The direct introduction of the sulfonic acid group is one of the most important reactions in industrial organic chemistry. Together with nitration and chlorination, it belongs to the important group of electrophilic aromatic substitution reactions. It gives high yields under relatively mild conditions and usually results in well-defined benzene derivatives.

Direct Sulfonation

Aqueous sulfuric acid, at various concentrations from about 76% up to 100 %, is frequently used as the sulfonating agent. In industry 100 % sulfuric acid is often called monohydrate. Oleum is a solution of sulfur trioxide in sulfuric acid; for sulfonation it is generally used at SO3 concentrations of 20 or 65wt % because the solidification points at these two concentrations are minimal (0 and 2 ?C, respectively). Other concentrations are less favorable with regard to storage and transportation because the solidification points are higher (for example, 45 % oleum solidifies already at 35 ?C).

Sulfonation with sulfuric acid is a reversible reaction (see also Figure 1): ArH+H2SO4 → ArSO3H+H2O
It can be shifted optimally to the right if the water of the reaction is bound or removed by distillation. Higher temperatures usually shift the equilibrium towards the right. However, both methods increase sulfone formation.
The water of the reaction can also be bound by adding thionyl chloride:

C6H6 +H2SO4 + SOCl2 ?→ C6H5SO3H+SO2 + 2 HCl

For example, the sulfonation of benzene with sulfuric acid and thionyl chloride gives a mixture consisting of 96.3 % benzenesulfonic acid, 2.7 % sulfuric acid, and 1.6% diphenyl sulfone [29]. Similar yields are obtained when sulfonating toluene or chlorobenzene in this way.

No reaction water is formed in sulfonations using chlorosulfuric acid or sulfur trioxide. However, increased sulfone formation occurs. As a sulfonating agent pure sulfur trioxide generally reacts too violently and leads to extensive side reactions including oxidation and sulfone formation. Hence it is used in complexed form, e.g., with pyridine, dioxane, trimethylamine, or dimethylformamide. Many industrial processes use sulfur trioxide gas as the sulfonating agent, normally mixed with an inert gas such as air, nitrogen, or carbon dioxide. Thus, the sulfonating agent is diluted or evenly distributed so that the heat of the reaction is quickly dissipated (see also →Surfactants). In the liquid phase, sulfur dioxide or dichloromethane is suitable as diluent. However, dichloromethane reacts with sulfur trioxide at temperatures above 0 ?C, and highly poisonous decomposition products are formed, such as bis(chloromethyl) ether. 1,1-Dichloroethane also reacts with sulfur trioxide at 80 ?C; the decomposition products include phosgene.

In stoichiometric amounts, chlorosulfuric acid serves as a sulfonating agent according to ArH + ClSO3H ?→ ArSO3H+ HCl

When using an excess of chlorosulfuric acid, sulfonyl chlorides are formed, see Section 1.9.1. The chlorosulfonation of toluene, for example, with chlorosulfuric acid at low temperature yields predominantly o-toluenesulfonyl chloride. This is the first step in the industrially important synthesis of saccharin (→Sweeteners).

Oxidation of Sulfur Compounds

Such sulfur compounds as thiophenols and diaryl disulfides can be oxidized with chlorine solution, permanganate, or nitric acid to form sulfonic acids. The process is industrially important wherever it is difficult or impossible to introduce the sulfonic acid group directly and the starting compounds are easily produced. An example of the use of halogens as oxidizing agents is given in.Diazo ReactionAryldiazonium halides are converted to aromatic sulfonic acids in glacial acetic acid containing SO2 in the presence of copper(I) chloride.

This reaction, which was first observed by L. Landsberg, permits the production of, for example, 1,2-benzenedisulfonic acid from orthoanilic acid with a yield of 68 %.

Sulfite Reaction

Aromatic halogen compounds can react with sulfite to form sulfonic acids if the halogen substituent is activated by nitro groups. For example, 1-chloro-2,4-dinitrobenzene reacts with sulfite to give 1,3-dinitro-4-benzenesulfonic acid. The reaction is catalyzed by copper ions. 2- Formylbenzenesulfonic acid and 2-sulfobenzoic acid are obtained similarly from the corresponding chlorine compounds.

The addition of hydrogensulfite to aromatic systems succeeds when their aromatic character is disturbed by the presence of certain substituents. The formation of 3-hydroxybenzenesulfonic acid from resorcinol and the sulfitation of m-dinitrobenzene with simultaneous reduction of a nitro group (Piria reaction) are examples:

Hydrogensulfite addition, followed by the formation of a benzenesulfonic acid, gives satisfactory yields in special cases only and has therefore acquired little industrial importance so far.