[37] _Ein natürliches System der Gift-Wirkungen_, München, 1893.
Loew considers that all substances which enter into combination with aldehyde or ketone groups must be poisonous to life generally. For instance, hydroxylamine, diamide and its derivatives, phenylhydrazine, free ammonia, phenol, prussic acid, hydric sulphide, sulphur dioxide and the acid sulphites all enter into combination with aldehyde.
So again the formation of imide groups in the aromatic ring increases any poisonous properties the original substance possesses, because the imide group easily enters into combination with aldehyde; thus piperidine (CH₂)₅NH is more poisonous than pyridine (CH)₅N; coniine NH(CH₂)₄CH-CH₂-CH₂CH₃, is more poisonous than collidine N(CH)₄C-CH-(CH₃)₂; pyrrol (CH)₄NH than pyridine (CH)₅N; and amarin,[38]
[38] Th. Weyl (_Lehrbuch der organischen Chemie_) states (p. 385) that amarin is not poisonous, but Baccheti (_Jahr. d. Chemie_, 1855) has shown that 250 mgrms. of the acetate will kill a dog, 80 mgrms. a guinea-pig; and that it is poisonous to fishes, birds, and frogs: hydrobenzamide in the same doses has no effect.
If the theory is true, then substances with “labile” amido groups, on the one hand, must increase in toxic activity if a second amido group is introduced; and, on the other, their toxic qualities must be diminished if the amido group is changed into an imido group by the substitution of an atom of hydrogen for an alkyl.
Observation has shown that both of these requirements are satisfied; phenylenediamine is more poisonous than aniline; toluylenediamine more poisonous than toluidine. Again, if an atom of hydrogen in the amido (NH₂) group in aniline be replaced by an alkyl, _e.g._ methyl or ethyl, the resulting substance does not produce muscular spasm; but if the same alkyl is substituted for an atom of hydrogen in the benzene nucleus the convulsive action remains unaffected.
If an acidyl, as for example the radical of acetic acid, enter into the amido group, then the toxic action is notably weakened; thus, acetanilide is weaker than aniline, and acetylphenylhydrazine is weaker than phenylhydrazine. If the hydrogen of the imido group be replaced by an alkyl or an acid radical, and therefore tertiary bound nitrogen restored, the poisonous action is also weakened.