[44] The union of carbon atoms in closed chains or rings was first suggested by Kekulé as an explanation of the structure and isomerism of the derivatives of benzene, C6H6, forming aromatic compounds (Note [26]).
[45] The following are the most generally known of the oxygenised but non-nitrogenous hydrocarbon derivatives. (1) the alcohols. These are hydrocarbons in which hydrogen is exchanged for hydroxyl (OH). The simplest of these is methyl alcohol, CH3(OH), or wood spirit obtained by the dry distillation of wood. The common spirits of wine or ethyl alcohol, C2H3(OH), and glycol, C2H4(OH)2, correspond with ethane. Normal propyl alcohol, CH3CH2CH2(OH), and isopropyl alcohol, CH3CH(OH)CH3, propylene-glycol, C3H6(OH)2, and glycerol, C3H3(OH)3 (which, with stearic and other acids, forms fatty substances), correspond with propane, C3H8. All alcohols are capable of forming water and ethereal salts with acids, just as alkalis form ordinary salts. (2) Aldehydes are alcohols minus hydrogen; for instance, acetaldehyde, C2H4O, corresponds with ethyl alcohol. (3) It is simplest to regard organic acids as hydrocarbons in which hydrogen has been exchanged for carboxyl (CO2H), as will be explained in the [following chapter]. There are a number of intermediate compounds; for example, the aldehyde-alcohols, alcohol-acids (or hydroxy-acids), &c. Thus the hydroxy-acids are hydrocarbons in which some of the hydrogen has been replaced by hydroxyl, and some by carboxyl; for instance, lactic acid corresponds with C2H6, and has the constitution C2H4(OH)(CO2H). If to these products we add the haloid salts (where H is replaced by Cl, Br, I), the nitro-compounds containing NO2 in place of H, the amides, cyanides, ketones, and other compounds, it will be readily seen what an immense number of organic compounds there are and what a variety of properties these substances have; this we see also from the composition of plants and animals.
[46] Ethylene bromide, C2H4Br2, when gently heated in alcoholic solution with finely divided zinc, yields pure ethylene, the zinc merely taking up the bromine (Sabaneyeff).
[47] Ethylene decomposes somewhat easily under the influence of the electric spark, or a high temperature. In this case the volume of the gas formed may remain the same when olefiant gas is decomposed into carbon and marsh gas, or may increase to double its volume when hydrogen and carbon are formed, C2H4 = CH4 + C = 2C + 2H2. A mixture of olefiant gas and oxygen is highly explosive; two volumes of this gas require six volumes of oxygen for its perfect combustion. The eight volumes thus taken then resolve themselves into eight volumes of the products of combustion, a mixture of water and carbonic anhydride, C2H4 + 3O2 = 2CO2 + 2H2O. On cooling after the explosion diminution of volume occurs because the water becomes liquid. For two volumes of the olefiant gas taken, the diminution will be equal to four volumes, and the same for marsh gas. The quantity of carbonic anhydride formed by both gases is not the same. Two volumes of marsh gas give only two volumes of carbonic anhydride, and two volumes of ethylene give four volumes of carbonic anhydride.
[48] The homologues of ethylene, CnH2n, are also capable of direct combination with halogens, &c., but with various degrees of facility. The composition of these homologues can be expressed thus: (CH3)_x(CH2)y(CH)zCr, where the sum of x + z is always an even number, and the sum of x + z + r is equal to half the sum of 3x + z, whence z + 2r = x; by this means the possible isomerides are determined. For example, for butylenes, C4H8, (CH3)2(CH)2, (CH3)2(CH2)C, (CH2)(CH2)2CH, and (CH2)4 are possible.
[48 bis] See also method of preparing C2H2 in Note [12 bis].
[49] This is easily accomplished with those gas burners which are used in laboratories and mentioned in the Introduction. In these burners the gas is first mixed with air in a long tube, above which it is kindled. But if it be lighted inside the pipe it does not burn completely, but forms acetylene, on account of the cooling effect of the walls of the metallic tube; this is detected by the smell, and may be shown by passing the issuing gas (by aid of an aspirator) into an ammoniacal solution of cuprous chloride.
[50] Amongst the homologues of acetylene CnH2n-2, the lowest is C3H4; allylene, CH3CCH, and allene, CH2CCH2, are known, but the closed structure, CH2(CH)2, is little investigated.
[51] The saturated hydrocarbons predominate in American petroleum, especially in its more volatile parts; in Baku naphtha the hydrocarbons of the composition CnH2n form the main part (Lisenko, Markovnikoff, Beilstein) but doubtless (Mendeléeff) it also contains saturated ones, CnH2n+2. The structure of the naphtha hydrocarbons is only known for the lower homologues, but doubtless the distinction between the hydrocarbons of the Pennsylvanian and Baku naphthas, boiling at the same temperature (after the requisite refining by repeated fractional distillation, which can be very conveniently done by means of steam rectification—that is, by passing the steam through the dense mass), depends not only on the predominance of saturated hydrocarbons in the former, and naphthenes, CnH2n, in the latter, but also on the diversity of composition and structure of the corresponding portions of the distillation. The products of the Baku naphtha are richer in carbon (therefore in a suitably constructed lamp they ought to give a brighter light), they are of greater specific gravity, and have greater internal friction (and are therefore more suitable for lubricating machinery) than the American products collected at the same temperature.
[52] The formation of naphtha fountains (which burst forth after the higher clay strata covering the layers of sands impregnated with naphtha have been bored through) is without doubt caused by the pressure or tension of the combustible hydrocarbon gases which accompany the naphtha, and are soluble in it under pressure. Sometimes these naphtha fountains reach a height of 100 metres—for instance, the fountain of 1887 near Baku. Naphtha fountains generally act periodically and their force diminishes with the lapse of time, which might be expected, because the gases which cause the fountains find an outlet, as the naphtha issuing from the bore-hole carries away the sand which was partially choking it up.