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
(ii)           3H2SO4    SO3H+ + H3O+ + 2HSO4-

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 :

mechanism of Sulphonation

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%)