CHAPTER VIII CARBON AND THE HYDROCARBONS

molecular structure and they also, like the polymerides, may be predicted from the above-mentioned conceptions, expressing the principles of the structure of the carbon compounds based on the law of substitution. According to it, for example, it is evident that there can be no isomerism in the cases of the saturated hydrocarbons C 2 H 6 and C 3 H 8 , because the former is CH 4 , in which methyl has taken the place of H, and as all the hydrogen atoms of methane must be supposed to have the same relation to the carbon, it is all the same which of them be subjected to the methyl substitution—the resulting product can only be ethane, CH 3 CH 3 ; the same argument also applies in the case of propane, CH 3 CH 2 CH 3 , where one compound only can be imagined. It is to be expected, however, that there should be two butanes, C 4 H 10 , and this is actually the case. In one, methyl may be considered as replacing the hydrogen of one of the methyls, CH 3 CH 2 CH 2 CH 3 ; and in the other CH 3 may be considered as substituted for H in CH 3 , and there it will consist of CH 3 CH CH 3 / CH 3 . The latter may also be regarded as methane in which three of hydrogen are exchanged for three of methyl. On going further in the series it is evident that the number of possible isomerides will be still greater, but we have limited ourselves to the simplest examples, showing the possibility and actual existence of isomerides. C 2 H 4 and CH 2 CH 2 are, it is evident, identical; but there ought to be, and are, two hydrocarbons of the composition C 3 H 6 , propylene and trimethylene; the first is ethylene, CH 2 CH 2 , in which one atom of hydrogen is exchanged for methyl, CH 2 CHCH 3 , and trimethylene is ethane, CH 3 CH 3 , with the substitution of methylene for two hydrogen atoms from two methyl groups—that is, CH 2 CH 2 / CH 2 , where the methylene introduced is united to both the atoms of carbon in CH 3 CH 3 . It is evident that the cause of isomerism here is, on the one hand, the difference of the amount of hydrogen in union with the particular atoms of carbon, and, on the other, the different connection between the several atoms of carbon. In the first case they may be said to be chained together (more usually to form an ‘open chain’), and in the second case, to be locked together (to form a ‘closed chai’ or ‘ring’). Here also it is easily understood that on increasing the quantity of carbon atoms the number of possible and existing isomerides will greatly increase. If, at the same time, in addition to the substitution of one of the radicles of methane for hydrogen a further exchange of part of the hydrogen for some of the other groups of elements X, Y ... occurs, the quantity of possible isomerides still further increases in a considerable degree. For instance, there are even two possible isomerides for the derivatives of ethane, C 2 H 6 : if two atoms of the hydrogen be exchanged for X 2 , one will have the ethylene structure, CH 2 XCH 2 X, and the other an ethylidene structure, CH 3 CHX 2 ; such are, for instance, ethylene chloride, CH 2 ClCH 2 Cl, and ethylidene chloride, CH 3 CHCl 2 . And as in the place of the first atom of hydrogen not only metals may be substituted, but Cl, Br, I, OH (the water radicle), NH 2 (the ammonia radicle), NO 2 (the radicle of nitric acid), &c., so also in exchange for two atoms of hydrogen O, NH, S, &c., may be substituted; hence it will be understood that the quantity of isomerides is sometimes very great. It is impossible here to describe how the isomerides are distinguished from each other, in what reactions they occur, how and when one changes into another, &c.; for this, taken together with the description of the hydrocarbons already known, and their derivatives, forms a very extensive and very thoroughly investigated branch of chemistry, called organic chemistry . Enriched with a mass of closely