These detailed notes cover aromatic chemistry, focusing on the bonding, electrophilic substitution, and Friedel-Crafts acylation reactions. The document discusses the stability of benzene and its derivatives and the reactions involving benzene rings.
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AQA Chemistry A-level
3.3.10: Aromatic Chemistry
Detailed Notes
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AQA Chemistry A-level
3.3.10: Aromatic Chemistry
Detailed Notes
This work by PMT Education is licensed under https://bit.ly/pmt-cc
https://bit.ly/pmt-edu-cc CC BY-NC-ND 4.0
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3.3.10.1 - Bonding
Benzene is an aromatic compound consisting of a ring of six carbon atoms with six
hydrogen atoms and a ring of delocalised electrons:
Example:
Each bond in the benzene ring has an intermediate length in between that of a double and
single bond.
The outer electron from the p-orbital of each carbon atom is delocalised to form the central
ring. This ring structure makes benzene very stable compared to other molecules of a similar
size.
Cyclohexatriene vs. Benzene
When benzene was first discovered its structure was not known. It was predicted from empirical
measurements that it had a structure similar to that of cyclohexatriene, with three double
bonds and three single bonds.
Example:
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Therefore the enthalpy change of hydrogenation for benzene was predicted to be -360kJmol-1,
three times the enthalpy change of cyclohexene.
Example:
It was later discovered that the enthalpy change of hydrogenation of benzene was actually
-208kJmol-1 leading to the conclusion of its different, unusual structure.
Arenes
Compounds that contain benzene as part of their structure are called arenes or aromatic
compounds. They have high melting points due to the high stability of the delocalised ring, but
low boiling points as they are non-polar molecules and often cannot be dissolved in water.
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3.3.10.2 - Electrophilic Substitution
The delocalised ring in benzene is an area of high electron density making it susceptible to
attack from electrophiles. When these species attack the electron ring, it is partially destroyed
then restored in the process of electrophilic substitution. This mechanism means aromatic
amines and nitrobenzene can be produced from benzene.
Mechanism
The electrophile is shown as A +.
Nitrobenzene
In this form of electrophilic addition, the electrophile is the NO2+ ion. This is a reactive
intermediate, produced in the reaction of concentrated sulfuric acid (H2SO4) with concentrated
nitric acid (HNO3).
Example:
When heated with benzene these reagents lead to the substitution of the NO2+ electrophile
onto the benzene ring, removing a hydrogen ion.
Mechanism
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This reaction shows a mono-substitution of a single NO2+ electrophile which takes place when
the reaction temperature is 55oC. At temperatures greater than this, multiple substitutions can
occur. It is vital that only one substitution occurs for the production of aromatic amines.
Friedel-Crafts Acylation
The delocalised electron ring in benzene can also act as a nucleophile, leading to the attack
on acyl chlorides. This reaction is known as Friedel-Crafts acylation.
In order for it to take place, a reactive intermediate must be produced from the acyl chloride
and an aluminium chloride catalyst.
Example:
This reactive intermediate is then attacked by the benzene ring.
Mechanism
At the end of the reaction, the H+ ion removed from the ring reacts with the AlCl4- ion to reform
the aluminium chloride, showing it to be a catalyst.
The product of this reaction is a phenylketone. In this case, the benzene group is called a
phenyl group. These molecules are commonly used in the industrial production of dyes,
pharmaceuticals and even explosives.
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