Aromatic Reactions
Most common reactions for aromatics
involve replacement of ring hydrogens
by other atoms or groups
(substitution reactions)
Mechanism of Electrophilic Aromatic
Substitution Reactions
• Electrophile – a reagent with a strong demand
for electrons
• Carbocation – a positive charged carbon
fragment
• Nucleophile – a negative ion, or one with an
unshared electron pair
• The aromatic ring acts as a “supplier” of
electrons to the electrophile (similar to how an
electrophile attacks an alkene double bond)
• The major difference with aromatics is that
the carbonium ion intermediate is resonance
stabilized
• Halogenation
.. ..
:Cl:Cl: + FeCl3
.. ..
..
nucleophile
:Cl+ + FeCl4..
electrophile – attacks a double bond
•
•
•
..
:Cl+
• the positive charge can be delocalized to the
carbon atoms that are ortho & para to the
position taken by the chlorine
• delocalized – an electron can move through
more than its two carbons
– this type of molecule is more stable
Carbocation intermediates
Resonance forms of a benzonium ion
(benzene ring with a positive charge)
1. Electrophilic Aromatic Substitution
Reactions
• a. Halogenation – requires the presence
of an iron catalyst (FeCl3 or FeBr3) to
convert Cl2 or Br2 (weak electrophiles)
to Cl+ or Br+ (strong electrophiles)
• Ex.; Chlorinate benzene
• b. Nitration - uses HNO3 in the
presence of H2SO4
• Ex.; Nitrate benzene.
• c. Alkylation (Friedel-Crafts Reaction)
- Discovered by Frenchman Charles Friedel
and American James Craft in 1877.
- Any alkyl halide in the presence of an
aluminum halide catalyst reacts with an
aromatic
- Ex.; React benzene with chloromethane.
- Ex.; React benzene with ethylchloride
• d. Sulfonation - an aromatic reacting
with H2SO4 in the presence of heat
• Ex.; Sulfonate benzene.
• A substituent that is already
on the benzene ring prior to
a substitution reaction
determines the position of
any entering substituent
Meta-directing groups
nitro
sulfonyl
carbonyl
cyano
- These direct the entering substituent to the
meta position relative to their position.
- Ex.; React nitrobenzene with methylchloride.
- Ex,; Chlorinate benzenesulfonic acid.
Ortho-para directing groups
• -Cl, -Br, -OH, -NH2, -OCH3, alkyl groups
• -these direct the entering substituents to
the ortho and para positions.
• Ex.; React chlorobenzene with
methylchloride.
• Ex.; Chlorinate phenol.
2. Catalytic Hydrogenation
• An addition reaction that requires 3 moles
of hydrogen, Ni, pressure and heat. The
reaction cannot be stopped with one or two
moles of hydrogen since the intermediate
cyclohexadiene or cyclohexene are more
easily hydrogenated than the aromatic
hydrocarbon.
• Ex.; Hydrogenate benzene.
• Catalytic hydrogenation is usually used to
make alkyl-substituted cyclohexanes
• Ex.; Synthesize ethylcyclohexane from
benzene in two steps.
3. Free Radical Substitution of
Aromatic Side Chains
• The side chain becomes substituted via the
free radical chain mechanism
• Substitution will always occur at the benzyl
carbon
• Ex.; Chlorinate toluene.
• Ex.; Brominate propylbenzene.
• Further reactions (ex; dichlorination or
dibromination) leads to replacement of the
remaining benzylic hydrogens.
• Ex.; Dichlorinate toluene.
4. Oxidation of Aromatic Side Chains
• The product will always be benzoic acid,
regardless of the length of the side chain.
• Reagents are KMnO4 and heat
• Ex.; Oxidize toluene.
• Ex.; Oxidize butylbenzene.
• If 2 or more alkyl groups are present, they
are all oxidized.
• Ex.; Oxidize p-dimethylbenzene.