Page 57 - Chemical and process design handbook

Speight_Part 1_S&V 11/7/01 3:01 PM Page 1.43

SULFONATION

Sulfonation is the introduction of a sulfonic acid group (–SO H) into an
3
organic compound as, for example, in the production of an aromatic sul-
fonic acid from the corresponding aromatic hydrocarbon.
ArH + H SO → ArSO H + H O
2 4 3 2
The usual sulfonating agent is concentrated sulfuric acid, but sulfur
trioxide, chlorosulfonic acid, metallic sulfates, and sulfamic acid are also
occasionally used. However, because of the nature and properties of sul-
furic acid, it is desirable to use it for nucleophilic substitution wherever
possible.
For each substance being sulfonated, there is a critical concentration
of acid below which sulfonation ceases. The removal of the water formed
in the reaction is therefore essential. The use of a very large excess of
acid, while expensive, can maintain an essentially constant concentration
as the reaction progresses. It is not easy to volatilize water from concen-
trated solutions of sulfuric acid, but azeotropic distillation can some-
times help.
The sulfonation reaction is exothermic, but not highly corrosive, so sul-
fonation can be conducted in steel, stainless-steel, or cast-iron sulfona-
tors. A jacket heated with hot oil or steam can serve to heat the contents
sufficiently to get the reaction started, then carry away the heat of reac-
tion. A good agitator, a condenser, and a fume control system are usually
also provided.
1- and 2-naphthalenesulfonic acids are formed simultaneously when
naphthalene is sulfonated with concentrated sulfuric acid. The isomers
must be separated if pure α- or β-naphthol are to be prepared from the
product mix. Variations in time, temperature, sulfuric acid concentration,
and acid/hydrocarbon ratio alter the yields to favor one particular isomer,
but a pure single substance is never formed. Using similar acid/hydrocar-
o
bon ratios, sulfonation at 40 C yields 96% alpha isomer, 4% beta, while at
0
160 C the proportions are 15% α-naphthol, 8.5% β-naphthol.

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