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Summary Chapter 25: Aromatic Chemistry A-Level Chemistry OCR A
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Chapter 25 : Aromatic CompoundsIntroducing Benzene
Benzene was discovered in 1825 by Michael Faraday, who isolated and identified it in an oily residue from the
gas that was used in street lighting.
Benzene is:
★ A colourless, sweet smelling, highly flammable liquid.
★ It is found naturally in crude oil, it is a component of petrol and it is also found in cigarette smoke.
★ It is classified as a carcinogen.
★ It is a hexagonal ring of six carbons.
★ It is classed as an aromatic hydrocarbon or arene.
Derivatives
Historically aromatic was used to classify the derivatives of benzene as many pleasant-smelling compounds
contained a benzene ring. However many odourless compounds have been found to contain a benzene ring
yet the term is still used to classify these compounds.
, Kekulé Model
Scientists tried to establish a structure for benzene looking at its molecular formula and the experimental
evidence available at the time. C6H6 suggested a structure containing many double bonds or a structure
containing double and triple bonds. Compounds containing multiple bonds were known to be very reactive,
however benzene appeared unreactive.
In 1865 Friedrich August Kekulé suggested that benzene was a ring of carbon atoms joined by alternative
single and double bonds. He claimed that he had thought of the ring shape of benzene whilst day-dreaming
about a snake seizing its own tail.
The model was not totally accepted as it was not able to explain all of its chemical and physical properties:
★ The lack of reactivity of benzene
If benzene contains double carbon bonds then it should decolourise bromine in an electrophilic
addition reaction. However:
- Benzene does not undergo electrophilic addition reactions.
- It doesn’t decolourise bromine under normal conditions so doesn’t have any double carbon
bonds in its structure.
★ The length of the carbon - carbon bonds in benzene
Using X-ray diffraction it is possible to measure bond lengths in a molecule. When benzene was
examined in 1929 by Kathleen Lonsdale it was found that all the bonds were 0.139 nm in length. This
is between a single bond 0.153 nm and a double bond 0.134 nm.
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