AWARDED A DISTINCTION.
UNIT 14B: Understand the reactions and properties of aromatic compounds
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B: Understand the reactions and properties of aromatic compounds
Benzene is built from hydrogen atoms (1s1) and carbon atoms (1s22s22px12py1). Each
carbon atom has to join to three other atoms (one hydrogen and two carbons) and doesn't
have enough unpaired electrons to form the required number of bonds, so it needs to promote
one of the 2s2 pair into the empty 2pz orbital.
(bonding in benzene - sp2 hybridisation and delocalisation, 2021)
The difference in benzene is that each carbon atom is joined to two other similar carbon
atoms instead of just one. Each carbon atom uses the sp2 hybrids to form sigma bonds with
two other carbons and one hydrogen atom. The next diagram shows the sigma bonds formed,
but for the moment leaves the p orbitals alone.
Benzene is a planar regular hexagon, with bond angles of 120°. This is easily explained. It is
a regular hexagon because all the bonds are identical. The delocalisation of the electrons
means that there aren't alternating double and single bonds. It is planar because that is the
only way that the p orbitals can overlap sideways to give the delocalised pi system.
With the delocalised electrons in place, benzene is about 150 kJ mol-1 more stable than it
would otherwise be. If you added other atoms to a benzene ring you would have to use some
of the delocalised electrons to join the new atoms to the ring. That would disrupt the
delocalisation and the system would become less stable. Since about 150 kJ per mole of
, benzene would have to be supplied to break up the delocalisation, this isn't going to be an
easy thing to do.
Sigma and pi:
When orbitals overlap covalent bonds gets formed. For example, an overlapping occurs when
sp2 from each carbon overlaps to form a single bond c-c resulting in a sigma bond. Pi bond is
achieved when the two 2p orbitals overlap to form a second bond. This results to the planar
arrangement having a rise around the C=C bonds.
Electron delocalisation:
The benzene theory suggested that six pi electrons were delocalised around the ring by
overlapping the orbitals resulting in there being no double bonds and equal bond length.
Additionally, by giving it a planar structure.
Kekulé proceeded to reason that the interchanging bonds continually oscillated so that
benzene was comprised of two rings with various alternate carbon-carbon bonds, which were
on inverse sides of equilibrium. A chemist by the name of Boffins found that instead of two
oscillating structures, benzene has two unique constructions however these exist together as
an intermediate with a lower energy than either of the two. The rationale for benzene's
absence of symmetry was additionally investigated utilising liquid benzene and it was
inferred that the overlapping vibrations and intermolecular forces altered the equilibrium of
benzene's two resonance structures, obliterating the focal point symmetry.
It is imperative to scrutinize the steadiness of benzene in Kekulé's construction as well, since
when benzene hydrogenation is noticed, the actual enthalpy energy secreted is less than the
original quantity. The hypothetical value for the hydrogenation of benzene, in view of
Kekulé's structure, can be calculated utilizing Hess' Law. Remember that the enthalpy of a
reaction is independent of the route that's followed, provided the products and reactants are
the same. Identified change that occurs in a system when matter is transformed by a given
chemical reaction, that the enthalpy of a reaction in cyclohexa-1, 2, 3-triene (Kekulé's version
of the benzene electron arrangement) was subtracted from the energy released when bonds
were broken to give the theoretical enthalpy of hydrogenation for benzene. 359.2KJ/mol is
calculated as the enthalpy of hydrogenation.
Using the method of Calorimetry, the hydrogenation enthalpy of benzene to be -208.5KJ/mol.
Recognised as the difference between the observed and the expected enthalpy of formation,
Resonance Energy can also be known as Delocalisation Energy.
(bonding in benzene - the Kekulé structure, 2021)
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