Introduction

Chlorosulfonic acid was first prepared by Williamson in 1854 by the action of phosphorus pentachloride on concentrated sulfuric acid and later by the direct action of hydrochloric acid on sulfur trioxide. Other methods of preparation include: distillation of fuming sulfuric acid (oleum) with phosphorus pentoxide in a stream of gaseous hydrogen chloride; the action of phosphorus trichloride or oxychloride, chlorine, thionyl chloride, or sulfur monochloride on concentrated or fuming sulfuric acid; passing a mixture of sulfur dioxide and chlorine into glacial acetic acid; or reaction of carbon tetrachloride with fuming sulfuric acid.

Chlorosulfonic acid is also named chlorosulfuric acid in Chemical Abstracts, but chlorosulfonic acid is the commercial name by which it is more widely known. Other names are: sulfuric chlorohydrin, sulfuric acid chlorohydrin, mono-chlorosulfuric acid, monochlorosulfonic acid, chlorohydrated sulfuric acid and sulfuryl hydroxychloride.

1 Manufacture

Modern chemical plants manufacture chlorosulfonic acid by the direct union of equimolar quantities of sulfur trioxide and dry hydrogen chloride gas. The process is a continuous flow operation and, since it is highly exothermic, heat removal is essential to maintain the reaction temperature at 50–80 °C. The sulfur trioxide may be used in the form of 100% liquid or as a dilute gaseous mixture from a contact sulfuric acid plant. Likewise, the hydrogen chloride may be 100% gas or in a diluted form. The chemical reactor may be a packed column cooled by a water-cooled condenser to moderate the vigour of the reaction and hence avoid decomposition of the product. The chemical plant must be composed of non-corroding materials such as glass, glass-lined steel, enamel or steel coated with polytetraethylene (PTFE) so that the chlorosulfonic acid is not much contaminated with iron. A typical analysis of commercial chlorosulfonic acid would be as follows: ClSO3H 98–99.5%; H2SO4 0.2–2%; free SO3 0–2%; HCl 0–0.5% and Fe 5–50 ppm. Chlorosulfonic acid may be stored and transported in steel containers, but in this case the iron content will be in the range 25–50 ppm. The annual production of chlorosulfonic acid increased substantially after World War II due to expansion of the synthetic detergent industry and of dyes, drugs and pesticides. Worldwide there are approximately twenty listed manufacturers of chlorosulfonic acid: in the USA the two major ones are EI DuPont de Nemours Co Inc (> 30 000 tyr-1) and the Gabriel Chemical Co (> 13 000 tyr-1). The price of chlorosulfonic acid has risen from approximately US$ 209 tr-1 in 1977 to US$ 389t-1 in 1991. Further details of the preparation of chlorosulfonic acid are given in Chapter 10, p 272.

2 Physical Properties

Chlorosulfonic acid (ClSO3H) is a colourless or straw-coloured liquid which fumes in air and decomposes slightly at its boiling point. The physical properties are shown in Table 1; these vary slightly from sample to sample reflecting the different amounts of the various impurities present, e.g. hydrogen chloride, sulfur trioxide and related compounds. It is difficult to prepare a really pure sample of chlorosulfonic acid because of its instability at the boiling point, even under reduced pressure, which tends to degrade rather than purify the molecule. Pure chlorosulfonic acid has been obtained by fractional crystallization. Chlorosulfonic acid is a strong acid which is toxic and corrosive and behaves as a dehydrating, oxidizing and chlorinating agent. It is soluble in halocarbons containing hydrogen; for instance, chloroform, dichloromethane and 1,1,2,2-tetrachloroethane but is only sparingly soluble in carbon tetrachloride and carbon disulfide. It is soluble in liquid sulfur dioxide, sulfuryl chloride, acetic acid, acetic anhydride, trifluoroacetic acid, trifluoroacetic anhydride and nitrobenzene.

The structure of chlorosulfonic acid 1 was proved by Dharmatti5 who showed magnetic susceptibility measurements that the chlorine atom was directly attached to the sulfur atom and further supporting evidence was obtained from Raman spectral studies.

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3 Chemical Properties

Chlorosulfonic acid is a powerful acid with a relatively weak sulfur-chlorine bond. It fumes in moist air producing pungent clouds of hydrogen chloride and sulfuric acid (Equation 1).

CISO3H + H2O [right arrow] H2SO4 + HCI (1)

When chlorosulfonic acid is heated it partially decomposes into sulfuryl chloride (SO2Cl2), sulfuric acid, sulfur trioxide, pyrosulfuric acid (H2S2O7), hydrogen chloride, pyrosulfuryl chloride (Cl2S2O5) and other compounds. At 170 °C, there is an equilibrium between chlorosulfonic acid, sulfuryl chloride and sulfuric acid (Equation 2). Sulfur dioxide and chlorine are not observed when chlorosulfonic acid is heated between 170 and 190 °C, but do appear at higher temperatures or when it is heated in a sealed tube (Equation 3).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (2)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (3)

When chlorosulfonic acid is treated with powerful dehydrating agents like phosphorus pentoxide, it is converted into its anhydride, pyrosulfuryl chloride (Cl2S2O5). Chlorosulfonic acid, by boiling in the presence of mercury salts or other catalysts, decomposes quantitatively into sulfuryl chloride and sulfuric acid. It functions as a chlorinating agent with sulfur, arsenic, antimony and tin and yields sulfur dichloride and the tetrachlorides of the other elements. With powdered tellurium or selenium, chlorosulfonic acid gives cherry-red or moss-green colours respectively and these can be used in spot tests for the acid. On heating with charcoal, it is decomposed with the evolution of sulfur dioxide, hydrogen chloride and carbon dioxide. In synthetic organic chemistry, chlorosulfonic acid can be used for sulfation of alcohols (Equation 4); sulfamation of amines (Equation 5); and the sulfonation and chlorosulfonation of aromatic compounds (Equations 6 and 7). In the latter reaction, there must be an excess (at least two equivalents) of the reagent present. All these reactions depend on the relative weakness of the sulfur-chlorine bond in chlorosulfonic acid. Chlorosulfonic acid only reacts slowly with saturated aliphatic hydrocarbons in the absence of a double bond or other reactive site, such as a tertiary...