Aromatic Chemistry: Electrophilic Substitution

Question 1

What type of mechanisms do benzenes undergo?

Answer 1

• Electrophilic substitution

Question 2

Why do benzenes tend to undergo electrophilic substitution reactions, rather than electrophilic addition reactions?

Answer 2

• Benzene rings is very stable due to delocalised ring of electrons
• Electrophilic addition reactions would destroy delocalised ring of electrons
• Instead, one of hydrogen atoms or other group bonded to ring is substituted by electrophile

Question 3

Draw a general mechanism for the reactions of aromatics using E to represent electrophile

Answer 3

Question 4

Explain what occurs at each step of the general mechanism for the reactions of aromatics

Answer 4

• Electron-dense delocalised ring of electrons attracts an electrophile
• Electrophile steals pair of electrons from centre of benzene ring and forms bond with one of carbons
• This partially delocalised ring of electrons and gives molecule positive charge
• To regain stability of benzene ring, carbon bound to electrophile loses a hydrogen to the ring
• Get substitution of H+ with electrophile

Question 5

What conditions are needed for the nitration of benzene?

Answer 5

• Concentrated nitric acid
• Concentrated sulfuric acid catalyst
• Maximum temperature of 50°C

Question 6

What is nitration of benzene?

Answer 6

• Addition of NO₂⁺ (nitronium ion) to benzene

Question 7

Why is sulfuric acid needed for the nitration of benzene?

Answer 7

• Acts as catalyst
• Generates electrophile (NO₂⁺)

Question 8

Give the overall word equation for the nitration of benzene

Answer 8

• Nitric acid + benzene → nitrobenzene + water
• Sulfuric acid above →

Question 9

Give the overall symbol equation for the nitration of benzene

Answer 9

• HNO₃ + benzene / C₆H₆ → nitrobenzene / C₆H₅NO₂ + H₂O

Question 10

Give the symbol equation for the formation of the electrophile, the nitronium ion, during the nitration of benzene

Answer 10

• HNO₃ + H₂SO₄ → HSO₄^- + NO₂⁺ + H₂O

Question 11

Draw the mechanism for the nitration of benzene

Answer 11

Question 12

What happens to the produced H+ ion made from the nitration of benzene?

Answer 12

• Reacts with HSO₄^- to reform H₂SO₄

Question 13

In which cases must we keep the temperature below 55°C during the nitration of benzene and why?

Answer 13

• Applies if only want one NO₂ group added (mono nitration)
• Above this temperature you’ll get multiple substitutions

Question 14

What are the uses of nitration reactions?

Answer 14

• Nitro compounds can be reduced to form aromatic amines, used to manufacture dyes and pharmaceuticals
• Explosives e.g TNT

Question 15

What are the reactions between benzenes and acyl chlorides known as?

Answer 15

• Friedel-Crafts acylation

Question 16

What conditions are needed for Friedel-Crafts acylation to occur?

Answer 16

• Aluminium chloride catalyst, AlCl₃
• Heated under reflux in non-aqueous solvent e.g dry ether

Question 17

Give the general reaction between acyl chlorides and benzene

Answer 17

• Acyl chloride + benzene → phenylketone + HCl
• pg 451 of CGP textbook

Question 18

Give the general equation for the formation of the carbocation during Friedel-Crafts acylation

Answer 18

• RCOCl + AlCl₃ → RCO⁺ + AlCl₄^-

Question 19

Why does a carbocation have to be formed during Friedel-Crafts acylation?

Answer 19

• Makes a stronger electrophile so it has a strong enough charge to react with the benzene ring

Question 20

Give the general mechanism between benzene and acyl chlorides

Answer 20