Aromatic Chemistry

Question 1

What are the 4 problems with Kekules suggested structure for benzene? (cyclic with a double bond every other bond)

Answer 1

1 - would expect electrophilic addition reactions, but doesn’t react
2 - Would expect 4 isomers of dibromobenzene, however 1,2 and 1,6 models were indistinguishable, so in reality only 3 (irregular = bonds)
3 - enthalpy changes of benzene didn’t match predictions based off double and single bond data
4 - x-ray crystallography should show = to be shorter than - –> actually showed perfect hexagon

Question 2

What did the equilibrium model for benzene suggest?

Answer 2

There were 2 forms that rapidly inter-converted

Question 3

What is the name of the accepted benzene model?

Answer 3

Delocalised Model

Question 4

What does the delocalised model for benzene show its structure to be?

Answer 4

A cyclic C hexagon, with p orbitals above and below. These orbitals are close enough to overlap, and so form a continuous pi bond above and below the planar of the ring

Question 5

What are the conditions for nitration?

Answer 5

conc. HNO3
conc H2SO4
50*C

Question 6

How is the nitronium ion generated for nitration?

Answer 6

1) HNO3 + H2SO4 –> H2NO3+ + HSO4-

2) H2NO3+ –> NO2+ + H2O

Question 7

What are the uses of nitrobenzenes?

Answer 7

As explosives (TNT)
To make aromatic compounds (amines, used to make dyes)

Question 8

What are the conditions for Friedel Crafts acylation?

Answer 8

AlCl3 catalyst, dry

Question 9

What are the reactants and products of Friedel-Crafts acylation?

Answer 9

Reactants - a benzene-based compound and either acyl chloride or an acid anhydride
Products - An aromatic ketone and either HCl or a carboxylic acid

Question 10

What type of reaction mechanism occurs in Friedel-Crafts acylation?

Answer 10

Electrophilic substitution

Question 11

How does the AlCl3 catalyst form the electrophilic ion necessary for Friedel-Craft acylation, and what is the name of this ion?

Answer 11

AlCl3 + RCOCl –> AlCl4- + RCO+

RCO+ –> Acyllium ion

Question 12

Describe and explain the difference in stability between the hypothetical cyclohexa-1,3,5-triene and benzene

Answer 12

Benzene is more stable
Has 3 double bonds rather than 1, and its double bonds are close enough to overlap, thus allowing the electrons to become delocalised in a pi orbital above and below the ring, which is more stable