O2: 27 Aromatic chemistry

Question 1

Describe the nature of the bonding in a benzene ring.

Answer 1

• planar structure
• bond length is in between single and double bonds

Question 2

Why is benzene more stable than the theoretical cyclohexa-1,3,5-triene?

Answer 2

Delocalisation of p electrons.

Question 3

What is the thermochemical evidence that proves benzene is more stable?

From enthalpies of hydrogenation.

Answer 3

When cyclohexene (with one double bond) is hydrogenated, the enthalpy change is -120 kJ mol-1.
Cylohexa-1,3,5-triene should have an enthalpy of hydrogenation of -360 kJ mol-1.
But benzene’s enthalpy of hydrogenation is -208 kJ mol-1, which proves it doesn’t have three double bonds and is more stable.

Question 4

Why do substitution reactions occur in preference to addition reactions with benzene?

Answer 4

The benzene ring is an area of high electron density so it would repel the lone pair.

Question 5

What does electrophilic attack on benzene rings result in?

Answer 5

Substitution.

Question 6

What is nitration used in?

Answer 6

The manufacture of explosives and formation of amines (dyes).

Question 7

Name, outline and give the reagents for the reaction mechanism of nitration.

Answer 7

Electrophilic substitution.
Conc HNO3, conc H2SO4.

Question 8

Write the equation for the formation of the nitronium ion (NO2+) in nitration.

Answer 8

HNO3 + 2H2SO4 -> NO2+ + H3O+ + HSO4-

Question 9

Name, outline and give the reagents for the reaction mechanism of Friedel-Crafts acylation.

Answer 9

Electrophilic substitution.
Acyl chloride/ acid anhydride and AlCl3.

Question 10

Write the general equation of Friedel-Crafts acylation, using AlCl3 as a catalyst.

Answer 10

RCOCl + AlCL3 -> RCO+ + AlCl4-
AlCl4- + H+ -> AlCl3 + HCl