A mixture of 5 parts of alcohol (90 to 95 per cent.) and 9 parts of strong sulphuric acid is heated in a still to a temperature of 130-140°. Ether is formed, and distils over, together with some water. With continuous addition of alcohol in properly regulated proportion to the hot mixture, the formation of ether is maintained, and the process goes on indefinitely. The stream of alcohol is adjusted so that the volume of the mixture remains constant; and the temperature is kept between the limits stated, since below 130° much unaltered alcohol passes over, and above 140° ethylene gas is produced, with destruction of a corresponding quantity of alcohol.

The distillate of crude ether thus obtained contains sulphur dioxide, water, alcohol, and so-called "oil of ether," a decomposition product of the side reactions which occur. To purify it from the acid, it is washed with soda solution or milk of lime; and to remove the greater part of the alcohol and water the washed ether is rectified by re-distillation.

Such ether serves for most technical purposes, but always retains a small quantity of alcohol. This can be almost entirety removed by washing with water. Treatment with sulphuric acid has been proposed for the purpose, but is not used commercially. By drying the ether over fused calcium chloride and fractionating, "dehydrated" ether is obtained,

The chemical reaction involved is expressed by the well-known equations: -

(i)

C2H5.OH

+

H1SO4

=

C2H5HSO4 + H1O; and

(ii)

C2H5.OH

+

C2H5.HSO4

=

(C2H5)2O + H1SO4.

Theoretically, therefore, 92 parts of (absolute) alcohol = 74 of ether.

The etherification vessel, constructed of lead-lined iron or other acid-resisting metal, may be heated by an external jacket of superheated steam, but in the more modern plants internal heating coils, made of lead, are employed. In one form of plant the etherifier is a cylinder of 1.8 m. diameter and 2 m. in height; it takes a charge of 3,200 kilos. of sulphuric acid (66° Be.) and 1,500 kilos. of 95 per cent. alcohol. Sometimes the acid and alcohol are mixed in a separate vessel, and the mixture, warmed by a steam coil, is passed into the etherifier by means of compressed air. In some installations the continuous supply of alcohol afterwards necessary is fed into the acid mixture in the etherifier through a series of vertical lead tubes, arranged in concentric circles.

The ether is freed from acid by passing the vapours through a lead-lined column, where they are washed by a solution of sodium carbonate; or alternatively, the condensed ether may be passed into a vessel containing milk of lime to effect the removal. To separate the bulk of the alcohol and water present, the ether is then distilled in a copper rectifying-still. If desired, the vapours before condensation may be further dehydrated by passing them through a drying-column containing calcium chloride.

Unaltered alcohol remaining in the etherification vessel is recovered periodically, together with alcohol and ether remaining in the fraction retained by the rectifying apparatus.

Other apparatus for the continuous production of ether works much on the principle of the Coffey still (p. 87), though the mechanical details are, of course, different. One of the most recent is Barbet's.1 This includes a tower divided into, say, twelve sections, each of which contains "plates" for dispersing the liquid, and is heated by a coil to a suitable temperature. Alcohol is introduced into the fourth section from the bottom, whilst sulphuric acid is fed into the top section. The alcohol meets the descending stream of hot sulphuric acid, and is partially converted into ether. Thus a mixture of ether and alcohol vapour is produced, which rises through the tower, and meets further hot acid in each successive section; the conversion of the alcohol into ether thus becomes more and more complete. The resulting ether passes to a condenser and is collected. The three lowest sections of the tower serve to remove alcohol and ether from the diluted sulphuric acid which reaches the bottom.

1 F.P. 479435, 1914; v. also B.P. 100406, 1916.

This apparatus can also be used for the manufacture of ethyl acetate or other ethyl esters.

Other processes have been proposed for the manufacture of ether. The following may be mentioned, though they do not appear to have come permanently into use on a large scale.

(A)

Esterification by means of benzenesulphonic acid.1 Here the ethyl ester of the acid is first formed, and reacts with more alcohol to form ether: -

(i).

C,H6SO3H + C2H5OH

=

C6H5SO3C2H5 + H1O; and

(ii).

C6H5SO3.C2.H5 + C2H5 OH

=

(C2H5)2O + C6H5 SO3H.

The benzenesulphonic acid is thus regenerated in the second stage. It then reacts again to form more ester; and so on. There are said to be no side-reactions in this process.

(6)

Ethylene, obtained from oil gas or other convenient economical source, is treated with sulphuric acid, forming sulphovinic acid as in Hennell's synthesis of alcohol. On adding a suitable proportion of water and heating, the alcohol formed reacts with some of the remaining sulphovinic acid, and ether is produced.2

Ordinarily the dehydrated, 'pure' ether obtained by the foregoing processes still contains traces of alcohol and water, which can be removed by distillation over metallic sodium. On a small scale this is effected by adding shavings of the metal to the ether as long as bubbles of gas are given off from the immersed sodium. When the action has ceased, the ether is distilled off, and collected in a receiver fitted with a drying tube to prevent absorption of moisture from the air during the operation.

Pure ether has the specific gravity 0720, and boils at 35°. There are, however, several qualities of ether recognised in trade, and containing various proportions of water, alcohol, and other substances; these admixtures modify the specific gravity and the boiling point very considerably.

In this country the bulk of the ether produced is made from industrial methylated spirit, but a certain amount is prepared with duty-paid rectified alcohol. The former product contains some methyl ether and methyl ethyl' ether, due to the presence of methyl alcohol in the industrial methylated spirit. Such ether is distinguished as "Ether,. Meth." Naturally the proportion of methyl compounds present will depend upon the particular fraction of the distillate represented by the ether: some kinds of "Ether, Meth." contain but little of the methyl ethers, other kinds include a considerable quantity.

1 Kraft and Roos, D.R.P., 69115.

2 P. Fritsche, Die Chem. Industrie, 1912, 35, 637.

The chief varieties of ether met with in this country are the following: -

(1). Ether pure, sp. gr. 0.720. - This is dehydrated, nearly pure ether produced from rectified spirit.

(2). Ether pure, sp. gr. 0.735. - Like the above, this is made from rectified spirit, but contains approximately 10 per cent. of alcohol and water, and is less purified.

(3). Ether, B.P. - This has sp. gr. 0.720, and boiling range from 34° to 36°. It approximates to No. 1, but is not necessarily made from pure alcohol, and is not quite so highly purified.

(4). Purified ether, B.P. - This has the same sp. gr. and boiling range as No. 3, but must conform to further tests designed to exclude aldehydes, peroxides, and methyl compounds.

(5). Ether Meth., sp. gr. 0.717. - Dehydrated, and practically free from water. Its low specific gravity is due to the presence of methyl ethers, which also give it a very low boiling point - about 20°. It is mainly used as a local anaesthetic.

(6). Ether Meth., sp. gr. 0.720. - This contains alcohol, water, and methyl compounds in varying proportions. It is used for ordinary laboratory purposes where a dehydrated ether is not required.

(7). Ether Meth. purif., sp. gr. 0.720. - This is dehydrated ether, nearly free from water and alcohol. Boiling range about 33° to 36°. Used as a laboratory solvent, and for technical purposes where a nearly pure ether is necessary.

(8). Ether Meth., sp. gr. 0.730. - Contains much water and alcohol. Used as a solvent in ordinary technical operations.

(9). Ether Meth., sp. gr. 0.735. - Used principally in ice-making apparatus. Rather cruder quality than No. 8.