A general method of obtaining methyl and ethyl sulphates is to treat the alcohols with concentrated sulphuric acid1 or with chlorosulphonic acid.2 The first method is convenient, but since the reaction is a reversible one it does not give the theoretical yield.

For the methyl hydrogen sulphate, 1 part by weight of methyl alcohol is mixed with 2 parts of strong sulphuric acid, and the mixture kept warm for some hours. It is then diluted with water, and barium carbonate added in excess, to neutralise the free acids. After filtration from the barium sulphate and excess of carbonate, the filtrate, which contains the barium salt of methyl hydrogen sulphate, is evaporated to a conveniently small bulk, and treated with just sufficient sulphuric acid to precipitate all the barium as sulphate, and then again filtered. An aqueous solution of the free methyl sulphuric acid is thus obtained, which can now be evaporated in a vacuum over strong sulphuric acid.

Methyl hydrogen sulphate is an oily liquid, which does not solidify on cooling to - 30°. On distillation, it decomposes into dimethyl sulphate and sulphuric acid.

Dimethyl sulphate, (CH3),SO4, is made by distilling monomethyl sulphate in a vacuum. To obtain a good yield, the monomethyl compound should be free from water and other impurities. It need not, however, be separately prepared. A mixture of 25 kilos. of sulphuric anhydride with 10 kilos. of nearly absolute methyl alcohol is made, the temperature being kept below 0°, and preferably between - 5° and - 10°. The mixture is then distilled in vacuo. An almost theoretical yield of 19 kilos. can be obtained.3

Alternatively, 100 parts of chlorosulphonic acid are run into 27 parts of methyl alcohol at a temperature of - 10° to - 15°, care being taken that the liquids are well mixed, and moisture excluded. The mixture is then distilled in a vacuum 4 (20 mm.; the temperature being about 140°). Yield, about 80 per cent. of the theoretical.

The local overheating which easily occurs may be avoided by diluting the methyl alcohol with carbon tetrachloride. An enamelled vessel fitted with a reflux condenser is charged with 6.4 kilos. of methyl alcohol (99 per cent. strength) and 20 kilos. of carbon tetrachloride. Into this mixture 24 kilos. of chlorosulphonic acid are slowly run, with stirring. The tetrachloride is then distilled off on a water-bath, and used again in the next charge: the residue is distilled in vacuo as before to obtain the dimethyl sulphate.1 Sulphuric anhydride may replace the chlorosulphonic acid.

1 Dumas and Peligot, Annalen, 1836, 15, 40. 2 Claesson, J. pr. Chem., 1879, 19, 231 et seq.

3 Merck, D.R.-P., 133542, 1901; Winther, " Patente," 1, 32.

4 Ullmann, Annalen, 1903, 327, 106. For laboratory details, see Lassar-Cohn, " Arbeitsmethoden," 1907, p. 294.

Haworth and Irvine have recently described a method of preparing dimethyl sulphate by the interaction of dimethyl ether and sulphur trioxide (B.P. 122498). See also p. 357.

Dimethyl sulphate finds considerable application as a methylating agent in the preparation of dyestuffs. It is a colourless, oily liquid, insoluble in water, and with a peppermint-like flavour. Boiling point 188°; sp. gr. 1.333 at 15°. It requires care in the handling, as it affects the respiratory organs strongly.2