Sulphonation
Sulphonation
Sulphonation
is the process of replacing a hydrogen atom by a sulphonic acid group, SO3H.
Sulphonation of a normal alkane from hexane onwards may be carried out by
treating the alkane with oleum (fuming sulphuric acid). The case of replacement
of hydrogen atoms is : Tertiary very much greater than secondary, and secondary
greater than primary; replacement of a primary hydrogen atom in sulphonation is
very slow indeed. Isobutane, which contains a tertiary hydrogen atom, is
readily sulphonated to give t-butylsulphonic acid :
(CH3)3CH + H2SO4/SO3 → (CH3)3CSO3H + H2SO4
Mechanism of sulphonation
This is not settled, one reason being the difficulty in
establishing the nature of the sulphonating species (cf. nitration).
Thus, various possibilities exist for sulphonation with concentrated sulphuric
acid, all arising from different equilibria, e.g.,
(i) 2H2SO4 ⇌ SO3 + H3O+ +HSO4-
(ii) SO3 +
H2SO4 ⇌ H2SO4(SO3)
or H2S2O7
External
work based on kinetic studies in concentrated sulphuric acid and in oleum
strongly favours the theory that the active species is sulphur trioxide. This
is also believed to be the case with aqueous sulphuric acid. If we accept this,
then it appears that the sequence of the steps and their relative rates depend
on the conditions. Thus, in concentrated sulphuric acid, the mechanism proposed
is :
On the other
hand, in oleum the mechanism proposed is :
When sulphur
trioxide is used in an organic solvent, the evidence is that the mechanism is
different from those given above, e.g.,
according to Cerfontain et al. (1968), sulphonation of p-dichlorobenzene
with sulphur trioxide in nitromethane proceeds as follows :
In addition
to the difficulties mentioned above, there are also the complications that sulphonation
is reversible and that isomerisation often occurs during the reaction.
These factors affect the isomer distribution under different conditions. Thus, e.g., Wanders et al. (1963) have
shown that toluene-p- and m-sulphonic both isomerise when heated
in aqueous sulphuric acid to yield a mixture of mainly m- and p-sulphonic
acid with only a small amount of the o-isomer. Also, the o-isomer
rapidly isomerizes to the p-isomer, but finally the same equilibrium mixture is
obtained as before (i.e., all three isomers). From these
results it appears that toluene-m-sulphonic acid is the most and the
o-acid the least thermodynamically stable isomer.
Benzenesulphonic
acid may be readily
prepared by heating benzene with concentrated sulphuric acid at 80oC:
C6H6 + H2SO4
=
C6H5SO3H +H2O (75-80%)
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