274 HISTORY OF CHEMISTRY. [LECTURE xiv. 



bromine two different classes of substitution products should 

 be obtainable from toluene, these classes being characterised 

 by the fact that in the one the halogen replaces a hydrogen 

 atom of the benzene (nucleus), and in the other a hydrogen 

 atom of the methyl (side-chain). Differences of this kind were 

 actually observed, 4 and Beilstein 5 showed that substances belong- 

 ing either to the one class or to the other are produced 

 according to whether chlorine acts in the cold or at the boiling 

 temperature. The members of the first of these groups of 

 chlorine compounds (of which, as di-substitution products of 

 benzene, three isomers exist) do not permit any exchange of 

 their chlorine for iodine, for cyanogen, or for hydroxyl or other 

 groups containing oxygen ; whereas the chlorine derivative of 

 the other group (which, as a mono-substitution product is the 

 only chlorine representative of the group) behaves like the 

 chloride of an alcohol radical, and can be converted, just as 

 easily as chlorides of this kind can, into an alcohol, an ether, 

 etc. 



The two formulae 



C ti H 4 Cl(CH 3 ) C H 5 CH,C1 



Chlorotoluenes. Benzyl chloride. 



indicate these differences, which arise, according to Kekule, 

 from the fact that the chlorine atom of the chlorotoluenes 

 stands in intimate relation to the carbon (being almost entirely 

 surrounded by it), whereas in benzyl chloride it is combined in 

 a manner similar to the halogen of the alkyl chlorides. An 

 explanation of an exactly similar kind is now furnished for 

 the essentially different behaviour of the phenols and of the 

 aromatic alcohols. Whereas in the former the hydroxyl group 

 replaces a hydrogen atom of benzene, in benzyl alcohol the 

 replaced hydrogen belongs to the methyl group : 



C 6 H 4 (OH)CH 3 C C H 5 CH 2 OH 



Cresols. Benzyl alcohol. 



On oxidation, the latter alone behaves as a primary alcohol 



4 Fittig, Annalen. 136, 301 ; Kekule, ibi-L 137, 192. 5 Beilstein and 

 Gcitner, ibid. 139, 331. 



