding electron pairs, three of which contribute to sigma bonds and
one to pi bonds.
These pi bond p-orbitals can overlap above and below the plane of the carbon ring to generate a
delocalized system with free-moving electrons from the p-orbitals. As a result, benzene contains three pi
bonds.
Delocalization of electrons:
Benzene is a cyclic compound composed of six hexagonally organised carbon atoms. Because each
carbon atom is sp2-hybridized, electrons can be delocalized in molecular orbitals that stretch all the way
around the ring, above and below the plane of the ring. The delocalized pi system is responsible for
benzene's distinct features, such as its stability and resistance to reactions. The Kekulé structure, which
indicates alternating double and single bonds between carbon atoms, has issues with benzene's
chemistry and form. The charge is distributed uniformly around the molecule by the delocalized pi
electrons, which stabilises it and makes it less prone to react. The delocalization explains why benzene
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,Mohammed Salam Unit 14: LA B
has a regular form, and the identical intermediate C-C bonds explain why benzene has a regular shape.
The chemical and thermodynamic reactivity of the benzene ring is altered by the delocalized electrons.
Several pieces of evidence reject Kekulé's benzene structure and support the recognised structure as
being very stable and consistently structured. These are some examples:
Reactivity with bromine: If Kekulé's structure is right, benzene will decolorize when it reacts with
bromine. However, because benzene is not extremely reactive, this supports Kekulé's concept. Bond
length statistics from X-rays: In benzene, the bond lengths are all the same length, which is contrary to
what would be predicted if Kekulé's model were right. If Kekulé's model is true, there will be two
different bond lengths corresponding to the ring's double and single bonds. The actual bond length is
between the single and double carbon bond lengths.
If Kekulé's model is true, the hydrogenation enthalpy of benzene is projected to be three times that of
cyclohexene. However, it is smaller and produces less energy than projected, indicating that the actual
benzene model is far more stable than Kekulé's model.
Absorption data from infrared spectroscopy: Data from infrared spectroscopy absorption demonstrate
that the C-H bonds in benzene are weaker than expected, supporting the delocalized hypothesis of
benzene.
The accepted structure of benzene is a delocalized model, in which electrons are not restricted to
specific bonds but are distributed throughout the molecule. This structure is exceedingly stable and
well-formed.
Explain the chemical reaction of benzene, phenol and methyl benzene:
Reaction of benzene R and C Equation Industrial importance
combustion Benzene and oxygen, C6H6 + 15/2 O2 = 6CO2 Benzene is used in the
supply of oxygen and + 3H2O manufacture of rubber
high temp and tyres. Some types
C6H6 + 9/2 O2 = 6Co + of lubricants contain
3H2O benzene. Benzene is
used in the
C6H6 + 3/2 O2 = 6C + manufacture of
3H20 pharmaceuticals. Most
products used in the
printing business
include benzene. There
are products that
include this chemical
and are specifically
used for cleaning
printing equipment,
making it last longer
2
, Mohammed Salam Unit 14: LA B
and work better.
hydrogenation Hydrogen gas, nickel C6H6 + 3H2 = C6H12 Benzene is used in the
catalyst 150 degrees at manufacture of colours
high pressure. Benzene is used in the
manufacture of
polymers, resins, and
synthetic fibres such as
nylon and Styrofoam.
nitration Concentrated nitric C6H6 + HNO3 + H2SO4 Benzene nitration is
acid, concentrated = C6H5NO2 + H20 utilised in the
sulphuric acid 50 – 60 manufacture of
degrees. numerous perfumes.
Nitrobenzene is used to
make explosives like
trinitrotoluene (TNT)
and picric acid.
Nitrobenzene is used to
make aniline, which is
used to make colours,
medicines, insecticides,
and synthetic rubber.
Nitrobenzene is also
used to make
lubricating lubricants,
which are utilised in
automobiles and
machinery.
alkylation Benzene, an alkyl CH4Cl + AlCl3 = CH4 + Alkylation of benzene
halide for example, AlCl4- C6H6 + CH4+ = with linear alkenes is
chlorobutane and C6H5CH4 used to produce linear
aluminium chloride alkyl benzenes (LABs)
catalyst. as the starting material
for the synthesis of
detergent
intermediates.
halogenation Chlorine/bromine in C6H6+ X2 = C6H5X Halogenation
the presence of intermediates can be
catalyst at room temp. found in items such as
resins and polymers,
fire retardants, and
petrol additives.
sulfonation Benzene and sulphuric C6H6 + H2SO4 = Benzenesulfonic acid is
acid/Sulphur trioxide. CH5SOH + H20 used in the
Benzene should be manufacture of
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