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Summary AQA Alevel Chemistry Reactions of Aromatics Notes
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AQA Alevel Chemistry Reactions of Aromatics Notes Electrophilic Substitution
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Reactions of AromaticsElectrophilic Substitution
The benzene ring is an area of high electron density so it attracts electrophiles.
Electrophiles are electron pair acceptors attracted to areas of high electron density.
The benzene ring is so stable that it does not undergo electrophilic addition which would
destroy the delocalised rings of electrons. Instead it undergoes electrophilic substitution where
one of the H atoms becomes substituted for the electrophile.
1. The electron dense region at the centre of the benzene ring attracts the electrophile.
2. The electrophile steals a pair of electrons from the centre of the benzene ring and forms a
bond with one of the carbons.
3. This partially breaks the delocalised electron ring giving the molecule a positive charge.
4. To regain stability the benzene ring loses a H from the carbon that the electrophile is
bound to.
5. Electrophilic substitution occurs as the H is replaced with an electrophile.
The Stages of Electrophilic Substitutions:
Generation of the electrophile.
The electrophile attack.
Regenerating aromaticity.
Nitration
When benzene is warmed with a concentrated nitric acid and concentrated sulphuric acid
(catalyst) nitrobenzene is produced.
Sulphuric acid acts as a catalyst which makes the nitronium ion which is the electrophile.
Once the nitronium ion forms it reacts with the benzene molecule to form nitrobenzene.
, Nitronium ion attracts the benzene ring.
An unstable intermediate forms.
H+ is lost.
The H+ ion reacts with H2SO4 to reform the catalyst.
If you only want one NO2 group added (mononitration) the temperature needs to be below
55degrees. Above this temperature lots of substitutions happen.
Sulphuric Acid is the catalyst.
The Nitric Acid is the base.
NO2 is the electrophile.
Uses of Nitration Reactions
Nitro-compounds can be reduced to form aromatic amines. They can manufacture dyes and
pharmaceuticals. Nitro-compounds decompose violently when heated so they are also used as
explosives such as TNT.
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