Note 37 bis — Chapter VI · TTHE COMPOUNDS OF NITROGEN WITH HYDROGEN AND OXYGEN

The property possessed by nitroglycerin (occurring in dynamite), nitrocellulose, and the other nitro-compounds, of burning with an explosion, and their employment for smokeless powder and as explosives in general, depends on the reasons in virtue of which a mixture of nitre and charcoal deflagrates and explodes; in both cases the elements of the nitric acid occurring in the compound are decomposed, the oxygen in burning unites with the carbon, and the nitrogen is set free; thus a very large volume of gaseous substances (nitrogen and oxides of carbon) is rapidly formed from the solid substances originally taken. These gases occupy an incomparably larger volume than the original substance, and therefore produce a powerful pressure and explosion. It is evident that in exploding with the development of heat (that is, in decomposing, not with the absorption of energy, as is generally the case, but with the evolution of energy) the nitro-compounds form stores of energy which are easily set free, and that consequently their elements occur in a state of particularly energetic motion, which is especially strong in the group NO2: this group is common to all nitro-compounds, and all the oxygen compounds of nitrogen are unstable, easily decomposable, and (Note 29) absorb heat in their formation. On the other hand, the nitro-compounds are instructive as an example and proof of the fact that the elements and groups forming compounds are united in definite order in the molecules of a compound. A blow, concussion, or rise of temperature is necessary to bring the combustible elements C and H into the most intimate contact with NO2, and to distribute the elements in a new order in new compounds.

As regards the composition of the nitro-compounds, it will be seen that the hydrogen of a given substance is replaced by the complex group NO₂ of the nitric acid. The same is observed in the passage of alkalis into nitrates, so that the reactions of substitution of nitric acid—that is, the formation of salts and nitro-compounds—may be expressed in the following manner. In these cases the hydrogen is replaced by the so-called radicle of nitric acid NO₂, as is evident from the following table:—

Caustic potash KHO. Glycerin C₃H₅H₃O₃.

Nitre K(NO₂)O. Nitroglycerin C₃H₅(NO₂)₃O₃.

Hydrate of lime CaH₂O₂ Phenol C₆H₅OH.

Calcium nitrate Ca(NO₂)₂O₂. Picric acid C₆H₂(NO₂)₃OH, &c.

The difference between the salts formed by nitric acid and the nitro-compounds consists in the fact that nitric acid is very easily separated from the salts of nitric acid by means of sulphuric acid (that is, by a method of double saline decomposition), whilst nitric acid is not displaced by sulphuric acid from true nitro-compounds; for instance, nitrobenzene, C₆H₅·NO₂. As nitro-compounds are formed exclusively from hydrocarbons, they are described with them in organic chemistry.

The group NO₂ of nitro-compounds in many cases (like all the oxidised compounds of nitrogen) passes into the ammonia group or into the ammonia radicle NH₂. This requires the action of reducing substances evolving hydrogen: RNO₂ + 6H = RNH₃ + 2H₂O. Thus Zinin converted nitrobenzene, C₆H₅·NO₂, into aniline, C₆H₅·NH₂, by the action of hydrogen sulphide.

Admitting the existence of the group NO₂, as replacing hydrogen in various compounds, then nitric acid may be considered as water in which half the hydrogen is replaced by the radical of nitric acid. In this sense nitric acid is nitro-water, NO₂OH, and its anhydride dinitro-water, (NO₂)₂O. In nitric acid the radical of nitric acid is combined with hydroxyl, just as in nitrobenzene it is combined with the radical of benzene.

It should here be remarked that the group NO₃ may be recognised in the salts of nitric acid, because the salts have the composition M(NO₃)_n, just as the metallic chlorides have the composition MCl_n. But the group NO₃ does not form any other compounds beyond the salts, and therefore it should he considered as hydroxyl, HO, in which H is replaced by NO₂.