HYDROCARBONS, the simplest of or ganic compounds, consisting solely of carbon and hydrogen. They are exceedingly numerous, and many of them occur in nature, both in petroleum, asphaltum and similar minerals of organic origin, and in the essential oils of plants. They are produced economically in the dry distillation of wood and peat, and are components of illuminating gas and the oils of coal tar. The hydrocarbons can be broadly divided into two general classes: (1) saturated hydrocarbons; (2) unsaturated hydrocarbons; —according to the way in which the carbon atoms that they contain are connected to one another. The first class contains only the single-linked carbon atoms; the second class comprises those in which pairs of carbon atoms arc united in doubles or threes. The first class includes only the paraffines.
In the fatty series, the atoms are all con nected in open chains, while in the aromatic series the carbon atoms are connected with one another in such a manner as to form closed rings. (See FATTY COMPOUNDS and AROMATIC COMPOUNDS). As a class, the hydrocarbons are insoluble in water; they cannot be saponified; and they are neutral, and do not combine with acids to form salts.
The classification of the hydrocarbons is as yet incomplete; but the greater number of those that are known can be included in one or an other of the following groups: 1. The Paraffins, having the general formula Ca H,"+'. The lowest member of this series is methane, or marsh gas, CH., and many other members of the series are known, each contain ing one carbon atom and two hydrogen atoms more than its immediate predecessor. See PARAFFINS.
2. The Ole fines, also known as Alkylenes, or Alkenes, having the general formula Cn 1-11n. Ethylene, Cal., is the simplest member of this series, which includes also propylene; the buty lenes ; amylenes; hexylenes; heptylenes, etc.
3. The Acetylenes, or Alkines, and the Ally lenes or Diolefines, having the general formula Cilitn-t. Acetylene gas, C,H, is the sim plest representative of this series, which in cludes also allene; divinyl, erythrene or pyr rolylene; piperylene; isoprene, diallyl, conylene, etc.
The foregoing all belong to the fatty sub division. At a red heat three molecules of acetylene unite to form one molecule of ben zene, thus linking the aliphatic or fatty series with the aromatic series. The latter class em braces: 4. The Benzene series, having the general formula known also as the cyclic hydrocarbons, and the terpenes.
5. The Terpenes, or alicyclic compounds, a class of substances having the general formula Colin, and including pinene, camphene, fen chene, sylvestrene, carvestrene, terpinolene, ter pinene, etc.
The known hydrocarbons having any one general formula may be ranged in a series, whose members exhibit a sort of regular pro gression in their properties. For example, the first eight members of the normal paraffin group may be arranged as follows: A series of this sort is said to be "homolo gous." The paraffins are said to be anormalD when they contain but two methyl groups (CH,,). Thus normal propane, C,H., has the constitu tional formula CH..CH..CH.; and it is not pos sible to arrange its atoms in any other way. In proceeding from this hydrocarbon to the next to the same series (that is, butane, CH,,,) we may substitute the methyl group, CH., for a hydrogen atom in the CH, of the propane, or for a hydrogen atom in one of the CH. groups. Hence we may have either of the two following constitutional formulas for the hydrocarbon bu tane: The latter compound, which contains only two CH, groups, is called "normal while the former, which contains three such groups, is known as aisobutane." Two butanes, each having the empirical formula C.H,o, are there fore possible, and both are actually known. When we pass to the higher members of the paraffin group we find that a far greater number of isomeric forms can exist, according to the way in which the new CH. group is introduced, in generating the new hydrocarbon from the one next below it in the general series. Thus oc tane, CH., can exist in 18 different isomeric forms, and tridecane, in no less than 802.
The saturated hydrocarbons are of the great est value in the arts, comprising a series of solvents (as naphtha, benzol, etc.) for fats, oils and resins, the mineral oils used for lighting and lubrication, motor fuels, vaseline, paraffin wax, the essential oils from which are derived perfumes and flavorings, the camphors, etc. Consult Norris, J. F., (Principles of Organic Chemistry' (New York 1912) ; Plimmer, R. H. A., 'Practical Organic and Bio-Chemistry' (London 1915) ; Pond, F. J., 'Heusler's Chem istry of the Terpenes) (Philadelphia 1902) ; Spielmann, P. E., 'Richter's Organic Chemistry' (Vol. I, London 1915).