Benzene - A2

Question 1

Explain why bromine reacts more readily with salicylic acid than with benzene

Answer 1

In salicylic acid lone pair of electrons on O the phenol Is partially delocalised into the ring
Electron density increases
Br2 electrophile is more polarised

Question 2

Describe with the aid of suitable diagrams showing orbital overlap, the difference in bonding between structure A and Structure B (A is alkene benzene) (B is benzene ring)

Answer 2

P orbitals overlap to form pi bonds
pi bonds are delocalised in structure b
pi bonds are localised between 3 carbons in structure A

Question 3

Structure A - enthalpy change of -119
Structure B - enthalpy change 0f -208
Suggest and explain which structure is a better representation of benzene

Answer 3

Structure B as the structure is more stable
(3 * -119 = -357)
Structure B is better because enthalpy change of hydrogenation for benzene is less exothermic than -357 kJ mol -1

Question 4

Explain the experimental evidence that led to the development of the updated model from the Kekule model of benzene. Describe the bonding in the updated model of benzene

Answer 4

The experimental evidence suggests that the carbon - carbon bond lengths are the same in benzene
Enthalpy change of hydrogen is less exothermic for benzene than kekule model
The bonding in modern model has p orbitals overlap to form pi bond. The pi electrons are delocalised.

Question 5

Give chemical explanation for the statement
The carbon carbon bonds in benzene are all the same length

Answer 5

Pi bonds in benzene are delocalised

Question 6

How To distInguish between phenol and alcohol

Answer 6

Bromine water

Question 7

Suggest one way or processing waste polymer I other than landfill and recycling

Answer 7

Combustion for energy
Organic feedstock

Question 8

Sulphuric acid is a catalyst
State one example of an organic reaction in which sulfuric acid is a catalyst

Answer 8

Elimination of water from alcohol
Nitration of benzene

Question 9

State the trend in the relative ease of nitration of phenol, benzene and benzoic acid

Answer 9

Phenol is most reactive in nitration
Benzoic acid is least reactive in nitration

Question 10

Describe in terms of overlap the similarities and differences between the bonding in kekule model and delocalised model of benzene

Answer 10

Kekule model and benzene have overlaps of p orbitals and pi bonds

Kekule model has alternating pi bonds. Delocalised benzene has pi ring system

Question 11

Describe two pieces of evidence to support the delocalised model of benzene

Answer 11

Less exothermic
All carbon carbon length the same

Question 12

What reacts with Ethanoic acid and with phenol

Answer 12

Aqueous potassium hydroxide
(Hydroxides)

Question 13

What is the bonding in benzene

Answer 13

Each carbon makes 2 sigma bonds to other carbon
Each carbon donates one electron into the delocalised pi system
6 electrons form a delocalised pi system across the ring
A p orbital from each carbon overlaps with two neighbouring p orbitals