Transcript Synthesis of Benzene Derivatives: Electrophilic Aromatic

15-8
Synthesis of Benzene Derivatives: Electrophilic
Aromatic Substitution
Benzene undergoes substitution reactions with electrophiles.
Electrophiles attack benzene by substituting for a hydrogen atom, not addition to
the ring.
Under the conditions of this type of reaction, ordinary non-aromatic conjugated polyenes
would polymerize rapidly.
Electrophilic aromatic substitution in benzene proceeds by addition of
the electrophile followed by proton loss.
Electrophylic aromatic substitution is a two-step process. The cationic
intermediate is resonance-stabilized:
The initial attack of the electrophile is endothermic because the sp3 carbon
generated interrupts the cyclic conjugation. The transition state is not aromatic.
The loss of the proton regenerates the sp2 carbon atom and aromaticity is
restored. This process is more favored than the nucleophilic trapping by the anion
accompanying E+.
The overall reaction is exothermic because the bonds formed are stronger than
the bonds broken.
15-9
Halogenation of Benzene: the Need for a Catalyst
Benzene in normally unreactive to halogens because they are not electronegative
enough to disrupt its aromaticity.
Halogens can be activated by Lewis acid catalysts, however, such as ferric halides
(FeX3) or aluminum halides (AlX3), to become much more powerful electrophiles.
This activated bromine complex can attack the benzene molecule, allowing the
other bromine atom to depart with the good leaving group FeBr4-
The FeBr4- next abstracts a proton from the cyclohexadienyl cation intermediate,
and in the process regenerates the original FeBr3 catalyst.
Bond energy calculations show that the electrophilic bromination of benzene is
exothermic:
Phenyl-H +112 kcal mol-1
Br-Br
+46 kcal mol-1
Phenyl-Br -81 kcal mol-1
H-Br
-87.5 kcal mol-1
---------------------------------------------------------
Reaction
-10.5 kcal mol-1
Fluorination of benzene is very exothermic (explosive).
Chlorination and bromination require an activating catalyst.
Iodination is endothermic and does not occur.
15-10 Nitration and Sulfonation of Benzene
Benzene is subject to electrophilic attack by the nitronium ion.
Benzene can be attacked by
concentrated nitric acid in the
presence of concentrated sulfuric acid.
The purpose of the sulfuric acid is to generate the more electrophilic nitronium ion:
The nitronium ion subsequently attacks the benzene molecule:
Aromatic nitration is the best way to introduce nitrogen-containing substituents
into the benzene ring.
The aromatic nitro group serves as a directing group in further substitutions and
as a masked amino function.
Sulfonation is reversible.
Fuming sulfuric acid (8% SO3 in concentrated H2SO4) reacts with benzene to form
benzenesulfonic acid.
Because the reaction of SO3 with water is so exothermic, the sulfonation of
benzene can be reversed by heating benzenesulfonic acid in dilute aqueous acid.
Because sulfonation is reversible, it can be used as a blocking group to control
further aromatic substitution and then later removed.
Benzenesulfonic acids have important uses.
Long chain-branched alkylbenzenes can be sulfonated to the corresponding
sulfonic acids.
After conversion to their sodium salts, these compounds can be used as synthetic
detergents.
Sulfonate detergents have now been replaced by more biologically friendly,
biodegradable detergents.
Sulfonation is often used to impart water solubility to organic compounds, as in
the manufacture of certain dyes.