Benzene and Phenol

Question 1

What are 3 pieces of evidence that disprove the kekulé model of benzene?

Answer 1

1. Benzene has no reaction with bromine water so there can’t be C=C double bonds.
2. X-ray diffraction measured bond lengths in benzene to be longer than single bonds, but shorter than double bonds - disproving the kekulé.
3. Hydrogenation enthalpies of benzene are less exothermic than expected, so the actual structure is more stable than the kekulé model.

Question 2

Describe the delocalised model of benzene.

Answer 2

Each carbon is only bonded to 3 other atoms, so there is one free electron per carbon which is in the p-orbital at a right angle to the plane.

Adjacent p-orbital electrons overlap sideways, both up and down directions, creating a ring of electron density.

Creates a system of π bonds, with delocalised electrons.

Question 3

What happens when phenol is dissolved in water?

Answer 3

Phenol is less soluble in water than alcohols because of its non-polar benzene ring.

When dissolved in water, phenol partially dissociates into phenoxide ions and H+ ions, making it a weak acid.

Question 4

Why is phenol more reactive than benzene?

Answer 4

In phenol, a lone pair of electrons on the oxygen is delocalised into the delocalised ring, so the electron density is higher than benzene, making phenol more susceptible to electrophilic attack.

Question 5

What directing group is NO2 and why?

Answer 5

3 - directing because it deactivates the benzene ring, so the reaction occurs less readily with electrophiles.

Question 6

What directing group is OH and NH2 and why?

Answer 6

2, 4 - directing because it activates the benzene, so the reaction occurs more readily with electrophiles.

Question 7

Benzene –> Nitrobenzene

Answer 7

Reagents - HNO3
Conditions - H2SO4 catalyst. 50°C

Benzene + HNO3 –> Nitrobenzene + H2O

Question 8

Benzene –> Bromo/Chloro Benzene

Answer 8

Reagents - Cl2
Conditions - AlCl3 or FeCl3 halogen carrier catalyst

Benzene + Cl2 –> Chlorobenzene + HCl

Question 9

Phenol/Phenylamine –> 2,4,6 - Tribromophenol/phenylamine

What observation is made in this reaction?

Answer 9

Reagents - Br2
Conditions - Room temp

Phenol + 3Br2 –> 2,4,6 - Tribromophenol + 3HBr

Reaction decolourises bromine and forms a white precipitate.

Question 10

Phenol –> Nitrophenol

Answer 10

Reagents - HNO3
Conditions - Room temp

Phenol + HNO3 –> 2-Nitrophenol OR 4-Nitrophenol + H2O

Question 11

Phenol –> Phenoxide Salt

Answer 11

Reagents - NaOH
Conditions - none

Phenol + NaOH –> Sodium Phenoxide + H2O

Question 12

Nitrobenzene –> Phenylamine

Answer 12

Reagents - 6[H], reducing agents
Conditions - Sn/conc HCl, excess NaOH(aq), Reflux

Nitrobenzene + 6[H] –> Phenylamine + 2H2O

Question 13

Benzene –> + Alkyl Group (Alkylation)

Answer 13

Reagents - Haloalkane
Conditions - AlCl3 halogen carrier catalyst

Benzene + Bromopropane –> Propylbenzene + HBr

Question 14

Benzene/Acyl Chloride –> Ketone (Aromatic) (Acylation)

Answer 14

Reagents - Acyl Chloride
Conditions - AlCl3 catalyst

Benzene + Acyl Chloride –> Aromatic Ketone + HCl

Carbon-halogen bond in acyl chloride breaks and forms carbon-carbon bond with benzene ring.