422 GIBBS. 



very large absorption band of aniline would suggest that the dynamic activity 

 of this compound is even greater than that of benzene itself. It has further been 

 stated 1 ' that the decrease in the basicity of the nitrogen atom by the replacing 

 of the aminic hydrogen by the methyl group, manifests itself in the absorption 

 spectra by a decrease in the persistence of the absorption band. 



'Thus, in aniline, the persistence of the absorption band or the relative change 

 of dilutions over which it is visible is 84 per cent, in methylaniline it is 73 per 

 cent, and in dimethylaniline it is only 50 per cent. The introduction, therefore, 

 of one methyl group reduces the persistence by 11 per cent and two methyl groups 

 decrease it by 34 per cent. It is very interesting to note that the basicity of 

 the nitrogen atom in dimethylaniline has been sufficiently reduced to allow the 

 spectrum to partake also of the first type. (Referring to ben&sne and mono allcyl 

 substitution products.) * * * The methyl and the dimethyl compounds in the 

 presence of sufficient hydrochloric acid (3 equivalents) give a very similar spectrum 

 to aniline hydrochloride, that is to say, the spectrum resembling the first type. 

 We may therefore conclude that the presence of unsaturated nitrogen in the a- 



position gives rise to a broad band with its head at -r-=3470 (A=288 ftu) and 

 of great persistence.' 



A comparison of the spectra of aniline and its hydrochloride and the mono 

 alkylated benzenes shows that in the hydrochloride the group — NtL.HCl behaves 

 almost exactly the same as a single alkyl group; "that is to say, the very 

 striking residual affinity of the — NH 2 group in aniline has entirely disappeared." 



The exact meaning of Baly and Collie's statements, concerning the 

 decrease in the basicit} r of the nitrogen atom of aniline due to the substitu- 

 tion of the animic hydrogen by methyl groups, is not clear. Walker 18 

 has shown that aniline, methylaniline and dimethylaniline are no excep- 

 tions to the general rule that the substitution of hydrogen by the positive 

 group methyl, produces an increase in the basicity of the compound as in 

 the substituted ammonias, methylamine, dimethylamine, trimethylamine, 

 the substituted phosphines and other compounds. In the increasing 

 order of the basicity the compounds referred to are aniline, methylaniline 

 and dimethylaniline. The persistence of the absorption band in the ultra 

 violet is in the inverse order of the basicity and also in the inverse order 

 of the stability of the group and its dynamic activity. 



Baly and his coworkers have shown that the aliphatic enol-keto tauto- 

 meters give no absorption bands when in the pure state and that the 

 absorption band indicates that the enolic and ketonic forms are coexistent 

 and in dynamic equilibrium with one another. In general, they state 

 that 



* * * An absorption band in the ultra-violet region of the spectrum is only 

 shown by compounds having a possibility of tautomerism. Such tautomerism is 

 not necessarily due to the presence of a labile atom, but may be of the same order 

 as that occurring in ring compounds of the aromatic type, in which the reversible 

 change of linking may take place periodically. 19 



17 Baly and Collie, Loo. cit. 



w Ztschr. f. phys. Chem. (1889), 4, 338. 



10 Baly and Desch. Astrophys. Journ. (1906), 23, 118. 



