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 hydrogen atom. Straight chain aliphatic alcohols, e.g. lauryl alcohol (dodecan-1-ol) can therefore be sulfated by chlorosulfonic acid without degradation or discolouration which often occurs with sulfur trioxide; this is important in the manufacture of hair shampoos, like sodium lauryl sulfate 2 (Equation 8); the sulfation reaction is often carried out in pyridine solution.

ROH + CISO3H [right arrow] ROSO3H + HCI (4)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (5)

ArH + ClSO3H [??] ArSO3H + HCl (6)

ArH + 2ClSO3H [??] ArSO2CI + H2SO4 + HCI (7)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (8)

With primary or secondary amines, chlorosulfonic acid yields the corresponding sulfamic acid (Equation 5); this reaction with cyclohexylamine afforded the artificial sweetener sodium cyclamate 3 (Equation 9). In contrast to alkanes, alkenes readily react with chlorosulfonic acid to give the alkyl chlorosulfonates; thus ethylene (ethene) is absorbed by chlorosulfonic acid to give ethyl chlorosulfonate 4 (Equation 10).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (9)

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (10)

Aromatic hydrocarbons also react smoothly with an equimolar amount of chlorosulfonic acid or an excess of the reagent to yield either the sulfonic acid or the sulfonyl chloride (Equations 6 and 7). The direct conversion of aromatic compounds into their sulfonyl chlorides (chlorosulfonation or chlorosulfonylation) is probably the most important reaction of chlorosulfonic acid because sulfonyl chlorides are intermediates in the synthesis of a wide range of sulfonyl derivatives. The process is of wide application because many substituents on the aromatic ring, e.g. alkyl, alkoxy, amide, carboxy, cyano, hydroxy, nitro and multiple bonds are unaffected by the reagent.

Chlorosulfonation is essentially an electrophilic substitution reaction, consequently the reaction is facilitated by the presence of electron-donor groups, like alkyl, alkoxy and hydroxy, when it proceeds under relatively mild conditions, e.g. the minimum excess (approx. two equivalents) of the reagent, temperatures of – 5 °C to 25 °C and an inert diluent such as chloroform. On the other hand, when electron-withdrawing groups, e.g. nitro, carbonyl or carboxy are present, the reaction requires more drastic conditions, e.g. a large excess of the reagent (five to ten equivalents) and heating to 100–150 °C.

The use of chlorosulfonic acid for the synthesis of organic sulfonyl chlorides has been reviewed. The work before 1943 is described with extensive references by Suter, by Jackson and, specifically for aromatic hydrocarbons, by Suter and Weston.

More recent reviews of sulfonation were carried out by Gilbert, Cerfontain, Andersen and Taylor and of chlorosulfonation by Bassin, Cremlyn and Swinbourne.

The use of chlorosulfonic acid for preparation of several important arylsulfonyl chlorides has been described. However, depending on the nature of the substrate and the experimental conditions, reaction with chlorosulfonic acid may also yield diaryl sulfones or chlorinated products.

Chlorosulfonic acid is widely used in organic qualitative analysis to prepare solid derivatives from liquid or low melting aromatic compounds, e.g. hydrocarbons, halides and ethers., The procedure involves conversion of the aromatic compound into the sulfonyl chloride, which is subsequently reacted with ammonia to yield the solid sulfonamide derivative which is suitable for melting point determination. Chlorosulfonic acid reacts with carboxylic acid anhydrides to give excellent yields of the acid chlorides (Equation 11).

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (11)

4 Safety

Chlorosulfonic acid reacts explosively with water producing fumes of hydrogen chloride and sulfuric acid; the pungent vapour is toxic and highly irritating to eyes, mucous membranes, skin and the respiratory tract.

When using the reagent, protective clothing, gloves and safety goggles are needed, because chlorosulfonic acid is highly corrosive and causes severe burns in contact with skin. Experiments using chlorosulfonic acid must be performed in an efficient fume cupboard. The acid is not itself flammable, but it can cause fires by contact with combustible materials because of the heat of reaction., Spills should be carefully diluted with large volumes of water. Absorption onto materials, such as expanded clay, diatomaceous earth, sand or soda ash mixture is effective, especially the latter since soda ash also partially neutralizes the acid.

