Aromatic Chemistry
(Mechanisms)

Electrophilic Substitution

arenes v stable
compounds

region of high electron density
above + below plane of molecule

  • substitution initiated by electrophile

electrophile
ion or molecule

  • with lone pair of electrons
  • that attack regions of high electron density

substitution reaction
where 1 atom/ group of atoms replaced by different atom/group of atoms

general
mechanism

Bromination

steps

  1. HED of delocalised ring
  • attracts E+
  • pair of e- from ring of delocalised pi e-
  • form bond w/ E+
  • breaking ring


  1. highly unstable intermediate produced
  • only partially delocalised e- system
  • w/ 4 delocalised e-


  1. in unstable intermediate
  • C-H bond breaks
  • 2 e- in bond move back to pi e- system
  • reforms stable delocalised ring


  1. Hydrogen lost as H+

Nitration

Acylation

Alkylation

monobromination
D: reaction in which 1 or more
Br atoms are added to compound

why not added across C=C bond?

  • partial C=C π bond requires additional energy
  • to overcome stability of delocalisation of π electrons

conditions

catalyst

polarises Br molecule
& Br+ acts as electrophile

  • iron (III) bromide
  • or iron (reacts w/ Br2 to form^^)
    2Fe + 3Br2 --> 2FeBr3

room temperature

equations

mechanism

Br+ ion

  • good electrophile
  • readily attacks electron rich regions
  • formed by delocalised π-electrons
  • above & below ring

step 1
formation of electrophile

  • Br-Br undergoes heterolytic fission
  • forms bromine cation - Br+

Br2 + FeBr3 ----> Br+ + FeBr4 -

step 2

  • Br+ forms coordinate bond w C atom
  • by accepting pair of delocalised π e- from benzene ring
  • benzene ring = POSITIVE CHARGE
  • break in delocalisation of π bonding
  • elimination of H+ = restores delocalisation


mechanism

step 3

  • reforming the catalyst
  • H+ eliminated from intermediate

H+ + FeBr4- ----> FeBr3 + HBr

called
'halogen carrier'

D: addition of 1 or more nitro (-NO2)
groups to a compound

benzene reacts with NITRATING MIXTURE

  • conc nitric acid
  • conc sulfuric acid (acts as catalyst)

equation
mononitration of benzene

nitric + sulfuric acid react = form Nitronium Ion (NO2+)

  • active nitrating species
  • acts as electrophile

mechanism

step 1
formation of nitronium ion (electrophile)


HNO3 + 2H2SO4 ---> NO2+ + 2H2O4- + H3O+ (hydroxonium ion)
or
HNO3 + H2SO4 ---> NO2+ + H2O4- + H2O

step 2
same as STEP 2 in Bromination

step 3

  • catalyst (sulfuric acid) reformed
  • when H+ eliminated from intermediate

H+ + HSO4- ---> H2SO4

conditions

conc sulfuric acid +
conc nitric acid

@ temp not exceeding 50°C

  • controls substitutions

H2SO4 = catalyst

  • increases rate of reaction
  • w/out being used up

D: process of
replacing H atom
by an acyl group

D: process of replacing H atom by alkyl group

  • benzene reacts with
  • acyl chloride
  • in presence of AlCl3 catalyst
  • form aromatic ketone

conditions

catalyst =
Aluminium Chloride
- AlCl3

anhydrous

  • prevents hydrolysis

electrophile
acylium ion
H3C------C+ ===== O

formed from

  • ethanoyl chloride
  • aluminium chloride catalyst

mechanism

step 1
formation of electrophile


CH3COCl + AlCl3 ---> H3C----C+====O + AlCl4-

step 2

  • acylium ion attacks delocalised π e- system in benzene ring
  • forms intermediate cation
  • which breaks down
  • forming phenylethanone + H+

step 3
catalyst regenerated


H+ + AlCl4- ---> AlCl3 + HCl

mechanism

  • benzene reacts with
  • halogenoalkane
  • in presence of AlCl3 catalyst
  • form alkyl benzene

conditions

catalyst - AlCl3

anhydrous

  • prevent hydrolysis

electrophile

  • carbocation ion
  • formed by reaction between
    • halogenoalkane
    • AlCl3 catalyst

equation

step 1
formation of electrophile
CH3Cl + AlCl3 ---> C+H3 + AlCl4-

step 2
same as acylation

step 3
catalysed regenerated
H+ + AlCl4- ----> AlCl3 + HCl

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