UNSATURATED COMPOUNDS. Thus far we have referred only to compounds in which the valen cies of the several atoms are completely satisfied, so that the molecules are incapable of taking on any more atoms. Thus, in marsh-gas, CIL. the four valencies of the carbon atom are evidently satisfied by four atoms of hydrogen. In ethane, three valencies of each carbon atom are evidently satisfied by hydrogen atoms, while the fourth valency of each carbon atom is satisfied by the other carbon atom. Compounds thus con taining the maximum possible number of atoms of hydrogen. or of other elements, are said to be saturated. Many other compounds, however. are known, in which this is not the case. Thus, while ethane, contains six hydrogen atoms, the gaseous compound known as ethylene contains only four hydrogen atoms in combination with two atoms of carbon, the formula of ethylene being Three different graphical formulas might rep resent a molecule of ethylene: viz.
According to the first, two valencies of each carbon atom are satisfied by the other carbon atom. According to time second, one valency of each carbon atom remains unsatisfied. Accord ing to the third. the carbon atom is not quadri valent, but trivalent. Now, the last two formu las must he rejected in view of the following fact: A great deal of ingenuity has been spent. by some of the most celebrated organic chemists, in efforts to produce a compound of molecular formula CH, and similar compounds, the exist ence of which would prove, either that one of the valeneies of a carbon atom may remain unsatis fied, or that carbon may occur in the tri-talent state; but all such efforts have invariably failed. The view corresponding to the first of the above graphical formulas has therefore been universally adopted, viz., that in ethylene, and in similar compounds, carbon atoms satisfy two of each other's valencies. This view is further strongly supported by the facts of what is now termed 'geometrical isomerism,' a discussion of which may be found under STEREO-CuENtisTRY. The formula of ethylene suggests that the molecule of this compound would not he broken up if one of the two bonds between the atoms of carbon were dissolved, and two univalent atoms of sonic element were linked on by the two carbon affinities thus set free. As a matter of fact.
ethylene combines directly with bromine and certain other elements, forming what is termed 'additive compounds,' the term suggesting that the atoms—say, of bromine—join the molecule of ethylene without displacing anything in it. Ethylene and its additive compound with bro mine are represented, respectively, by the fol lowing two formulas: In eases like that of ethylene, the correspond ence between theory and experiments is again perfect. Whenever, namely, the formula of a compound is found, by the structural hypothesis, to contain a double bond between two carbon atoms, the compound represented by the formula is invariably found to he capable of forming ad ditive products. Compounds like ethylene are, for obvious reasons, termed com pounds. In view of the perfect correspondence between theory and fact, such compounds may be defined either theoretically or empirically, as follows: (I) Unsaturated compounds are com pounds in whose molecules there is at least one pair of earbon atoms linked by a double bond; or else, (2) unsaturated compounds are compounds capable of forming additive products with bro mine or other elements, i.e. capable of uniting with bromine without at the same time losing any of their own constituents.
The term doubly unsaturated compounds is often applied to acetylene and its derivatives, whose fornmlas are found to contain carbon atoms linked to each other by a triple bond, while the compounds themselves are found ca pable of taking on tvvice as umeh bromine as the corresponding quantities of compounds like ethylene. Further. in doubly unsaturated eom pounds this formation of additive products with bromine or other elements is found to take place in two steps, as is shown, in the ease of acetylelle, by the following chemieal equations: The following graphical formulas exhibit clear ly the relation between the molecules of acetylene and of its additive products bromine: An important property of acetylene and many of its derivatives is their capacity for forming certain metallic Compounds (see ACETYLENE). By this property many doubly unsaturated com pounds may be readily distinguished from the unsaturated compounds of the ethylene series.