3.10.2 electrophilic substitution

what happens when benzene undergoes electrophilic mono-substitution?

• catalyst generates electrophile with + charge

• an electron pair from delocalised ring is donated to electrophile

• ring is partially destroyed

• electrophile is substituted onto the benzene ring, removing a hydrogen ion and restoring the ring

what happens when benzene undergoes nitration?

give the overall equation.

• nitronium ion electrophile generated

• H atom on benzene is substituted for nitronium ion

• benzene + HNO₃ → nitrobenzene + H₂O

  • H₂SO₄ above arrow

why is nitration an important step in synthesis?

• produces nitrobenzene and aromatic amines from benzene

• nitrobenzene used in the manufacture of explosives, pesticides, dyes and pharmaceuticals

• aromatic amines used in the manufacture/making of dyes, surfactants/fabric softeners, detergents and hair conditioners

what are the conditions for nitration?

• conc H₂SO₄ and conc HNO₃ to generate NO₂⁺ electrophile

  • H₂SO₄ is the catalyst, HNO₃ acts as base when generating electrophile

• temperature 50 - 55°C

why shouldn’t nitration be carried out at a temperature higher than the required reaction temperature?

• reaction must be a mono-substitution

• only one substitution must occur for the production of aromatic amines

• at temperatures higher than 50 - 55°C, multiple substitutions/more nitrations can occur

why shouldn’t nitration be carried out at room temperature?

• a relatively high activation energy is required to form the unstable intermediate with a partially broken delocalised ring

• reaction would be slow at room temp

in nitration, how is the H₂SO₄ used as a catalyst?

it generates the electrophile, the nitronium ion, from nitric acid

HNO₃ + 2 H₂SO₄ → NO₂⁺ + 2 HSO₄^- + H₃O⁺

in nitration, how is the H₂SO₄ catalyst regenerated?

H⁺ lost from benzene reacts with the hydrogen sulfate ion

H⁺ + HSO₄^- → H₂SO₄

why is phenylamime a weak base?

• lone pair on N is (partially) delocalised / spread into ring

• so lone pair is less available to accept H⁺

why is amine B a stronger base than amine A?

amine B: 

• positive inductive effect / alkyl group is electron-releasing

• so lone pair on N is more available to donate to H⁺

amine A:

• lone pair on N is partially delocalised / spread into ring

• so lone pair is less available to donate to H⁺

what happens when benzene undergoes Friedel-Crafts acylation?

give the overall equation.

• acylium ion electrophile generated

• H atom on benzene is substituted for acyl group

• benzene + acyl chloride → phenylketone + HCl, e.g.

  benzene + ethanoyl chloride → phenylethanone + HCl

  • exception: benzene + methanoyl choride → benzenealdehyde + HCl 

• benzene + acid anhydride → phenylketone + carboxylic acid

  • AlCl₃ above arrow

why are Friedel-Crafts acylation reactions important steps in synthesis?

• produces aromatic ketones (phenylketones), which are commonly used in industrial production of:

  • dyes

  • pharmaceuticals

  • explosives

  • fragrances

• reaction increases C chain length

what are the conditions for Friedel-Crafts acylation?

• anhydrous

• AlCl₃ (halogen carrier catalyst)

• heat under reflux

in Friedel-Crafts acylation, how is AlCl₃ used as a halogen-carrier catalyst?

it generates the electrophile, an acylium, from the acyl chloride or acid anhydride

in Friedel-Crafts acylation, how is the halogen-carrier catalyst regenerated?

the H⁺ ion lost from the benzene ring reacts with the AlCl₄^- to reform the AlCl₃ catalyst

H⁺ + AlCl₄^- → AlCl₃ + HCl

what happens when benzene undergoes Friedel-Crafts alkylation?

give the overall equation.

• alkyl ion electrophile generated

• H atom on benzene is substituted for alkyl group

• benzene + haloalkane → alkylbenzene + HCl, e.g.

  benzene + chloromethane → methylbenzene + HCl

  • AlCl₃ above arrow

why are Friedel-Crafts alkylation reactions important steps in synthesis?

reaction increases C chain length

in Friedel-Crafts alkylation, how is AlCl3 used as a halogen-carier catalyst?

it generates the electrophile, a carbocation, from the haloalkane

haloalkane + AlCl₃ → alkyl ion + AlCl₄^-

e.g. CH₃Cl + AlCl₃ → CH₃⁺ + AlCl₄^-

in Friedel-Crafts alkylation, how is the halogen-carrier catalyst regenerated?

the H⁺ ion lost from the benzene ring reacts with the AlCl₄^- to reform the AlCl₃ catalyst

H⁺ + AlCl₄^- → AlCl₃ + HCl

what effect does adding an alkyl group have on benzene?

• the ring becomes more reactive, so further alkylations are likely to occur

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• electrophilic substitution reactions happen faster if benzene has an alkyl group

  • alkyl group is electron-releasing / has positive inductive effect / increases electron density of benzene ring

  • so electrophiles will be more attracted / benzene ring will be a better nucleophile

how does benzene react with bromine?

• bromonium ion electrophile generated

• H atom on benzene substituted for Br

• benzene + Br₂ → bromobenzene + HBr

  • FeBr₃ or AlCl₃ above arrow

what are the conditions for the bromination of benzene?

• room temp and pressure

• anhydrous FeBr₃ or AlBr₃ - halogen carrier catalyst

  • can be formed by adding metallic Fe or Al to reaction mixture

for the bromination of benzene:

1. give an equation to show how the halogen-carrier catalyst generates the electrophile

2. give an equation to show how the halogen-carrier catalyst is regenerated

1. Br₂ + AlBr₃ → Br⁺ + AlBr₄^-

2. H⁺ + AlBr₄^- → AlBr₃ + HBr