Chapter_15 - Chapter 15 Reactions of Aromatic Compounds...

Chapter 15
Reactions of Aromatic Compounds

Chapter 15 2

Electrophilic Aromatic Substitution
➨
Arene (Ar-H) is the generic term for an aromatic hydrocarbon
★ The aryl group (Ar) is derived by removal of a hydrogen atom from an arene
➨
Aromatic compounds undergo electrophilic aromatic substitution
(EAS)
★ The electrophile has a full or partial positive charge

Chapter 15 3
 A General Mechanism for Electrophilic Aromatic
Substitution: Arenium Ion Intermediates
➨
Benzene reacts with an electrophile using two of its π electrons
★ This first step is like an addition to an ordinary double bond
➨
Unlike an addition reaction, the benzene ring reacts further so that
it may regenerate the very stable aromatic system
➨
In step 1 of the mechanism, the electrophile reacts with two π
electrons from the aromatic ring to form an arenium ion
★ The arenium ion is stabilized by resonance which delocalizes the charge
➨
In step 2, a proton is removed and the aromatic system is
regenerated

Chapter 15 4

Halogenation of Benzene
➨
Halogenation of benzene requires the presence of a Lewis acid
➨ Fluorination occurs so rapidly it is hard to stop at
monofluorination of the ring
★ A special apparatus is used to perform this reaction
➨ Iodine is so unreactive that an alternative method must be used

Chapter 15 5
➨
In the step 1 of the mechanism, bromine reacts with ferric bromide
to generate an electrophilic bromine species
➨
In step 2, the highly electrophilic bromine reacts with π electrons
of the benzene ring, forming an arenium ion
➨
In step 3, a proton is removed from the arenium ion and
aromaticity is regenerated
★
The FeBr
3
catalyst is regenerated

Chapter 15 6
 Nitration of Benzene
➨ Nitration of benzene occurs with a mixture of concentrated nitric
and sulfuric acids
★
The electrophile for the reaction is the nitronium ion (NO
2
+
)

Chapter 15 7

Sulfonation of Benzene
➨
Sulfonation occurs most rapidly using fuming sulfuric acid
(concentrated sulfuric acid that contains SO
3
)
★ The reaction also occurs in conc. sulfuric acid, which generates small quantities
of SO
3
, as shown in step 1 below

Chapter 15 8
➨
Sulfonation is an equilibrium reaction; all steps involved are
equilibria
★ The sulfonation product is favored by use of concentrated or fuming sulfuric acid
★ Desulfonation can be accomplished using dilute sulfuric acid ( i.e . with a high
concentration of water), or by passing steam through the reaction and collecting
the volatile desulfonated compound as it distils with the steam

Chapter 15 9
 Friedel-Crafts Alkylation
➨ An aromatic ring can be alkylated by an alkyl halide in the
presence of a Lewis acid
★ The Lewis acid serves to generate a carbocation electrophile

Chapter 15 10
➨
Primary alkyl halides probably do not form discreet carbocations
but the primary carbon in the complex develops considerable
positive charge
➨ Any compound that can form a carbocation can be used to
alkylate an aromatic ring

Chapter 15 11
