Arenes
Reactions & Mechanisms
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We already know that The delocalisation of some of the bonding electrons
of benzene has some important consequences:
Benzene reacts with only very reactive electrophiles
Benzene is much more stable than expected.
The extra stability means that benzene will less readily undergo addition reactions.
The more loosely held electrons are open to attack by electrophiles. Hence, the
characteristic reaction of benzene is electrophilic substitution.
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Resistance to Addition
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Addition across a double bond usually leads to a more stable structure, saturated
compound since single bonds are stronger than double bonds
However, benzene is extra stable due to delocalisation. Addition would lead to a
loss of delocalisation and a less stable product. (add diagram)
Electrophilic substitution: during substitution reactions the delocalisation and
extra stability is retained. (add diagram)
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Naming of Aromatic Compounds
• Draw out the structures of the following Arenes.
There are no rules to learning them-just plenty of
revision!
• Benzene
• Methylbenzene
• Chlorobenzene
• Nitrobenzene
• Phenol
• 1,2 dichlorobenzene
• Phenylamine
Reactions
• 1. Combustion
• What would you expect to see?
• Benzene burns in oxygen with a smoky flame
due to its high carbon content. This is a
general test between aromatic and aliphatic
compounds
• Write a balanced equation for the reaction
• 2C6H6(l) + 15O2(g) 12CO2(g) + 6H2O(g)
Nitration &
Halogenation
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Both of these reactions involve:
Initial attack by an electrophile (often a cation), to form an intermediate addition compound.
Loss of hydrogen ion, H+, leaving a substitution product.
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Now write an equation for the nitration reaction
State the Conditions
Do the same for Halogenation
Nitration
A mixture of conc. Sulphuric acid and Nitric acid below 50°C, react exothermically with benzene to
form mononitrobenzene. At temperatures above 50°C, dinitrobenzene will be formed
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Halogenation
Chlorine or bromine react with benzene at room temperature in the dark. However, a suitable
catalyst (a halogen carrier) such as anhydrous AlCl3, anhydrous AlBr3, iron filings or iodine crystals
must be present.
The reaction is exothermic, forming mono-chloro or bromobenzene. Further substitution occurs if
the reaction time is long, and products up to C6Cl6 and C6H3Br3 can be obtained. This is not a
catalyst free reaction.:
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