Note 26 — Chapter XI · THE HALOGENS: CHLORINE, BROMINE, IODINE, AND FLUORINE

With the predominance of the representation of compound radicles (this doctrine dates from Lavoisier and Gay-Lussac) in organic chemistry, it was a very important moment in its history when it became possible to gain an insight into the structure of the radicles themselves. It was clear, for instance, that ethyl, C2H5, or the radicle of common alcohol, C2H5·OH, passes, without changing, into a number of ethyl derivatives, but its relation to the still simpler hydrocarbons was not clear, and occupied the attention of science in the ‘forties’ and ‘fifties.’ Having obtained ethyl hydride, C2H5H = C2H6, it was looked on as containing the same ethyl, just as methyl hydride, CH4 = CH3H, was considered as existing in methane. Having obtained free methyl, CH3CH3 = C2H6, from it, it was considered as a derivative of methyl alcohol, CH3OH, and as only isomeric with ethyl hydride. By means of the products of metalepsis it was proved that this is not a case of isomerism but of strict identity, and it therefore became clear that ethyl is methylated methyl, C2H5 = CH2CH3. In its time a still greater impetus was given by the study of the reactions of monochloracetic acid, CH2Cl·COOH, or CO(CH2Cl)(OH). It appeared that metalepsical chlorine, like the chlorine of chloranhydrides—for instance, of methyl chloride, CH3Cl, or ethyl chloride, C2H5Cl—is capable of substitution; for example, glycollic acid, CH2(OH)(CO2H), or CO(CH2·OH)(OH), was obtained from it, and it appeared that the OH in the group CH2(OH) reacted like that in alcohols, and it became clear, therefore, that it was necessary to examine the radicles themselves by analysing them from the point of view of the bonds connecting the constituent atoms. Whence arose the present doctrine of the structure of the carbon compounds. (See Chapter VIII., Note 42.)