6.1.1 - Aromatic Compounds
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What was Kekule’s idea?
A structure which had 6 carbons in a ring with an alternating double and single bonds.
What was the issue with bromine water and Kekule’s idea?
The substance did not react with bromine water like an alkene, so no addition reaction was occurring.
What was the issue with the bonds and Kekule’s idea?
Bond lengths for double and single bonds are different but in benzene they are all equal forming a regular hexagon.
What was the issue with enthalpy of hydrogenation and Kekule’s idea?
The enthalpy of hydrogenation isn’t 3 times the hydrogenation of cyclohexene, which suggested it is more stable than Kekule’s structure.
What is a pi bond?
Sideways overlap of p orbitals above and below the nucleus.
What are sigma bonds?
Head on overlap of orbitals directly between the bonding atoms.
What is the actual structure of benzene?
6 pi electrons forming a delocalised pi bond above and below the ring.
Describe the carbons in benzene.
Each carbon atom is bonded to 2 other carbon atoms and 1 hydrogen atom. These 3 bonds are called sigma bonds.
What is formed around each carbon?
A planar hexagonal ring with a trigonal planar shape around each carbon.
What is left in the structure?
A 4th electron on each carbon in a p-orbital above and below the plan of carbon atoms.
What do the pi orbitals do?
Overlap sideways in both directions to form a delocalised pi system. The 6 electrons are now spread above and below the ring, they are delocalised so there is less repulsion between them, making benzene more stable.
What are arenes?
Hydrocarbons containing one or more benzene rings.
What is an aromatic compound?
Any compound with a benzene ring.
In the nitration of benzene what is added to the benzene ring?
NO2
What are the reagents and conditions for the nitration of benzene?
Conc HNO3 in the presence of conc H2SO4 catalyst. Reflux at 50C.
What is reflux?
Continuous process of evaporation and condensation to prevent volatile reagents escaping.
What may happen if the temperature is increased in the nitration of benzene?
Further substitution.
What is added to the benzene ring during the halogenation of benzene?
A halogen
What are the reagents and conditions for the halogenation of benzene?
A halogen in the presence of a halogen carrier catalyst, (e.g AlCl3 or iron chloride or iron metal when adding chlorine).
What happens during alkylation?
When a hydrogen is replaced by an alkyl group.
What are the reagents and conditions for alkylation?
Haloalkane in the presence of a halogen carrier catalyst.
How can you limit multiple substitutions occurring?
Use excess benzene.
What is an acyl group?
An alkyl R group attached to a C=O
What are the conditions for acylation?
Halogen carrier catalyst, reflux 60C for 30minutes, anhydrous conditions.