Unit 14 Assignment 2 – Distinction (Learning Aim B)
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Course
Unit 14 - Applications of Organic Chemistry
Institution
PEARSON (PEARSON)
BTEC Applied Science Unit 14 Assignment B - Aromatic Ring Chemistry for Designer Chemicals. Benzene and monosubstituted benzene compounds included as well as substitution and addition reactions of benzene included. Referenced throughout. Assignment has been awarded a DISTICTION.
This report provides knowledge about benzene as it explains accepted structure of benzene in terms
of hybridisation, sigma and pi bonding, and delocalisation of electrons as well as the chemical
properties of benzene, phenol, and methylbenzene and why these properties make them important
in industrial reactions. It will also present a comparison on the mechanism for the addition and
substitution reactions of benzene and then provide an analysis of the effects of different
monosubstituents on the benzene ring.
Kekulé Structure
The kekulé structure was introduced by a German chemist named August Kekulé and it was the first
sensible structure of benzene that allowed further study to be initiated to get a benzene structure
that would have no problems. The benzene structure that he presented consisted of carbon atoms
arranged in a hexagon shape where there would be alternating double and single bonds between
each carbon atom. The remaining bond from each carbon was made with a hydrogen atom for it to
have a total of 4 bonds each. The molecular formula of the kekulé structure is C 6H6. The image below
shows the kekulé structure including all the double and single bonds between the carbon atoms as
well as the hydrogen atoms:
The image below shows another way of showing the kekulé structure:
However, there were three problems with the kekulé structure consisting of the bond length and
shape, reactivity, and thermochemical data.
The bond length and shape were a problem because the hexagon shape of the kekulé structure
would be irregular due to the carbon-carbon single and double bonds having different bond lengths.
The carbon-carbon single bonds would have a bond length of 0.154nm whereas the carbon-carbon
double bonds would have a bond length of 0.133nm. This would cause the kekulé structure to have
alternating longer and shorter sides, which is different to how the real benzene is. In the real
benzene structure, the carbon-carbon bonds all have the same bond length being 0.139nm allowing
the hexagon shape to be regular. Due to the hexagon shape being regular, the bond angle would
also the same and that would be 120°. This information about benzenes bond length and shape is
confirmed by x-ray crystallography, which usually consists of exposing a crystallised sample of a
molecule such as benzene to an x-ray camera. When x-ray crystallography was carried out on
benzene, all of the carbon and hydrogen atoms and their bonds were shown in one plane where
they all had the same bond length and shape.
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