Page:The New International Encyclopædia 1st ed. v. 10.djvu/425

* HYDBOCARBONS. 369 HYDROCARBONS. methyl group, CH3, in place of a liydrogen atom in the member immediately preceding it. Thus, by substituting CH3 in place of one H in ethane, CjH„ we get propane, C.llsCHa or C3II,. The transformation of ethane into propane may be actually eti'ected as follows: First, mono-iodo- ethane (C^HjI) is obtained by substituting one iodine atom in lieu of one alom of hydrogen in ethane (C.Hj) ; then mono-iodo-ethane is treated v.ilh methyl iodide (C'HjI) and metallic sodium. The iodine is tlius all removed, and the remain- ing C2H5 group unites with CH3 group to form the compound propane (CjHsCHj or CjH,). The first four members of the above homolo- gous series are liquefiable gases, and each mem- ber is more easily liquefied than the one pre- ceding it; the ne.xt eleven members are liquids, each having a higher hoiling-point than the one preceding it; finally, the higher members are solids, each having a higher melting-point than the one preceding it. The lower members burn with a pale, scarcely luminous flame; the higher members give a bright light, and paraffin wa.x, which is a mixture of solid paraffin hydrocarbons, is used for the manufacture of candles. The paraffins are all colorless, chemically inert, and insoluble in water. The composition of any single paraffin hydro- carbon is expressed, of course, by its own molecu- lar formula. Thus, the formula OjH, shows the chemical composition of ethane; the formula CjHg shows the composition of propane, etc. On the other hand, the composition of the paraffin hydrocarbons in general is expressed by a typical formvla, in which algebraic symbols stand for the numbers of atoms of the component elements. This typical (genei-al) formula is, Cj, H„„ + -. The formula permits readily to calculate the number of hydrogen atoms in any one of the members of the above homologous series, if the number of its carbon atoms is given. Thus, if »t = 1, then 2re-f2 = 4, and C„ H j„+2 becomes CH4 (the formula of methane). The highest member of the series that is actually known to chemists contains 60 carbon atoms, and hence, the typical formula tells us, the number of hydrogen atoms in the molecule of that hydrocarbon (called hexn- contane) is 2)!. + 2 = 2 X 60 -f 2 = 122, and its molecular formula is therefore CmH,;,. It is a well-known fact that compounds exist which have the same chemical composition and yet differ in their physical and chemical properties. Such compounds are termed 'isomeric,' and their nni- tual relations are explained on the assumption that the atoms in their molecules, though the same in kind and number, are differently grouped. In the above homologous series, the first three members have no such isomers, i.e. only one variety of each could be obtained; the fourth membcT, butane, has two isomers (called butane and isobiitnne) ; the fifth member, pen- tane. has three isomers; the sixth, hexane. has five isomers, etc. The higher the molecular weight, the greater the number of isomeric for- mulas which could be constructed according to the structural theniy. Not all of these theo- retically possible hydrocarbons have been ac- tually prepared in Ihe laboratory. Most of them have no practical value and are interesting only inasmuch as they go to prove the validity of thij 'structural theory' of compounds; but the num- her of cases of isomerism in which the theory has been found to hold true is so great that chemists are no longer anxious to obtain new proofs by artificially producing all of the pos- sible isomeric compounds. See Cabbon Com- pounds. (2) The Olefins, or hydrocarbons of the ethy- lene series, have the characteristic property of directly taking on bromine and other elements to form additire products. This shows that the combining capacity of the carbon contained in them is not completely satisfied by their hydro- gen ; so they are said to be 'unsaturated com- pounds.' When arranged in order of their molecular weights, they form the following homologous series : Ethylene, CaH.,. moU-cMilar weiprht 28. Propylene, CgHg, raoleoular weight 42. Butylenes, C^Hg, molecular weight 56. Amylenes, CgHjo, molecular weight 70. Hexylenes, CaHij. molecular weight 84, etc. In this series, too, each member contains one carbon and two hydrogen atoms more than the member immediately preceding it; that is to say, the difference in molecular weight between any two consecutive members of the series amounts to 14. Here, too, as in the paraffin series, a cer- tain amount of regularity is found in the varia- tion of the physical properties of the compounds on passing up the series. The first four mem- bers are gases, and each one is more easily lique- fied than the one preceding it. He.xylene and the following 13 members are liquids, each one boil- ing at a higher temperature than the one pre- ceding it. The higher members are solids, and each one has a higher melting temperature than the one preceding it. The olefins are colorless and insoluble in water. They contain a high percentage of carbon and burn with a luminous ilame. The composition of all the olefins is expressed by the typical formula of the series. C H,^. The ratio of the numbers of hydrogen and car- bon atoms is the same throughout the series; showing that the different members of the series have all the same percentage composition. They are, however, not isomeric, in the ordinary' sense of the term ; for their molecules evidently con- tain different numbers of atoms. Thus ethylene (C,H, ) contains two carbons and four hydro- gens; propylene (C.H,), three carbons and six hydrogens, etc. But, beginning with butylene, each single member represents several compounds which nuist he considered as isomeric, since they have precisely the same molecular composition and yet differ in their properties. The struc- tural theory lets us foresee the existence of three isomeric butylenes. five amylenes. thirteen hexy- lenes, etc. Again, as in the parnffin series, the higher the molecular weight, the greater the number of isomers possible, according to both theory and experience. Nascent hydrogen adds itself readily to the olefins (unsaturated), yielding hydrocarbons of the paraffin series (saturated). (3) The Arrttilenc Tltidrncnrbotifi. when ar- ranged in the order of their molecular weights, form the following homologous series: Acetylene. C2H2. molecular weight 20. Allylene. C3H.1, molecular weight 40. Crotonylene. C4H6. molecular weight 54, etc. Like any other homologous series, the acety- lene series presents a certain amount of regu- larity in the variation of the physical properties