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Lecture notes 14.2 Aromatic ring chemistry for designer molecule
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14.2 Aromatic ring chemistry for designer chemicalsP2. Explain the structure of benzene using sigma and pi bonding, providing
evidence for the structure.
Benzene (C6H6) is an aromatic compound, that is very stable and is the simplest arene
hydrocarbon, which is a nonpolar liquid at room temperature and was once widely used
as a solvent until it was discovered to be carcinogenic- a compound that can cause
cancer. The activation energy is extremely high, conveying any chemical reaction
happening is quite slow.
There are three ways of representing the structure of benzene.
Firstly, benzene is shown as an unsaturated aromatic compound that is flat and planar
with three double bonds which are highly reactive. The structure shown has a six-carbon
ring which is represented by a hexagon showing 3-double bonds. Although the
hydrocarbon itself is far more stable and less reactive than the Kekule structure suggests.
The carbon-to-carbon bonds are represented by equal single and dashed line.
Secondly, is the resonance structure of benzene which shows it is only one isomer.
Here the skeletal notations of carbon are not drawn at all. The straight lines represent
carbon-carbon bonds and carbon atoms are assumed to be where the lines meet. Here
neither hydrogen nor C-H bonds are drawn, as the lines shown drawn within the
pentagon are meant to represent the hydrogen atoms.
The last image conveys what is shown during an electrophilic substitution reaction. This
shows the benzene ring, within the pentagon. Here, the electrophile attacks the
electron-rich benzene ring with its six delocalized electrons.
Hydrogenation is an additional reaction in which hydrogen atoms are added all the way
around the benzene ring. Even though hydrogenation of benzene does happen, it only
takes place under environments of either extreme heat or high pressure. This is
because benzene is stabilized by the delocalization of electron density. On account of
allure extreme separation energy, the response will not happen at a big rate under usual
environments that are an extreme strength condition (extreme heat) must be
secondhand for the backlash to really happen. Even though the backlash is exothermic;
it is not almost as exothermic according to the highest authority. Though the backlash is
exothermic; this is occasionally on account of the evidence that benzene is fragrant.
, Π bond delocalization is found within the delocalization is found in the cyclic molecule
of benzene (C6 H6) which consists of 6 carbon atoms bound together in a hexagonal
ring.
Each carbon has a single hydrogen atom attached to it. The lines in this figure represent
the σ bonds in benzene. The basic ring structure is composed of σ bonds formed from
overlap of sp2 hybrid orbitals on adjacent carbon atoms. The unhybridized carbon pz
orbitals project above and below the plane of the ring. They are shown here as they
might appear if they did not interact with one another.
But what happens, of course, is that the lobes of these atomic orbitals meld together to
form circular rings of electron density above and below the plane of the molecule. The
two of these together constitute the "second half" of the carbon-carbon double bonds in
benzene. This computer-generated plot of electron density in the benzene molecule is
derived from a more rigorous theory that does not involve hybrid orbitals; the highest
electron density (blue) appears around the periphery of the ring, while the lowest (red) is
in the "doughnut hole" in the center. Note, however, that molecules that include 𝜋
bonding can also be electrophiles, particularly when the 𝜋 bond involves an
electrophilic atom.
Sigma bonding (ó) is comprised of sigma bonds which are the strongest covalent
bonds, owing to the direct lapping of the sharing orbitals. The electron sharing in a
𝜎 bonds are generally appertained to as 𝜎 electrons. Generally, all single bonds are
sigma bonds. They can be formed via the following combinations of infinitesimal
orbitals.
P3. Explain the chemical properties of industrially important benzene and
monosubstituted benzene compounds.
The molecular formular for benzene is C6H6, in benzene, each Carbon atom has 1
Hydrogen atom attached to it, making it the simplest arene compound. When it reacts
with a halogen or haloalkane, the electrophile is substituted with the Hydrogen atom. It
does not undergo electrophilic addition as there is no true C=C bond. Only one 1,2
disubstituted product exists, all C6H6 atoms
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