Aromatic Chemistry: Electrophilic Substitution Flashcards

1
Q

What type of mechanisms do benzenes undergo?

A
  • Electrophilic substitution
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2
Q

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

A
  • 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
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3
Q

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

A
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4
Q

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

A
  • 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
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5
Q

What conditions are needed for the nitration of benzene?

A
  • Concentrated nitric acid
  • Concentrated sulfuric acid catalyst
  • Maximum temperature of 50°C
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6
Q

What is nitration of benzene?

A
  • Addition of NO2+ (nitronium ion) to benzene
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7
Q

Why is sulfuric acid needed for the nitration of benzene?

A
  • Acts as catalyst
  • Generates electrophile (NO2+)
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8
Q

Give the overall word equation for the nitration of benzene

A
  • Nitric acid + benzene → nitrobenzene + water
  • Sulfuric acid above →
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9
Q

Give the overall symbol equation for the nitration of benzene

A
  • HNO3 + benzene / C6H6 → nitrobenzene / C6H5NO2 + H2O
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10
Q

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

A
  • HNO3 + H2SO4 → HSO4- + NO2+ + H2O
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11
Q

Draw the mechanism for the nitration of benzene

A
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12
Q

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

A
  • Reacts with HSO4- to reform H2SO4
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13
Q

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

A
  • Applies if only want one NO2 group added (mono nitration)
  • Above this temperature you’ll get multiple substitutions
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14
Q

What are the uses of nitration reactions?

A
  • Nitro compounds can be reduced to form aromatic amines, used to manufacture dyes and pharmaceuticals
  • Explosives e.g TNT
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15
Q

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

A
  • Friedel-Crafts acylation
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16
Q

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

A
  • Aluminium chloride catalyst, AlCl3
  • Heated under reflux in non-aqueous solvent e.g dry ether
17
Q

Give the general reaction between acyl chlorides and benzene

A
  • Acyl chloride + benzene → phenylketone + HCl
  • pg 451 of CGP textbook
18
Q

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

A
  • RCOCl + AlCl3 → RCO+ + AlCl4-
19
Q

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

A
  • Makes a stronger electrophile so it has a strong enough charge to react with the benzene ring
20
Q

Give the general mechanism between benzene and acyl chlorides

A