This section is from the book "Alcohol, Its Production, Properties, Chemistry, And Industrial Applications", by Charles Simmonds. Also available from Amazon: Alcohol: Its Production, Properties, Chemistry, And Industrial Applications.
The manufacture of this has been described in detail by J. Walter.5 It is obtained for technical purposes by heating 80 kilos. of pure aniline, 78 of methyl alcohol, and 8 of sulphuric acid (66° Be. = sp. gr. 1.841) in an autoclave to 230-235° for nine to ten hours. The autoclave is of cast iron, enamelled inside, and the requisite temperature is obtained by means of a bath of molten alloy, in which the autoclave is immersed. The alloy is composed of lead, 71 per cent., and tin, 29 per cent. During the heating, the pressure in the autoclave rises to 30-32 atmospheres by the end of six hours, remains constant for another three hours or so, and then gradually falls.
After the reaction is completed, the vessel is allowed to cool down well, preferably during the whole of the next day, when it is opened, and the issuing vapours passed through a condenser to recover the methyl alcohol present in them. Methyl ether, formed as a by-product, is also present; it is either allowed to escaps in the gaseous form, or is recovered as dimethyl sulphate by absorbing it with fuming sulphuric acid.
1 Cain, " Intermediate Products," p. 60.
2 Dammer, "Chemische Technologie," 2, 948.
3 D.R.P., 250236.
4 See also a paper by Frankland, Challenger, and Nicholls. Trans. Chem. Soc, 1919, 115, 198.
5 Chem. Zeit, 1910, 34, 641, 667, et seq.
The contents of the autoclave are then passed into a still, together with a quantity of caustic soda solution corresponding with the amount of sulphuric acid used. The* liberated dimethylaniline is next distilled off in a current of steam, and passes into a separating vessel, from which the condensed water can be run off. The yield of the dimethyl product is about 98 kilos., or 92 per cent. of the theoretical.
The chief impurities present in commercial dimethylaniline are aniline and the monomethyl compound, with some higher boiling condensation-products arising from the use of impure methyl alcohol. When necessary, the various impurities can be removed by fractional distillation, or by freezing-out methods.
It is important to use alcohol as free as possible from acetone, as the presence of this compound diminishes the yield, and renders the product unsuitable for some purposes.
Dimethylaniline is a nearly colourless, oily liquid, which melts at 0.5°, and boils at 192.5°. Sp. gr. 0962 at 15°/15°. It is used in the production of several dyestuffs, e.g., methyl violet, methyl orange, Meldola's blue, crystal violet, and malachite green, and also for the making of nitrosodimethylaniline and dimethyl-ra-aminophenol. It is also said to be employed in Germany for the production of ' methyl rubber," in order to increase the elasticity of the product.
A catalytic process for the production of methyl anilines has recently been described. A mixture of the vapours of methyl alcohol and aniline is passed over alumina heated to a temperature of 400-430°. This gives a mixture of monomethylaniline and dimethylaniline, with only traces of aniline remaining in the mixture. On adding further methyl alcohol and repeating the operation, dimethylaniline is given (Mailhe and de Godon).1 This process dispenses with the use of autoclaves and high pressures, and moreover, the methyl alcohol need not be specially purified from acetone, nor the aniline from water.
A similar method is said to be equally satisfactory for the preparation of the methyl toluidines. If the vapours of either o-, m-, or p-toluidine and methyl alcohol are passed over the aluminium oxide at 350-400° a mixture of methyl- and dimethyl-toluidines is obtained. A second treatment converts this mixture wholly into the dimethyl toluidine.2