Step 3: Formation of a Substituted productrather than kinetic, control can be achieved at high temperatures. Hence, directive effects
are not expected to play a key role in determining the proportions of isomeric products of
high-temperature sulfonation.
Aromatic sulfonic acids can be intermediates in the preparation of dyes and many phar-
maceuticals. Sulfonation of aniline produces p-aminobenzenesulfonic acid or sulfanilic acid,
which is a zwitterionic compound with an unusually high melting point. The amide of this
compound and related compounds form a large group of sulfa drugs (a type of antibiotic).
Overall reaction: ArH + SO 3 →ArSO 3 H
83.3.3 Electrophilic aromatic nitration
Nitration occurs with aromatic organic compounds via an electrophilic substitution mech-
anism involving the attack of the electron-rich benzene ring by the nitronium (nitryl) ion.
Benzene is commonly nitrated by refluxing with a mixture of concentrated sulfuric acid
and concentrated nitric acid at 50°C. The sulfuric acid is regenerated and hence acts as a
catalyst.
Selectivity is always a challenge in nitrations. Fluorenone nitration is selective and yields a
tri-nitro compound or tetra-nitro compound by tweaking reaction conditions just slightly.
Another example of trinitration can be found in the synthesis of phloroglucinol. Other
nitration reagents include nitronium tetrafluoroborate which is a true nitronium salt. This
compound can be prepared from hydrogen fluoride, nitric acid and boron trifluoride. Aro-
matic nitro compounds are important intermediates for anilines; the latter may be readily
prepared by action of a reducing agent.
Overall reaction: ArH + HNO 3 →ArNO 2 + H 2 O
83.3.4 Friedel-Crafts alkylation
Figure 175 Friedel-Crafts alkylation of benzene with methyl chloride
The Friedel-Crafts reactions, discovered by French alkaloid chemist Charles Friedel and his
American partner, James Crafts, in 1877, is either the alkylation or acylation of aromatic
compounds catalyzed by a Lewis acid. They are very useful in the lab for formation of
carbon-carbon bonds between an aromatic nucleus and a side chain.
Source of electrophile
Friedel-Crafts alkylation is an example of electrophilic substitution in aromatic compounds.
The electrophile is formed in the reaction of an alkyl halide with a Lewis acid. The Lewis
acid polarizes the alkyl halide molecule, causing the hydrocarbon part of it to bear a positive
charge and thus become more electrophilic.
CH 3 —Cl + AlCl 3 →CH 3 ++ AlCl 4 −
or