Page:The New International Encyclopædia 1st ed. v. 04.djvu/235

CARBON COMPOUNDS. 195 CARBON COMPOUNDS. be found discussed iu the article Stereo-Chemis- TRT. ) The isomerism of carbon compounds shows phiinly that the composition of a substance does not entirely determine its chemical individuality. For two or more molecules composed of the same kind and number of atoms may represent very difl'erent compounds. It is therefore evident that the character of a com])ound nnist depend to a great extent also upon tlie manner in which the atoms are held in combination within its molecule. This, at least, is the onlj- possible e.planation that presents itself to the mind, as- suming that substances are really made up of atoms and molecules; and without this assump- tion, i.e. without the atomic hypothesis, isomer- ism could not be explained at all. After, there- fore, the composition and molecular weight of a newly isolated compound have been determined, a further and much more difBcult problem remains to be solved; viz. to determine the chemical con- stitution of the compound, i.e. the manner in which the atoms are arranged in its molecules. This problem is solved by combining the results of a careful study of the chemical and physical properties of the compound with a theoretical assumption first induced independently by Ke- kule and Couper. The following jiaragraphs may convey an idea as to how this is done. Gr.phical Formvlas. The theoretical as- sumption just referred to is ( 1 ) that an atom of carbon is quadrivalent, i.e. has four times the combining cajiacity of an atom of hydrogen, and, therefore, can hold in combination four 'univa- lent' atoms like those of hydrogen and chlorine, or two 'divalent' atoms like those of oxygen; (■2) that two or more carbon atoms may be directly combined with one another and may thus partly satisfy one another's combining capacity. This assumption, together with a knowledge of the valencies (combining capacities) of other elements, makes it easy to determine a priori the several different combinations that are pos- sible with a given set of atoms, each arrange- ment being represented by a scheme, called a graphical formula, in which the atoms are repre- sented by the chemical symbols of the elements, and their combining capacities by dashes that link together the symbols. The set of atoms CH4 can be represented, on the above assumption, by only one graphical formula, viz. • n H— C— H I H The sets C2H0 and CH, can likewise be repre- sented each by onlv one graphical formula, viz. H n ' H H H H— C— C— H and H H H It H The set C^H.o can be represented by two differ- ent graphical formulas, viz. H H H H H H H H— C— C— C— H I 'I H 11 H-C— C— C— ('^-H and I I H H H-C I H H I I •C-H -C-H I H Similarly, the set Ct.11,2 can be represented by three diti'erent grapliical formulas; the set C,H„ by /lie formulas, etc. When a compound is discovered whose molecu- lar formula can be represented by only one such graphical scheme, the case is simple, and the structure of the molecule becomes known at once. Furtlier, in such cases the inference from llie theory is that only one compound of the given molecular formula is cajiable of existence, in this manner, for instance, the theory gives us the constitution of marsh-gas, CH„ of ethane, C^Ha (a constituent of coal-gas), of propane, CjHs, etc. ; the verdict of the theory being, further, that only one compound CH„ only one compound C-He, and only one compoimd CJlg, are capable of existing. The fact that the most careful researches have actually led chemists to the discovery of one, and only one, compound cor- responding to each of these molecular formulas, speaks strongly in favor of the structural theory. -Again, in cases in which more than one graph- ical fornnila can be constructed from a given set of atoms, the number of compounds actually known is generally the same as the nimiber of formulas. Thus, we have seen that two different stnictural formulas correspond to the molecular formula CiH,o; and, as a matter of fact, two, and only two, compounds of the formula C,H,o can be obtained, viz. the sub- stances known, respectively, as butane and iso- butane, which have the same composition and the same molecular weight, yet differ considerably in their physical and chemical jjroperties. Similar- ly, three dili'erent compounds of the formula CjH,, are known, five different com])ounds of the formula CcH,„ etc. In eases in which the num- ber of isomeric compounds actually known was less than the number indicated as possible by the structural theory, earnest research has finally led to the discovery of the wanting isomerides. The correspondence between the chemical prop- erties of a compound and the relations exhibited by its graphical formula is usually capable of experimental demonstration. In the case of marsh-gas the graphical formula is symmetrical, because the doctrine of valency assumes no differ- ence between the- several valencies of an atom. We may, accordingly, expect that the four por- tions of hydrogen contained iu the compound exercise precisely the same function and are in all respects identical. If there were two dif- ferent positions iu the graphical formula, which we will call positions A and B, then we could obtain, for example, one derivative by substi- tuting clilurine for hydrogen in position A while leaving hydrogen in position B; and we could obtain a different derivative by substituting chlorine in position B while leaving hydrogen in A. In other words, two difl'erent mono- chloro-substitution products of marsh-gas would be possible. But the mono - chloro - substitution product has been obtained by many different methods, and the product was always found the same. -Ml efforts to produce two difl'erent deriva- tions have failed. The conclusion is that the several portions of hydrogen contained in marsh- gas have really the same function. When we examine the graphical formula of ethane, CM„ (see above), we find again that the several positions of the hydrogen atoms are identical: each hydrogen atom is, namely, at-