The Principles of Chemistry, Volume I

by Dmitry Ivanovich Mendeleyev

[37] Although the methods of formation and the reactions connected with hydrocarbons are not described in this work, because they are dealt with in organic chemistry, yet in order to clearly show the mechanism of those transformations by which the carbon atoms are built up into the molecules of the carbon compounds, we here give a general example of reactions of this kind. From marsh gas, CH₄, on the one hand the substitution of chlorine or iodine, CH₃Cl, CH₃I, for the hydrogen may be effected, and on the other hand such metals as sodium may be substituted for the hydrogen, e.g. CH₃Na. These and similar products of substitution serve as a means of obtaining other more complex substances from given carbon compounds. If we place the two above-named products of substitution of marsh gas (metallic and haloid) in mutual contact, the metal combines with the halogen, forming a very stable compound—namely, common salt, NaCl, and the carbon groups which were in combination with them separate in mutual combination, as shown by the equation:

CH₃Cl + CH₃Na = NaCl + C₂H₆.

This is the most simple example of the formation of a complex hydrocarbon from these radicles. The cause of the reaction must be sought for in the property which the haloid (chlorine) and sodium have of entering into mutual combination.

[38] When m = n - 1, we have the series C_nH₂. The lowest member is acetylene, C₂H₂. These are hydrocarbons containing a minimum amount of hydrogen.

[39] For instance, ethylene, C₂H₄, combines with Br₂, HI, H₂SO₄, as a whole molecule, as also does amylene, C₅H₁₀, and, in general, C_nH_2n.

[40] For instance, ethylene is obtained by removing the water from ethyl alcohol, C₂H₅(OH), and amylene, C₅H₁₀, from amyl alcohol, C₅H₁₁(OH), or in general C_nH_2n, from C_nH_2n+1(OH).

[41] Acetylene and its polymerides have an empirical composition CH, ethylene and its homologues (and polymerides) CH₂, ethane CH₃, methane CH₄. This series presents a good example of the law of multiple proportions, but such diverse proportions are met with between the number of atoms of the carbon and hydrogen in the hydrocarbons already known that the accuracy of Dalton's law might be doubted. Thus the substances C₃₀H₆₂ and C₃₀H₆₀ differ so slightly in their composition by weight as to be within the limits of experimental error, but their reactions and properties are so distinct that they can be distinguished beyond a doubt. Without Dalton's law chemistry could not have been brought to its present condition, but it cannot alone express all those gradations which are quite clearly understood and predicted by the law of Avogadro-Gerhardt.

[42] The conception of the structure of carbon compounds—that is, the expression of those unions and correlations which their atoms have in the molecules—was for a long time limited to the representation that organic substances contained complex radicles (for instance, ethyl C₂H₅, methyl CH₃, phenyl C₆H₅, &c.); then about the year 1840 the phenomena of substitution and the correspondence of the products of substitution with the primary bodies (nuclei and types) were observed, but it was not until about the year 1860 and later when on the one hand the teaching of Gerhardt about molecules was spreading, and on the other hand the materials had accumulated for discussing the transformations of the simplest hydrocarbon compounds, that conjectures began to appear as to the mutual connection of the atoms of carbon in the molecules of the complex hydrocarbon compounds. Then Kekulé and A. M. Butleroff began to formulate the connection between the separate atoms of carbon, regarding it as a quadrivalent element. Although in their methods of expression and in some of their views they differ from each other and also from the way in which the subject is treated in this work, yet the essence of the matter—namely, the comprehension of the causes of isomerism and of the union between the separate atoms of carbon—remains the same. In addition to this, starting from the year 1870, there appears a tendency which from year to year increases to discover the actual spacial distribution of the atoms in the molecules. Thanks to the endeavours of Le-Bel (1874), Van't Hoff (1874), and Wislicenus (1887) in observing cases of isomerism—such as the effect of different isomerides on the direction of the rotation of the plane of polarisation of light—this tendency promises much for chemical mechanics, but the details of the still imperfect knowledge in relation to this matter must be sought for in special works devoted to organic chemistry.

[43] Direct experiment shows that however CH₃X is prepared (where X = for instance Cl, &c.) it is always one and the same substance. If, for example, in CX₄, X is gradually replaced by hydrogen until CH₃X is produced, or in CH₄, the hydrogen by various means is replaced by X, or else, for instance, if CH₃X be obtained by the decomposition of more complex compounds, the same product is always obtained.

This was shown in the year 1860, or thereabout, by many methods, and is the fundamental conception of the structure of hydrocarbon compounds. If the atoms of hydrogen in methyl were not absolutely identical in value and position (as they are not, for instance, in CH₃CH₂CH₃ or CH₃CH₂X), then there would be as many different forms of CH₃X as there were diversities in the atoms of hydrogen in CH₄. The scope of this work does not permit of a more detailed account of this matter. It is given in works on organic chemistry.