CD - Benzene and Reactions of Aromatic Compounds

Question 1

Describe Kekulé’s structure for benzene

Answer 1

Kekulé suggested that the structure of benzene involved a 6 carbon ring with alternating double and single bonds which continuously switched

Question 2

Give evidence why Kekulé’s structure for benzene was wrong

Answer 2

If his model were correct, there would always be 3 bonds with length C-C and three bonds with C=C length. The x-ray diffraction studies have shown that benzene all the carbon-carbon bonds in benzene are the same length

Question 3

Describe the delocalised model for benzene

Answer 3

A 6 carbon ring with one p-orbital electron donated from each carbon which combine to form a delocalised ring of electrons above and below the planar carbon ring

Question 4

State why the delocalised model for benzene explains the substitution reaction with bromine

Answer 4

If benzene had double bonds, Bromine would react via electrophilic addition, and only one product would be formed.
If you add bromine to benzene, nothing happens unless you use a catalyst, they react by electrophilic substitution.

Question 5

Explain why benzene reacts by substitution rather than addition

Answer 5

• The electrons in the delocalised ring can repel each other further away than if they were localised into double bonds. This makes benzene more stable
• An addition reaction would need to take electrons from the delocalised ring to form a bond and the stability of benzene would decrease. Instead, in substitution, a hydrogen atom is just swapped for something else, so the stability of the delocalised electrons is preserved

Question 6

Give the method and equation for the reaction of nitrobenzene

Answer 6

Warm benzene with conc. Nitric acid and Sulfuric acid
HNO₃ + H₂SO₄ → H₂NO₃⁺ + HSO₄⁻
H₂NO₃⁺ → NO₂⁺ + H₂O

Question 7

What temperature must the reaction be below if you only want mononitration of benzene

Answer 7

Below 55°C

Question 8

Describe the mechanism for the mononitration of benzene

Answer 8

• The nitronium ion attacks the benzene ring
• An unstable intermediate is formed with a broken ring of electrons by a positive charge, and a bond to both the nitronium ion and a hydrogen
• The hydrogen ion is lost (reacts with HSO₄⁻ to reform the sulfuric acid catalyst)

Question 9

Give 2 ways of making benzenesulfonic acid

Answer 9

Either:
1) Boil benzene under reflux with conc. sulfuric acid for several hours
OR
2) Warm benzene to 40°C with fuming sulfuric acid for half an hour

Question 10

What is the electrophile in the reaction between sulfuric acid and benzene?

Answer 10

SO₃

Question 11

Give the equation for the equilibrium of aqueous sulphuric acid

Answer 11

H₂SO₄ ⇌ H₂O + SO₃

Question 12

Give the mechanism for the substitution of SO₃ and benzene

Answer 12

1) The SO₃ attacks benzene, drawing a pair of electrons from the delocalised ring
2) The -ve Oxygen atom on SO₃⁻ takes an H atom from benzene. The pair of electrons in the C-H bond move to the delocalised ring
3) Benzenesulfonic acid is formed

Question 13

How can you make a compound with a halogen a stronger electrophile?

Answer 13

By using a halogen carrier

Question 14

What halogen carrier would you use for compound containing Chlorine and what would it form?

Answer 14

Aluminium Chloride

RCl + AlCl₃ → R⁺ + AlCl₄⁻

Question 15

What halogen carrier can be used for Chlorine or Bromine and what does it form?

Answer 15

Iron
2Fe + 3Cl₂ → 2FeCl₃
2Fe + 3Br₂ → 2FeBr₃

Question 16

How would you alkylate benzene?

Answer 16

By reacting a halogen-alkane with benzene and a halogen carrier (e.g. Iron)

Question 17

What is a Friedel-Crafts alkylation?

Answer 17

The addition of an alkyl group by substituting benzene with a halogen-alkane using a halogen carrier (AKA a Friedel-Crafts catalyst)

Question 18

Give equation for the Friedel-Craft alkylation of benzene with RCl

Answer 18

C₆H₆ + RCl + AlCl₃ [under reflux] → C₆H₅R + HCl

Question 19

How would you acylate benzene and name the products?

Answer 19

By reacting an acylchloride with benzene and AlCl₃.

This produces phenylketones and HCl (AlCl₃ is regenerated)

Question 20

Give the name for CH₃CO⁺

Answer 20

An acylium ion

Question 21

How would you polarise a dimer (such as Br₂)?

Answer 21

A Halogen Carrier