Page:The New International Encyclopædia 1st ed. v. 04.djvu/238

* CARBON COMPOUNDS. 198 CARBON COMPOUNDS. ence of which would prove, either that one of the valencies of a carbon atom may remain unsatis- fied, or that carbon may occur in the tri-talcnt slate i but all such ellorts 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 sipported by the facts of what is now termed 'geometrical isomerism,' a discussion of which may be found under Stereo-Chemistry. The fornnil.x of ethylene suggests that the molecule of this comi>ouiul would not be broken up if one of the two bonds between the atoms of carbon were dissolved, and two univalent atoms of some 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 arc represented, respectively, by the fol- lowing two formulas: H H H— C— Br 1 H— C 1 hJ H— C— Br i H Ethylene Ethylene di-bromide In cases like that of ethylene, the correspond- ence between theorj' and experiments is again perfect, ^'ilenever, namely, the formula of a comptnind is found, by the structural hypotliesis, to contain a double bond between two carbon atoms, the compound I'epresented by tlie fornuila is invariably found to be capable of forming ad- ditive products. Compounds like ethylene are, for obvious reasons, termed uiisaturuted com- pounds. In view of the perfect correspondence between theory and fact, such compoimds may be defined either theoretically or empirically, as follows: (1) Unsaturated compounds are com- pounds in whose molecules there is at least one pair of carbon atoms linked by a double bond; or else, ("2) unsaturated compounds are coni])ounds capable of forniing 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 doiihbj unsaturated compounds is often applied to acetylene and its derivatives, whose formulas are found to contain carbon atoms linked to each other by a triple bond, while the comjioimds themselves are found ca- pable of taking on twice as much bromine as the corresponding quantities (if coniiiounds like ethylene. Further, in doubly iiisaturated com- pounds this formation of additive products with bromine or other elements is found to take place in two steps, as is shown, in the case of acetylene, by the following chemical equations: I CjH, + Brj = CjH.Br^ Acetylene Bromine Acetylene di-bromlde II CjHjBr^ + Br, = CjH^Br., Acetylene Bromine Acetylene di-bromlde tetra-bromlde The following graphical formulas exhibit clear- ly the relation between the molecules of acetylene and of its additive products ith bromine : H H H i C— Br Br— C— Br 'il d-Br Br— C— Br A ! ii i Acetylene Acetylene Acetylene di-bromide tetra-bromide An important property of acetylene and many of its derivatives is their capacity for forming certain metallic compounds (see Acetyij;ne). By this projierty many doubly unsaturated com- pounds may be readily distinguished from the unsaturated compounds of the ethylene series. Bexzexe and Its Derivatives. One of the most important of the compounds of carbon is ben- zene (q.v. ), a liquid hydrocarbon found in coal- tar. Thousands of substances are derived from benzene, thousands find extensive application in the arts. (See Coal-Tar Colors.) An exact knowledge of the constitution of benzene is there- fore highly desirable for theoretical as well as for immediate practical reasons, and so chemists have now for many years diligently searched for a graphical formula that might be in perfect cor- respondence with all the chemical properties of benzene. A perfect formula has not as yet been devised. Nevertheless, four diU'erent formulas, each of which exhibits well sonic of the proper- ties of benzene, liave been proposed and are com- monly referred to, respectively, as "Kekule's formula,' 'the prism formula,' 'the diagonal for- mula,' and 'the centric formula.' The last of these is now supposed to be the best, but the first is still commiuily used by chemists and therefore requires explanation in the present sketch. It is as follows: H A H-C C-H H I H KekulC'"t* fornuila for benzene Unlike the fiuiuulas already considered in this article, the formula of benzene has its carbon atoms arranged, not in the form of a chain, but in a ring, with alternating single and double bonds between the atoms. The complete demon- stration of this formula formed one of the most beautiful cliapters of organic chemistry. Noth- ing astonishes the young stvulent more than to find that in this demonstration an atom of bro- mine, substituted for one of the hydrogen atoms of benzene, is made to travel round the molecule, exchanging places successively with eacli of the hydrogen atoms, and that this migration of the bromine atom is demonstrated beyond the slight- est possibility of doubt. The demonstration can- not be repeated here, and an account of it must be sought for hi one of the larger w(uks on organic chemistry (or consult Marckwald's lecture on the theory of benzene published in Ahrcns's t^ammlung chcmischcr und clicmisch- tcchnischer VorlriKje. Stuttgart, 18!t7). What requires explanation in the present article ia the