Alkanes sulfochlorination

Sulfochlorination of Alkanes. Sulfochlorination of straight-chain alkane mixtures with S02 and Cl2 under light illumination slightly above ambient temperature yields sec-alkyl sulfochlorides.103,196 Polysulfochlorination is prevented by running the reaction at low conversions (about 30%). The product is hydrolyzed to yield sodium alkylsulfonates used in detergents. 

Because at 80°C, S — 0.5, and at 100°C no sulfochlorination at all takes place, the process is run between 35 °C and 40°C. The selectivity also depends on the molar ratio of sulfur dioxide to chlorine. Provided that there is a fine dispersion of the reacting gases in the alkane for a molar ratio of 3, the selectivity approaches 0.98 (Fig. 5). Sufficient turbulence in the reaction medium (supported by additional mechanical energy) means that the sulfochlorination itself proceeds at the gas-liquid interface. 

The radical chain mechanism of the sulfochlorination is very similar to that of the chlorination. Accordingly, in normal cases the regioselectivities of the sulfochlorination and the chlorination are equal. For example, (-1) substituents decrease the reactivities of the adjacent C-H bond. This influence can even be observed at the y position. Thus, the consecutive second sulfochlorination affords no geminal or vicinal disulfochlorides in the product. Where there are differences between the regioselectivities of sulfochlorination and chlorination (as in the case of isoalkanes), it is because under the conditions of sulfochlorination, chlorination also takes place to a considerable extent. Figure 6 shows the main components of a sulfochlorination mixture. Today the kinetics and the regioselectivity of the sulfochlorination of /z-alkanes are so well known that the kinetic modeling of the concentration-conversion curves is possible for all partners of the reaction [12]. 

A modern sulfochlorination is run continuously (Fig. 7). To limit the formation of di- and polysulfochlorides (Fig. 8) up to eight reactors are connected in a cascade. For a 25% alkane conversion, a residence time of 4-5 h is necessary if the circulating and cooling systems are appropriately designed. Since a lower gas flow rate promotes the formation of di- and polysulfochlorides, it makes no sense to run a sulfochlorination plant much below name-plate capacity. 

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 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... 

Of the large number of possible ways of synthesizing alkanesulfonates, only sulfochlorination of alkanes (conversion with sulfur dioxide and chlorine to form alkane sulfonyl chlorides and their saponification with sodium hydroxide Table 1, entry 10) and sulfoxidation (reaction with sulfur dioxide and oxygen and neutralization of the sulfonic acids Table 1, entry 11) are of industrial importance [18]. 

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... 

A patent has been lodged covering the sulfochlorination of gaseous alkanes. " Photochemical bromination using N-bromosuccinimide of the arenes (136) has been examined under a nitrogen atmosphere. The remote functionalization of the alcohol (137) has been carried out using DIB/Ii/cyclohexane. The product obtained from this, after oxidation, is the iodoketone (138, 84%). ... 

Sulfochlorination under the above conditions is smoothest and most unambiguous for normal alkanes and for cycloalkanes of simple structure branched alkanes and alkylated cycloalkanes are less suitable because the hydrogen on tertiary carbon atoms is not replaced. 

Introduction. Alkanesulfonates are anionic surfactants. They are produced by Bayer through the sulfochlorination process. Optimum properties are obtained when a mixture of straight-chain alkanes with 14 -17 carbon atoms is used. 

Conventional Method of Recovering the Alkanesulfonates. The reaction product of sulfochlorination (obtained in ca. 30 - 33 % yield) is hydrolyzed at ca. 100 C with aqueous sodium hydroxide, and the mixture obtained is cooled in several stages. Most of the unreacted alkanes separate in the hot phase and are recycled to the production process. The aqueous hydrolyzed solution contains residual alkanes, alkanesulfonates, and sodium chloride produced from reaction of the sulfonylchloride with sodium hydroxide solution. 

Ecological Aspects of the Modified Process. Conventional treatment of the hydrolysis mixture leads to a wastewater stream whose organic load, consisting of lost product and emulsified starting material, is ca. 20 g/L COD. The organic components also contain a certain amount of combined chlorine formed as a result of side reactions with the alkane chains during sulfochlorination. 

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. 

The main products of the sulfochlorination process are the alkane sulfochlorides, which in turn are saponified to the desired alkane sulfonates. This process was first discovered in the 1930s, and industrially realized shortly afterwards. Sulfochlorination is a radical chain reaction, initiated by irradiation with short-wavelength light. 

The yield of alkane sulfochloride depends on the molar ratio of SO2 to CI2, the purity of the feedstock and the reaction temperature. Optimal conditions in the technical process are 35-40°C and a molar ratio of SO2 to CI2 of 3 1. Nevertheless, there are side-reaction such as the chlorination of the alkane, chlorination of the formed alkane sulfochlorides, and the formation of sulfuric acid from traces of water. In addition, a second sulfochlorination reaction may take place. Thus, the main components of the sulfochlorination process are the desired alkane monosulfochloride, di- and trisulfochlorides and chlorinated products of these species, as well as monochlo-ralkanes and dichloralkanes. 

Sulfochlorination was the first successful procedure for preparing alkane sulfonates. Nowadays, however, these materials are mainly produced by the sulfoxidation process. 

Berthold, H., Matte, E., Mbrlein, J., Rose, F., Scholz, J., Werner, W. and Wirth, D., Preparation of alkane-sulfonate solutions with high viscosity from sulfochlorinated alkanes, German (East) DD 292,448 DD 292,449, 1991. 

Alkylsulfonates are also important anionic detergents but less so for the production of solid washing powders as they cannot readily be processed in spray drying processes. For their production long-chain alkanes are treated with either SO2/CI2 (sulfochlorination) or with SO2/O2 (sulfoxidation). Both processes require operation at low alkane conversion to avoid undesirable multi-sulfonation. 

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