aromatic chemistry [O2] PAPER 2

Question 1

name and outline the mechanism, and state the reagents and conditions, for the nitrification of benzene.

Answer 1

• electrophilic substitution
• conc. HNO3
• conc. H2SO4
• 50°C

Question 2

name and outline the mechanism, and state the reagents and conditions, for the Friedel-Crafts acylation of benzene.

Answer 2

• electrophilic substitution
• acyl chloride/acid anhydride
• AlCl3 catalyst
• anyhydrous

Question 3

generation of electrophile for the nitration of benzene?

Answer 3

Question 4

In practice, ethanoic anhydride is used in the industrial synthesis rather than ethanoyl chloride.

Give one reason why ethanoyl chloride is not used in the industrial synthesis.

Answer 4

corrosive

Question 5

Describe the shape and bonding of benzene.

Answer 5

• C6H6
• each C has three covalent bonds
• planar, 6-carbon ring with 120° bond angles
• C—C bonds are equal in length
• spare electrons in p orbital are delocalised and overlap to form π bonds, leading to electron clouds above and below the plane of the ring

Question 6

Data about the hydrogenation of cyclohexene and of benzene are given.
Compare the stability of benzene with that of the hypothetical cyclohexa-1,3,5-triene molecule.

Use the data in your answer.

Answer 6

• Expected ∆H° hydrogenation of cyclohexatriene = -360 kJmol-1
• ∆H° hydrogenation of benzene is less exothermic by 152 kJmol-1
• benzene is more stable than cyclohexatriene

Question 7

The enthalpy of hydrogenation of cyclohexa-1,3-diene is not exactly double that of cyclohexene.
Suggest a value for the enthalpy of hydrogenation of cyclohexa-1,3-diene and justify your value.

Answer 7

• value reasonably more than -240 kJmol-1, e.g. -235 kJmol-1
• the double bonds are close enough to allow some delocalisation (since the double bonds are separated by one single bond)

Question 8

generation of electrophile for the Friedel-Crafts acylation of benzene?

Answer 8

Question 9

uses of nitrobenzene compounds?

Answer 9

• explosives
• make aromatic amines (used for dyes)

Question 10

Suggest why sodium benzoate is soluble in cold water but benzoic acid is insoluble in cold water.

Answer 10

• sodium benzoate is soluble as it is ionic
• benzoic acid is insoluble as, despite the ability of the COOH to form hydrogen bonds, the benzene ring is non-polar

Question 11

Answer 11

A

Question 12

reduction of nitrobenzene to phenylamine - reagent and conditions?

Answer 12

• Sn
• HCl

Question 13

Suggest why sodium benzoate is soluble in cold water but benzoic acid is insoluble in cold water. [2 marks]

Answer 13