Benzene and Arenes

Question 1

Describe the structure of benzene

Answer 1

A ring of delocalised electrons above and below the plane of the molecule
A continuous overlap of pi bonds, making the structure very stable

Question 2

Compare the enthalpy of hydrogenation of benzene and 3x C=C

Answer 2

Less exothermic for benzene, therefore it’s more stable
As more energy is needed to break the continuous pi bonds

Question 3

Why is benzene resistant to bromination?

Answer 3

Breaking the continuous pi bond is not energetically favourable.
For alkenes, it has a localised pi bond, when broken they form two sigma bonds

Question 4

Compare the bromination of phenol and benzene

Answer 4

Phenol is more reactive than benzene. The lone pair of e- on the oxygen are fed into the ring of delocalised electrons. This increases the e- density of the ring of delocalised electrons, so its more susceptible to attacks by electrophiles

Question 5

Reaction of benzene with bromine

Answer 5

Electrophilic substitution
Reagents: AlBr3 or FeBr3
AlBr3 + Br2 –> Br+ + AlBr4-

Question 6

Nitration of benzene

Answer 6

Electrophilic substitution
Reagents: Conc H2SO4 and Conc HNO3
H2SO4 + HNO3 –> NO2+ + H2O + HSO4-

Warmed over 60°

Question 7

Alkylation of benzene

Answer 7

Electrophilic substitution
Reagents: Alkyl group + FeCl3
E.g:
CH3Cl + FeCl3 –> CH3+ + FeCl4-

Question 8

Acylation of Benzene

Answer 8

Electrophilic substitution
Reagent: Acyl Chloride + FeCl3

E.g:
CH3COCl + FeCl3 –> CH3C+O + FeCl4-

Question 9

Nitration of Phenol

Answer 9

Reagents: HNO3 acid
Room temperature

Produces a mixture of 2-nitrophenol or 4-nitrophenol
Observation: brown mixture