Free-radical reactions sulfochlorination

The usual way to achieve heterosubstitution of saturated hydrocarbons is by free-radical reactions. Halogenation, sulfochlorination, and nitration are among the most important transformations. Superacid-catalyzed electrophilic substitutions have also been developed. This clearly indicates that alkanes, once considered to be highly unreactive compounds (paraffins), can be readily functionalized not only in free-radical from but also via electrophilic activation. Electrophilic substitution, in turn, is the major transformation of aromatic hydrocarbons. 

The most important applications of photochemical reactions in organic technol-ogy/synthesis are in the field of free radical reactions such as halogenation (mainly chlorination), sulfochlorination, sulfoxidation, and nitrosation. There is an increasing trend now toward development of nonradical photoreactions, particularly in the synthesis of vitamins, drugs, and fragrances. Table 26.2 lists representative syntheses of commercial value that have been carried out with light (see, among others, Coyle, 1986 Pfoertner, 1991, for more examples). 

Sulfochlorination of Paraffins. The sulfonation of paraffins using a mixture of sulfur dioxide and chlorine in the presence of light has been around since the 1930s and is known as the Reed reaction (123). This process is made possible by the use of free-radical chemistry and has had limited use in the United States. Other countries have had active research into process optimization (124,125). 

Radiation-induced substitution reactions have been reviewed by Wilson (1972) with examples of nitration, nitrosation, sulfochlorination, and others. These generally proceed by a free-radical mechanism. The free radicals are generated by the action of radiation on the reagent, which is present in large excess—for example,... 

Alkanesulfonyl chlorides are prepared in good yields by the photoreaction of sulfur dioxide and chlorine with saturated hydrocarbons.89 157 158 This sulfochlorination, known as the Reed reaction,159 presumably proceeds by a free-radical mechanism 158 160... 

The free-radical-induced reaction of alkanes with sulfuryl chloride characteristically results in the chlorination of hydrocarbons. However, when pyridine is added to the irradiated reactants, sulfochlorination occurs in quite satisfactory yield. For example, the irradiated reaction of cyclohexane and sulfuryl chloride in the presence of pyridine resulted in a 54.8% yield of cyclohexanesulfonyl chloride and only 9.4% of chlorocyclohexane.161... 

Photochemical sulfochlorination of saturated hydrocarbons has been used in the production of long-chain alkanesulfonyl chlorides, again as precursors to the salts as surfactants. These processes involve free radical intermediates. Analogous to the sulfoxidation of long-chain alkanes, these processes also provide mixtures of alkanesulfonyl chloride and chloroalkane products. Nonetheless, pure products can be obtained for short C1-C3 alkyl chain lengths. A high-pressure gas phase process for the reaction of CH4 with CI2 and excess SO2 is used in one commercial manufacturing route to... 

Several sulfonations using compounds of SOj (sulfochlorination, sulfoxidation, the addition of bisulfite to an olefinic bond) proceed by free radical chain reactions which characteristically require catalysts for initiation. Many catalysts have been proposed for these three types of sulfonation. As mentioned in the discussion of these reactions, common catalysts used are, respectively, light, acetic anhydride, and peroxides. In the second case, the acetic anhydride is not strictly a catalyst since it is used in stoichiometric amount and yields an acetate derivative intermediate. 

Alkane sulfonates are mainly produced by two methods, i.e. the process of sulfochlorination and the process of sulfoxidation. In both processes, a radical chain reaction takes place and therefore the alkane starting material must be free from branched compounds, olefines and aromatics because such compounds will act as chain-stoppers in the chain reactions. Highly pure straight alkanes of paraffin fractions from 13 to 18 carbon atoms are used, with an optimum in the range of 14-17 carbons in the chain. 

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