RADICAL, or RADICLE, in chemistry (and particularly in organic chemistry), a name given to certain molecular groups whose con stituent atoms possess such relations and affini ties that the groups themselves persist and maintain their integrity, even when the com pounds in which they occur undergo profound chemical change. The following example will illustrate the nature of a radical more dearly: Ordinary alcohol (ethyl alcohol), when free from water, has the chemical formula C.H.O. If this be saturated with hydrochloric acid gas in the presence of chloride of zinc, a compound having the formula C.H.C1 is formed; and by treating the alcohol with iodine under proper conditions, an analogous compound having the formula Cilia is obtained. When this last sub stance is heated with water and metallic zinc, we obtain the hydrocarbon gas ethane, C.H.. These reactions, and others of a similar nature, suggest that the molecular group C.H. persists through all of these changes; and this hypothesis is found to be justified in every way. The group C.H. is, therefore, a and to it the name Sehylp has been given. The gas ethane, is the hydride of ethyl, according to this view, and its formula may be written C.H.H. Similarly, C.H.C1 and C,HsI are called ethyl chloride and ethyl iodide, respectively. Ethyl alcohol, in the same} way, is regarded as the hydrate of the radical ethyl, and its formula is written CH..0H. Ethyl is thus seen to be a monovalent radical with a high degree of sta bility, behaving, in its chemical relations, very much like the alkali metals, potassium and so dium. Many compounds are known in which two or more molecules of ethyl occur, with their combined valencioi satisfying a single divalent atom or radical. Ordinary ether, for example, is the oxide of, ethyl, its formula being (C2114).0. The analogy of this substance with the oxide of potassium, 1{..0, is complete.
If the persistence of a radical were abso lute, so that we could never destroy the rad ical by any chemical or physical means, we ob viously could not distinguish it from an element; and it is possible that some of the bodies that we now recognize as elements are merely radi cals of extraordinary persistence, which we may some day succeed in breaking up into their con stituents. This is pure speculation, however, and until we know more about the ultimate con stitution of matter, we are to regard a ((radical' merely as a convenient conception, enables us to simplify chemical formulae. and to form a dear general view of the analogies and chemical relations that subsist among various chemical compounds. Radicals may be classed
as ((acid* or compounds. according as they have anacid or a basic character. Thus the radical SO., which occurs in sulphuric acid and in the sulphates, and which is called (sulohion,' is an acid radical, while ethyl, Ca., is basic.
Among the simpler inorganic radicals that are of frequent occurrence, the following are monovalent: Hydroxyl (OH), the °nitro* group •(NO2), the anitroso* group (NO), the (ainido' group (NH.) and cyanogen (CN). The following are divalent: Sulphion (SO4), sulphuryl (SO.), the group (NH) and the group (SO). In organic chem istry many radicals are recognized. The com moner basic radicals of the fatty series may be regarded as derived from the saturated hydro carbons (or paraffins) by the removal of one or more atoms of hydrogen. Thus we have the following series, which are derived in the man ner described, the radicals in each column being obtained from the paraffin at the head of that column by the removal of hydrogen atoms: CHa Calla Ca* Cagle Cana Methane Ethane Propane Butane Pentane CH, Calla Cab C4FIs ain Methyl Ethyl Propyl Butyl CH, Galt Calf. C4Ha Wise ' Methylene Ethylene Profcg Cab Ceene Butylene Pentylene CH Calf, llo Methenyl Ethenyl Propenyl Butenyl Pentenyl C C,H, Calla Calls CaH, (Carbon) Ethane Propane Butane Pentine The names here assigned to the radicals are due to Hofmann, and the system in accordance with which they are formed will be sufficiently evident, without further explanation. Hof mann's names are now commonly adopted, but some of the foregoing radicals are still known by other names also. Thus Hofmann's tetisenyP is often called "(vinyl)); and, similarly, his tpropenyl,) tbutenyl,° tethine,° tpropine,)) and tbutine° are respectively known as ugly ceryl,* "acetylene,* tallylenet and tcrotonylene.° The radicals tabulated above and which are of constant occurrence in organic chemistry contain only hydrogen and oxygen. Many or ganic radicals are known, however, which, con tain other elements also. The first oxygen bearing group to be recognized as a radical was C.H.CO, which occurs in benzoic acid, and in many other compounds that are re lated to the oil of bitter air on This• radical was recognized by Liebig and \,\ olikr, in 1832. Many such radicals are now recognized, es pecially in the organic acids and their deriva tives. As an example, "acetyl° may be cited. This has the composition C21-130, and acetic acid, C21-1,0.0H, is its hydrate.