5 Uses

Chlorosulfonic acid is employed in the manufacture of synthetic detergents such as sulfates of alkenes or unsaturated oils, polyoxypropylene glycol, long chain alcohols, alkylarenes or alkyl diphenyl ethers. It is also extensively used in the manufacture of sulfonamide antibacterials (sulfa drugs), diuretics and other pharmaceuticals, pesticides, artificial sweeteners (saccharin), disinfectants (chloramine and dichloramine T), plasticizers, dyes and pigments, sulfonyl polymers as plastics, and ion exchange resins. Chlorosulfonic acid is an oxidizing and dehydrating agent and functions as a catalyst in the esterification of aliphatic alcohols, alkylation of alkenes, and synthesis of alkyl halides from alkenic halides and isoalkanes containing tertiary hydrogen. It is used as a vulcanization accelerator, a source of anhydrous hydrogen chloride and in the tanning, textile and paper industries.

Overall, the approximate breakdown of the commercial applications of chlorosulfonic acid is as follows: detergents 40%; pharmaceuticals 20%; dyes 15%; pesticides 10% and miscellaneous uses, e.g. plasticizers, ion-exchange resins, etc. 15%.

Sulfonation and Chlorosulfonation of Organic Compounds

1 Introduction

Sulfonation is a bimolecular electrophilic substitution reaction (S2) which may be depicted in general terms as shown (Equation 1):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII] (1)

The general SE2 mechanism involves addition of the electrophile (E+) to the aromatic nucleus to form the σ-complex 1 which subsequently loses a proton to yield the substitution product 2. In this process either the first addition step or the second stage may be rate determining. In sulfonation, the sulfonic acid group is very bulky so that in the SE2 reaction k-1 becomes large in comparison with k2 which results in a kinetic isotope effect. The latter causes increasing resistance to sulfonation when a degree of steric hindrance is present in the aromatic substrate. In sulfonation, the ortho:para substitution ratios vary widely according to the reaction conditions which indicates that different electrophilic species may be involved.

Sulfonation, unlike nitration, is a reversible process which enables the sulfonic acid group (SO3H) to be employed in organic synthesis as a blocking and orientation-directing group. An example is provided by the synthesis of o-nitroaniline 3 from acetanilide 4 (Equation 2).

[FORMULA NOT REPRODUCIBLE IN ASCII] (2)

In acetanilide 4 the first sulfonation step causes the bulky sulfonic acid group to predominantly enter the para-position due to steric hindrance of the o-positions by the acetamido substituent. hi the second nitration stage, the nitro group enters the aromatic nucleus ortho to the electron-donating acetamido group. In the last stage, the sulfonic acid moiety is removed by heating with dilute sulfuric acid (protiodesulfonation) which demonstrates the unique reversibility of sulfonation. The large size of the sulfonic acid group makes sulfonation especially sensitive to steric factors in the aromatic substrate and the severe resistance to sulfonation may cause the initial sulfonation products to rearrange to the most thermodynami-cally stable isomer in which steric hindrance is a minimum. The classic example of this phenomenon is the thermal rearrangement of naphthalene -1-sulfonic acid 5 into the 2-sulfonic acid 6 (Equation 3).

[FORMULA NOT REPRODUCIBLE IN ASCII] (3)

Thus, sulfonation of naphthalene with concentrated sulfuric acid at 80 °C afforded 96% of the 1-sulfonic acid 5, whereas at 165 °C 85% of the 2-sulfonic acid 6 was obtained.

In the sulfonation of monosubstituted benzenes by concentrated sulfuric acid, electron-donating substituents (X) such as acetamido, alkoxy, hydroxy or alkyl groups facilitate the reaction and, in such cases, sulfonation occurs under relatively mild conditions (0–35 °C) to give a mixture of the o- and p-sulfonic acids1 (Equation 4).

[FORMULA NOT REPRODUCIBLE IN ASCII] (4)

The p-isomer generally predominates because of the large steric size of the sulfonic acid group and is almost the sole product when other large substituents are present as in the case of acetanilide 4. With benzenes containing electron-withdrawing substituents (Y), such as carbonyl, carboxy, nitro or sulfonic acid groups, sulfonation is more difficult and demands comparatively forcing conditions (a large excess of the sulfonating reagent and temperatures . 100 °C) then the resultant product is mainly the m-sulfonic acid (Equation 5).

[FORMULA NOT REPRODUCIBLE IN ASCII] (5)

The sulfonation of halobenzenes is anomalous because, although the halogen atom exerts a powerful electron-withdrawing (-I) inductive effect with deactivation, sulfonation of halobenzenes occurs in the o/p-positions and not in the m-position as would be anticipated for an electron-withdrawing group. The observed orientation of sulfonation is due to electron onation involving the electromeric (+E) effect from the lone electron pairs on the halogen atom in the presence of the electrophilic reagent. A similar effect is also observed with substituents of the type -CH=CHR (where R = an electron-withdrawing group such as nitro).