This oxidation-product of alcohol is employed industrially in the preparation of certain aniline dyestuffs, and also in making paraldehyde, which has some application in medicine. The two chief sources of commercial aldehyde are the by-products of wood distillation, and the by-products of alcohol rectification. As the boiling point of aldehyde is only 21°, the compound is found in the first runnings of the distillates. From these it is separated by fractional distillation and rectification. The aldehyde for technical purposes obtained from wood is usually of about 95 to 96 per cent. purity.

Another method of obtaining acetaldehyde utilised in recent years is the catalytic oxidation of ethyl alcohol. A current of air is bubbled through warm alcohol, and the issuing vapours are led over a heated finely-divided metal serving as catalyst, in essentially the same manner as is described in the case of methyl alcohol. A process not involving alcohol may be briefly mentioned, namely, the decomposition of lactic acid into aldehyde and formic acid brought about by heating it with dilute sulphuric acid. The lactic acid solution employed is obtained by fermenting a " mash ' of potatoes with the lactic acid bacillus.

It is quite possible that these various methods of obtaining aldehyde for industrial purposes will sooner or later be superseded by processes based upon the oxidation of acetylene. In one such process this gas is led into a hot dilute sulphuric acid solution, containing not more than about six per cent. of the acid, together with mercuric oxide. The acetylene is oxidised to acetaldehyde, which forthwith distils from the hot liquid, so that the process is a continuous one. The mercuric salt is gradually reduced to metallic mercury, which can be re-converted into the oxide electro-lytically, and used again. A good yield of pure aldehyde is said to be obtained.2

1 Zeitsch. angew. Chem., 1911, 51, 2429. 2 F.P , 455370.

On a small scale aldehyde is prepared chemically as follows. Two hundred and ten grams of potassium bichromate, broken into pieces about the size of a pea, are placed in a large round-bottomed flask of about 3 litres capacity, together with 840 grams of water. The flask is fitted with a tap-funnel, and is connected with a condenser and receiver, the latter being well cooled with ice. Keeping the flask cooled, a mixture of 210 grams of alcohol (90 per cent.) and 280 grams of strong sulphuric acid, also well cooled, is slowly added through the funnel, and the mixture well shaken. Much heat is evolved, and aldehyde distils over, with more or less water, acetal, and alcohol. Towards the end of the operation the heat of a water-bath may be applied to the flask. The distillate is rectified on the water-bath at about 50°, the vapours being led through a reflux condenser, which retains water and alcohol, into receivers containing anhydrous ether. The ethereal solution of aldehyde is now saturated with gaseous ammonia (dried by passing over lime), when the aldehyde-ammonia separates out as a white, crystalline mass. This is separated, drained, washed with ether, and dried. Pure aldehyde may be obtained from it by dissolving it in an equal weight of water, and distilling the solution on a water-bath with dilute sulphuric acid (1.5 parts of cone, sulphuric acid to 2 parts of water). The distillate is collected in a cooled receiver, and dehydrated with calcium chloride.

Acetaldehyde is a colourless, mobile liquid, sp. gr. 07951 at 10°, and boiling point 21°. It serves for the preparation of paraldehyde as already mentioned. By the action of aluminium alcoholate it is converted into ethyl acetate.1 Thus ethyl acetate can be produced from acetylene.