into two groups: (1) those exhibiting properties closely analogous to the aliphatic series—the polymethylenes (q.v.), and (2) a series exhibiting properties differing in many respects from the aliphatic and polymethylene compounds, and characterized by a peculiar stability which is to be associated with the disposition of certain carbon valencies not saturated by hydrogen—the “aromatic series.” There also exists an extensive class of compounds termed the “heterocyclic series”—these compounds are derived from ring systems containing atoms other than carbon; this class is more generally allied to the aromatic series than to the aliphatic.
We now proceed to discuss the types of aliphatic compounds; then, the characteristic groupings having been established, an epitome of their derivatives will be given. Carbocyclic rings will next be treated, benzene and its allies in some detail; and finally the heterocyclic nuclei.
Accepting the doctrine of the tetravalency of carbon (its divalency in such compounds as carbon monoxide, various isocyanides, fulminic acid, &c., and its possible trivalency in M. Gomberg’s triphenyl-methyl play no part in what follows), it is readily seen that the simplest hydrocarbon has the formula CH4 named methane, in which the hydrogen atoms are of equal value, and which may be pictured as placed at the vertices of a tetrahedron, the carbon atom occupying the centre. This tetrahedral configuration is based on the existence of only one methylene dichloride, two being necessary if the carbon valencies were directed from the centre of a plane square to its corners, and on the existence of two optical isomers of the formula C.A.B.D.E., C being a carbon atom and A.B.D.E. being different monovalent atoms or radicals (see Stereo-Isomerism). The equivalence of the four hydrogen atoms of methane rested on indirect evidence, e.g. the existence of only one acetic acid, methyl chloride, and other monosubstitution derivatives—until the experimental proof by L. Henry (Zeit. f. Phys. Chem., 1888, 2, p. 553), who prepared the four nitromethanes, CH3NO2, each atom in methane being successively replaced by the nitro-group.
Let us now consider hydrocarbons containing 2 atoms of carbon. Three such compounds are possible according to the number of valencies acting directly between the carbon atoms. Thus, if they are connected by one valency, and the remaining valencies saturated by hydrogen, we obtain the compound H3C·CH3, ethane. This compound may be considered as derived from methane, CH4, by replacing a hydrogen atom by the monovalent group CH3, known as methyl; hence ethane may be named “methylmethane.” If the carbon atoms are connected by two valencies, we obtain a compound H2C:CH2, ethylene; if by three valencies, HC⫶CH, acetylene. These last two compounds are termed unsaturated, whereas ethane is saturated. It is obvious that we have derived three combinations of carbon with hydrogen, characterized by containing a single, double, and triple linkage; and from each of these, by the substitution of a methyl group for a hydrogen atom, compounds of the same nature result. Thus ethane gives H3C·CH2·CH3, propane; ethylene gives H2C:CH·CH3, propylene; and acetylene gives HC⫶C·CH3, allylene. By continuing the introduction of methyl groups we obtain three series of homologous hydro-carbons given, by the general formulae CnH2n+2, CnH2n, and CnH2n−2, each member differing from the preceding one of the same series by CH2. It will be noticed that compounds containing two double linkages will have the same general formula as the acetylene series; such compounds are known as the “diolefines.” Hydrocarbons containing any number of double or triple linkages, as well as both double and triple linkages, are possible, and a considerable number of such compounds have been prepared.
A more complete idea of the notion of a compound radical follows from a consideration of the compound propane. We derived this substance from ethane by introducing a methyl group; hence it may be termed “methylethane.” Equally well we may derive it from methane by replacing a hydrogen atom by the monovalent group CH2·CH3, named ethyl; hence propane may be considered as “ethylmethane.” Further, since methane may be regarded as formed by the conjunction of a methyl group with a hydrogen atom, it may be named “methyl hydride”; similarly ethane is “ethyl hydride,” propane, “propyl hydride,” and so on. The importance of such groups as methyl, ethyl, &c. in attempting a nomenclature of organic compounds cannot be overestimated; these compound radicals, frequently termed alkyl radicals, serve a similar purpose to the organic chemist as the elements to the inorganic chemist.
In methane and ethane the hydrogen atoms are of equal value, and no matter which one may be substituted by another element or group the same compound will result. In propane, on the other hand, the hydrogen atoms attached to the terminal carbon atoms differ from those joined to the medial atom; we may therefore expect to obtain different compounds according to the position of the hydrogen atom substituted. By introducing a methyl group we may obtain CH3·CH2·CH2·CH3, known as “normal” or n-butane, substitution occurring at a terminal atom, or CH3·CH(CH3)·CH3, isobutane, substitution occurring at the medial atom. From n-butane we may derive, by a similar substitution of methyl groups, the two hydrocarbons: (1) CH3·CH2·CH2·CH2·CH3, and (2) CH3·CH(CH3)·CH2·CH3; from isobutane we may also derive two compounds, one identical with (2.), and a new one (3) CH3(CH3)C(CH3)CH3. These three hydrocarbons are isomeric, i.e. they possess the same formula, but differ in constitution. We notice that they may be differentiated as follows: (1) is built up solely of methyl and ·CH2· (methylene) groups and the molecule consists of a single chain; such hydrocarbons are referred to as being normal; (2) has a branch and contains the group ⫶CH (methine) in which the free valencies are attached to carbon atoms; such hydrocarbons are termed secondary or iso-; (3) is characterized by a carbon atom linked directly to four other carbon atoms; such hydrocarbons are known as tertiary.
Deferring the detailed discussion of cyclic or ringed hydrocarbons, a correlation of the various types or classes of compounds which may be derived from hydrocarbon nuclei will now be given. It will be seen that each type depends upon a specific radical or atom, and the copulation of this character with any hydrocarbon radical (open or cyclic) gives origin to a compound of the same class.
It is convenient first to consider the effect of introducing one, two, or three hydroxyl (OH) groups into the –CH3, >CH2, and ⪫CH groups, which we have seen to characterize the different types of hydrocarbons. It may be noticed here that cyclic nuclei can only contain the groups >CH2 and ⪫CH, the first characterizing the polymethylene and reduced heterocyclic compounds, the second true aromatic compounds.