1. Field of the Invention
This invention relates to an improved process for the production of light-colored lower alkanesulfonic acids by reaction of the corresponding alkyl halides with alkali metal sulfite in aqueous solution at elevated temperature and pressure and subsequent separation of the alkali metal halide, release of the alkanesulfonic acid formed and recovery thereof.
2. Statement of Related Art
More particularly, the invention seeks to provide high-purity alkanesulfonic acids which are suitable for use, for example, in the field of industrial detergents. This requires above all the maintenance of low maximum levels of chloride ion contamination in the alkanesulfonic acid. At present, industrial quantities of highpurity alkanesulfonic acids are not available at acceptable prices. Accordingly, the object of the invention is to provide access to the particularly interesting class of C.sub.1-6 alkanesulfonic acids, preferably the corresponding free sulfonic acids containing 1 to 4 carbon atoms, which are suitable for use in practice precisely because of their reasonable price. Methanesulfonic acid is particularly important in this regard. The starting materials required for its production are available in large quantities as inexpensive chemicals.
Short-chain alkanesulfonic acids can be produced by oxidation of the corresponding mercaptans with nitric acid or peracids and with iodine in the presence of bromide ions in dimethyl sulfoxide. Unfortunately, these processes only give moderate yields. In addition, mercaptans are too expensive for industrial processes.
In addition, short-chain alkanesulfonic acids can be produced from alkanes by sulfochlorination and subsequent saponification of the alkanesulfonyl chloride or by sulfoxidation. However, the sulfonation reaction takes place at the hydrocarbon chain in statistical distribution and, in addition, may even take place several times, so that 1-sulfonic acids cannot be selectively produced.
The reaction of olefins with sodium hydrogen sulfite and subsequent release of the alkanesulfonic acids also gives unwanted secondary products.
The reaction of short-chain alkyl halides with sodium sulfite to form the corresponding alkyl sodium sulfonates by the so-called Strecker synthesis is known from B1. Chem. Soc. Japan 32, 850 (1959). However, the release of the short-chain alkanesulfonic acids from the sodium salts obtained is not described. In fact, it also involves difficulties because both the short-chain alkanesulfonic acids and their salts and also the sodium salts formed as secondary reaction products are highly soluble in water. Accordingly, the desired alkanesulfonic acid cannot simply be extracted from the reaction mixture formed.
Applicants' earlier patent application DE 3812846 A1 "A process for the production of C.sub.1-6 alkanesulfonic acids" relates to a process for the production of linear or branched C.sub.1-6 alkanesulfonic acids from their alkali metal salts which is characterized in that the alkali metal salts are reacted with hydrogen chloride in a solution or suspension in C.sub.1-4 monoalkanols, the alkali metal chlorides precipitated are separated and the C.sub.1-6 alkanesulfonic acids are isolated from the monoalkanol phase. The Examples of this earlier application describe the production of the particular sodium salts of the alkanesulfonic acids in question by the Strecker synthesis by reaction of the corresponding alkyl chlorides with sodium sulfite in aqueous solution at 150.degree. C. over a reaction time of 8 hours. The corresponding sodium salt of methanesulfonic acid is prepared by a 12-hour reaction in an autoclave at 100.degree. C. Comparatively limited final reaction pressures are established.
By contrast, the problem addressed by the present invention was firstly to establish reaction conditions under which the alkali metal salts and particularly the sodium salts of the lower alkanesulfonic acids in question, particularly methanesulfonic acid, could be produced considerably more economically and, secondly, to provide a simple method of separating the free alkanesulfonic acid from the aqueous solutions of the sodium salts formed, preferably without having to use auxiliary solvents in the form of the lower monoalkanols from the teaching of the earlier application cited above.