552 CHEMISTRY [ORGANIC. units of atomicity into the molecule, and if we suppose these carbon atoms to be saturated by hydrogen, we shall have an ascending series of hydrocarbons, each member of which differs from the one beneath it by + CH 2 . Such series are known as homologous series. examples : CH 4 , Methane. C a H 6 , Ethane. C 3 H 8 , Propane. C 4 H 10 ,Tetrane. C 5 H 12 ,Pentane. CTT nil on-Pi CH 4 0, Methyl alcohol. C 2 H 6 0, Ethyl C 3 H 8 0, Propyl C 4 Hi 0, Butyl C 5 H 12 0, Amyl The following are CH 2 2 , Formic acid. C 2 H 4 S , Acetic ,, C 3 H 6 2 , Propionic,, C 4 H 8 2 , Butyric ,, C 6 H 10 O 2 , Valeric Isologous Series. An inspection of the foregoing formulae will show that hydrocarbons of the general formula C B H 0n+2 are the only saturated compounds, i.e., the only compounds in which the atom-fixing powers of the carbon atoms are completely satisfied. If an odd number of hydrogen atoms be withdrawn, a compound having a certain odd number of unsatisfied units of atomicity (i.e., a " perissad " radicle) is the result, and such compounds are incapable of existing in the free state. From this and previous considerations it follows, that " all hydrocarbons contain an even number of atoms of hydrogen," and further, " that the sum of the atoms of monad and triad elements contained in the molecule of a carbon compound must also always be an even number " (Schorlemrner). Compounds containing an even number of unsatisfied units of atomicity, although non-saturated, are capable of existing in the free state. Thus, commencing with the saturated hydrocarbons of the C n H 2n+2 series, hydrogen atoms can be withdrawn by pairs, giving rise to a descend- iag series of hydrocarbons, each member of which differs from the one below it by + H 2 . Such series are termed isologous series. The following table shows at a glance the relationship between homologous and isologous series, and at the same time the system of nomenclature used : [sologous Series. Isologous Series. Isologous Series. Isologous Series. Name of Com pound. For mula. Name of Com pound. For mula. Name of Com pound. For mula. Name of Com pound. For mula. Methane CH 4 Ethane C 7 H S Propane C,II R Butane C 4 H, Methene CH 2 Ethene C 2 H 4 Propene C,H S Butene C 4 H g Homo- Ethine CjHj Propine C,H 4 Butine C<H 6
logous
Propone C,H 7 Butone C 4 H, Series. Butune C 4 H 2 Organic Radicles. The meaning of the term radicle has already been explained (p. 474), so that it is here only necessary to point out the part played by such unsaturated groups of atoms in the formation of organic compounds. It has already been mentioned that an odd number of hydrogen atoms withdrawn from a saturated hydrocarbon of the C B H 2B+2 series leaves an unsaturated group having an odd number 1 of unsatisfied units of atomicity. For example : CH, EL Methane. (CH 3 ) Methyl. C 2 H 6 -H = (C 2 H 5 ) Ethane. Ethyl. C B H 2B+2 -H = (C B H 2B+1 ) (C 2 H 4 )"-H=(C 2 H 3 ) " Ethene. Ethenyl. (C 3 H c r-H=(C 3 H 5 ) " Propene. Propenyl. (C,H 2B )"-H = (C B H 2B _ : Thus the C B H 3B+2 hydrocarbons may be conveniently regarded for some purposes as hydrides of C B H 2B+1 radicles, a series the members of which enter largely into the com position of organic compounds of all classes. Perissad radicles are incapable of existing in the free state because on isolation two semi-molecules unite ; for instance 1 The names of perissad radicles are made to end in yl. H H C. H H C H H H H H C C H H H H I I _ C G H H H Dimethyl = ethane. Diethyl = tetrano. Generally 2(C B H 2B+1 ) = C 2B H 4B+2 . It is by no means necessary that a group of atoms should be capable of isolation in order to constitute such group an organic radicle. Any unsaturated group which through several reactions remains unchanged may be so regarded. 2 (See, for example, the previously quoted reac tions of acetic acid.) The following are additional examples of organic radicles : Cyanogen N=C ; in the free state, N=C C^N- Acetyl (C 2 H 3 0) = CO CEL Carboxyl (COOHV: in the free state, oxalic acid, COOH While hydrocarbon radicles are positive (see p. 476), cyanogen and the oxygen-containing or acid radicles are negative. Fatty and Aromatic Groups. In order to assist in bringing the vast numbers of organic compounds within the scope of some system of classification, chemists fre quently adopt the convenient division of them into fatty and aromatic groups. It will be seen subsequently that most organic com pounds may be regarded as derived by substitution from hydrocarbons. 3 Starting, then, with the saturated hydro carbons C B H 2n+2 , the isologous series, down to C n H 2B _ 6 , and some of the members of the C n H 2M _ 4 series, with their derivatives, constitute the fatty group, so called because many of its members exist in fatty bodies. The hydro carbons of the C B H 2M _ 6 , &c., series, with their derivatives, are termed the aromatic group, because many of the com pounds are obtained from balsams, essential oils, gum- resins, and other aromatic substances. The chief charac teristic of the aromatic group is the comparative stability of its compounds, for, whereas the artiad (i.e., even) radicles of the fatty group act as unsaturated groups entering freely into direct combination with other elements, the aromatic radicles act more like saturated groups entering into direct combination only with difficulty, and forming substitution compounds with comparative ease. The division here made between fatty and aromatic substances must not be regarded as one having a sharply defined boundary line. In point of fact, the two series merge into one another, and compounds belonging to one group can be transformed into compounds of the other. Thus benzene, the typical hydrocarbon of the aromatic group, can be formed directly from ethine or acetylene, a hydrocarbon of the fatty group, by the polymerization effected by heat : 3C 2 H 2 Ethine. C 6 H 6 . Benzene. Also acetone, a substance directly obtainable from the 2 So numerous, indeed, are these radicles in Organic Chemistry that this branch of the science has been named the " Chemistry of com pound radicles." 3 " We may therefore define that part of our science which is gene rally known as Organic Chemistry as the Chemistry of the Hydro
carbons and their Derivatives." Schorlemrner.