3.3.10.2 electrophilic substitution

define an electrophile

electron pair acceptor

what mechanism does benzene undergo?

electrophilic substitution

what are the two electrophiles we need to know?

• ⁺NO₂ (nitronium ion)

• RCO⁺ (acylium ion)

  • [R−C≡O ⁺]

outline the steps of the general mechanism

1. (generation of electrophile): a catalyst is used to produce the electrophile, E⁺

2. electrophilic substitution

3. (regeneration of catalyst): the H⁺ liberated from the benzene reforms the catalyst

draw + outline the general electrophilic substitution for benzene

• 2 electrons from the delocalised ring form a new bond with the E⁺

• 2 electrons from C-H bond go back to the ring to complete it

outline nitration of benzene

• benzene reacts with a mixture of concentrated HNO₃ + concentrated H₂SO₄ (catalyst) at 50℃ to form nitrobenzene

• mechanism is electrophilic substitution

give the overall equation for the nitration of benzene

describe step 1 (generation of electrophile) of nitration of benzene + give the 2 equations

1. H₂SO₄ protonates the HNO₃ as it is a stronger acid

   • H₂SO₄ + HNO₃ → HSO₄⁻ + H₂NO₃⁺

2. the protonated nitric acid breaks down + forms the nitronium ion (nitric cation) + water

   • H₂NO₃⁺ → NO₂⁺ + H₂O

combine the 2 equations of step 1 to give an overall equation for the generation of the electrophile

draw the mechanism for the nitration of benzene

give the equation for the regeneration of the catalyst

give 2 uses of nitrobenzene

1. synthetic uses

2. making TNT

nitrobenzene can be used in the preparation of what?

azo dyes

describe how nitrobenzene is used in the preparation of azo dyes

give the overall equation

• nitro group is reduced to an amine

• reagent: HCl

• condition: Sn catalyst

nitration can occur more than….

more than once

methylbenzene is more ____ than benzene, what does this mean?

• reactive

• this means that the nitration reaction can be carried out at a lower temperature

why is methyl benzene more reactive than benzene?

• methyl group pushes electron density on the ring, increasing its electron density

• this increases the ring’s attraction to the electrophile

why does benzene not undergo electrophilic addition instead of substitution?

because this would involve permanently breaking the ring of delocalised electrons, which would make benzene unstable (the ring gives it stability)

give another reaction that benzene undergoes

Friedel-Crafts acylation reactions

what mechanism does Friedel-Crafts acylation involve?

electrophilic substitution

outline what Friedel-Crafts acylation involves

benzene reacts with acyl chlorides (RCOCl) in the presence of AlCl₃ to form a ‘phenyl ketone’

give the overall general equation for the reaction between acyl chlorides and benzene

what are the 3 steps of friedel-crafts acylation?

1. generation of electrophile

2. electrophilic substitution

3. regeneration of catalyst

outline step 1 (generation of electrophile) + give the equation for it

• AlCl₃ removes the Cl from the acyl chloride

• ROCl + AlCl₃ → RC⁺O + AlCl₄⁻

what is the electrophile involved in this mechanism? give its structure

the acylium ion is the electrophile which attacks the benzene molecule

outline the general mechanism of electrophilic substitution for acylation

give the equation for step 3 (regeneration of the catalyst)