FATTY liYnmnoc.annoss. These form an impor tant class, and are subdivided into the paraffin, olefin, and acetylene series.
(1) The Paraffins. or saturated hydrocarbons of the methane series. are very stable com pounds and do not react even with the strongest acids and alkalies; hence their name, paraffins. derived from the Latin paruin affinis, which means 'having little affinity.' Large quantities of them occur in nature as petroleum, natural gas, ozokerite. etc. They are also obtained by the destructive distillation of coal, cannel, shale, etc. When they are arranged in the order of their molecular weights, the following 'homolo gous series' is obtained: :Methane, Cli„ molecular weight Ethane, molecular weight 30. Propane, COI., molecular weight 44. Butanes, molecular weight its.
Pentanes, cot molecular weight 72.
Hexanes, molecular weight fM.
Heptaues, molecular weight 100. etc.
The difference in molecular weight between any two consecutive members of the series is evi dently the same. It amounts to 14. the sum of the 'weights' of one carbon atom (which is 12) and two hydrogen atoms (which is 2). To obtain the formula of any member, we may substitute a methyl group, CIL, in place of a hydrogen atom in the member immediately preceding it. Thus, by substituting CIL in place of one II in ethane, we get propane, CACI1, or C,11.. The transformation of ethane into propane may be actually effected as follows: First. mono-iodo ethane is obtained by substituting one iodine atom in lieu of one atom of hydrogen in ethane ; then mono-iodo-ethane is treated with methyl iodide (CI1J) and metallic sodium. The iodine is thus all removed, and the remain ing group unites with CH, group to form the compound propane (C,ILCH, or 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 next eleven members are liquids, each having a higher boiling-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 wax, 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 shows the chemical composition of ethane; the formula shows the composition of propane, etc. On the other hand, the composition of the paraffin hydrocarbons in general is expressed by a typical formula, in which algebraic symbols stand for the numbers of atoms of the component elements. This typical (general) formula is. C. 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 n = 1, then 2n + 2 = 4, and H 2n+2 becomes CH, (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 heam contane) is 2a+ 2 = 2 X 60 + 2 = 122, and its molecular formula is therefore 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 nm 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 member, butane, has two isomers (called butane and isohutane) ; 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 he constructed according to the structural theory. Not all of these theo retically possible hydrocarbons have been ac tually prepared in the laboratory. Most of them have no practical value and are interesting only inasmuch as they go to prove the validity of the `structural theory' of compounds; hut the num ber 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 CARBON COM POUNDS.