(2) The Olefins, or hydrocarbons of the ethy lene series, have the characteristic property of directly taking on bromine and other elements to form additive 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, COI,. molecular weight 28. Propylene. molecular weight 42. Butylenes, 4:,H„,, molecular weight 56. Amylenes, molecular weight 70. Hexylenes, molecular w eight 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. Hexylene 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 flame.
The composition of all the olefins is expressed by the typical formula of the series. 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 eon tain different numbers of atoms. Thus ethylene contains two carbons and four hydro gens; propylene (C,IL), three carbons and six hydrogens, etc. But, beginning with butylene, each single member represents several compounds which must 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 paraffin 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. Hydrocarbons. when ar ranged in the order of their molecular weights, form the following homologous series: Acetylene. C2H 2. molecular weight 20.
Allylene, 0311t, molecular 40.
Crotonylene, C4 IT 6. 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 of the compounds with increase of their molec ular weight.
The acetylenes are unsaturated; in fact, 'doubly unsaturated' compounds (see ACETY LT:xEI, as is shown by their capacity for taking on bromine and other elements to form additive products. The acetylene hydrocarbons burn with a very smoky flame, unless by some method enough oxygen is supplied to burn up completely the large amount of carbon they contain. The general formula of the series is Nascent hydrogen adds itself readily to the acetylene hydrocarbons, yielding still unsaturated hydro carbons of the ethylene series.
Anom ATI C II YDROCARItoN 5. The principal groups into which these Hydrocarbons are sub divided include the Benzene series, the Naphtha lene series, and the .4 nthracene series. The typical formulas representing these three series are, respect ively, C. C. and Other hydrocarbons. not included in these series. are represented by the formulas „, etc.; but these are not important enough to lie mentioned here. The most important members of the benzene series are benzene (C„11„), toluene and xylene The most important substance of the naphthalene series is naphthalene itself : that of the anthracene series, anthra cote (('„II,,,). All these are described in special articles. Suffice it to mention here that the chief source of aromatic hydrocarbons is the coal-tar (q.v.) obtained as a by-produet in the manufac ture of coal-gas. and that it is principally from these hydrocarbons that innumerable artificial dyestuffs, drugs. etc.. are now made on a very large industrial scale. See also CannoN Com The hydrocarbons should not he confounded with the 'carbohydrates,' like sugar, starch, or cellulose, which contain besides carbon and hy drogen also oxygen.