Organic compounds containing three or four atoms of nitrogen united to one another (the triazo- and tetrazo-compounds) are numerous and often stable. Thus diazo-amido-compounds, RN=N-N-NHR, contain the triazene group. Compounds containing the cyclic triazene group or azide ring,
may be made by a variety of reactions, such as those which involve the condensation of a diazo-compound with one containing a single nitrogen atom, involving elimination of water or halogen hydracid, or both. Thus:
C6H5-NH-NH2 + NOCl = C6H5-N3 + H2O + HCl.
Phenyl hydrazine. → Phenyl azide.
The hydrolysis of such compounds containing the azide ring leads to the formation of hydrogen azide or azoimide, N3H, which was first prepared in this way by Curtius in 1890.
The reaction by which this compound is generally prepared is that discovered by Wislicenus in 1892, and is best carried out as follows: Dry ammonia is passed over sodium at 250° to 350° C. or allowed to bubble through the molten metal, which is converted into the amide. The reaction is complete in about five hours.
2Na + 2NH3 = 2NaNH2 + H2.
Nitrous oxide is then passed over the melted amide in a nickel dish at 190° C. until no more ammonia is evolved.
NaNH2 + N2O = H2O + NaN3.
The sodium azide, NaN3, is then distilled with sulphuric acid (1:1). The greater part of the hydrazoic acid (azoimide) distils below 45° C. in the form of a 91 per cent, solution, which is the first quarter of the distillate. The distillation is continued until the addition of silver nitrate gives no further precipitate of silver azide.
Hydrazoic acid is also obtained when hydrazine is oxidised by a compound containing a third nitrogen atom, such as NCl3, or better, nitrous acid, or by hydrogen peroxide.
Solutions containing 5 grams of hydrazine sulphate and 3.3 grams of potassium nitrite are mixed in the cold, and distilled after the effervescence has ceased. If silver nitrite is used, the hydrazoic acid is precipitated as silver azide:
2N2H4.H2SO4 + 2HNO2 = 2N3H + 4H2O + H2SO4.
Properties of the Free Acid
The anhydrous acid is prepared by removing the last quantities of water from the concentrated distillate with anhydrous calcium chloride. It is a colourless, mobile liquid which boils at +37° C. It freezes at a low temperature, and the solid melts at -80° C. The molecular weight deduced from the density of the vapour, about 25° C. above the boiling-point of the liquid, corresponds to the simplest formula, N3H. The acid as well as many of its salts are liable to explode when heated or struck. The heat of formation, calculated from the heat of combustion of ammonium azide, NH4N3, is -62.100 Cals. per mol. It mixes with water and alcohol.
The aqueous solution reacts acid. Its conductivity, determined at concentrations of from 0.1 to 0.001N, gives a mean dissociation constant of about 1.8×10-5. This increases somewhat with increase of concentration, resembling in this respect the constant of a stronger acid.
Solutions containing rather less than 30 per cent, of the acid give off its vapour, which produces dense fumes of ammonium azide when brought into contact with ammonia vapour. The vapour of hydrazoic acid has a most unpleasant smell, and causes inflammation of the mucous membrane when inhaled. The solution is poisonous and attacks the skin. Even in small quantities (0.1 per cent.) it is poisonous to plants, and will prevent the germination of seedlings.
Although the solution is fairly stable, yet on long boiling with mineral acids it is decomposed into nitrogen and ammonium salts, a decomposition which is much accelerated by the presence of platinum black.
When the solution is electrolysed hydrogen is evolved at the kathode, and nitrogen, probably by a secondary oxidation of the compound (as in the case of solutions of ammonia), at the anode.3,4 Strong oxidising agents such as potassium permanganate convert it into water and nitrogen.
With nitrous acid, hydrazoic acid gives nitrous oxide in addition to nitrogen:
N3H + HNO2 = N2 + N2O + H2O.
It is easily reduced by sodium amalgam, by zinc and acid, by ferrous hydroxide, etc., to ammonia and hydrazine:
N3H + 6H = NH3 + N2H4.
It is also reduced by polysulphides or H2S:
H2S + N3H = S + N2 + NH3.
Hydrazoic acid dissolves the more electro-positive metals, magnesium, aluminium, zinc, and iron, with evolution of hydrogen in most cases. The nascent hydrogen is partly, or it may be wholly, used to reduce the hydrazoic acid to ammonia. Thus in the case of copper no hydrogen is set free:
Cu + 3N3H = Cu(N3)2 + N2 + NH3.
In several of its reactions hydrazoic acid shows a similarity to nitric acid, as is seen by comparing the corresponding reactions with copper and hydrogen sulphide:
3Cu + 8HNO3 = 3Cu(NO3)2 + 2NO + 4H2O;
3H2S + 2HNO3 = 3S + 2NO + 4H2O.
Salts of Hydrazoic Acid
The azides of many metals have been prepared in the crystalline state. In these the acid is always monobasic; the formulae therefore correspond to those of the halides, and the properties are often intermediate between those of a chloride and of a bromide.
The azides of the alkalies, LiN3, NaN3, KN3, and NH4N3, are easily soluble in water and also soluble in alcohol. Those of the alkaline earths, namely, MgN6, CaN6, etc., are also soluble. Other azides, such as CuN6 (red-brown), AgN3 (white), HgN3 and HgN6, PbN6 and TiN3, are sparingly soluble or almost insoluble in water. The remaining metals mostly give basic salts.
Lead azide is well known as a detonator, and is used in the fuses of shells.
The formulae of many salts have been proved by the electrolysis of solutions of NH4N3 in liquid ammonia.
The azides of the alkali and alkaline earth metals are not explosive, and some may even be melted without decomposition. The azides of Na, K, Rb, and Cs give up their nitrogen quietly at about 300° to 350° C.; those of Ca, Sr, Ba do so at about 100° to 120° C. Consequently they have been used as a source of pure nitrogen.
A few double salts have been prepared, compounds of alkali azides with those of other metals, chiefly belonging to Group VIII., e.g. NH4N3.CoN6 (blue), are known.
Azides of the Halogens
Iodine Azide
This compound has been prepared by bringing an aqueous suspension of silver azide into contact with an ethereal solution of iodine. On evaporation of the ether in a current of air, colourless crystals are left which have a penetrating odour similar to that of cyanogen iodide, and which are easily decomposed by explosion into iodine and nitrogen. The compound may be hydrolysed by water, giving azoimide and hypoiodous acid:
N3I + H2O = N3N + HOI.
The iodine is therefore the electro-positive part of the molecule.
Bromine Azide
Bromine Azide has also been prepared by the action of bromine vapour on sodium or silver azide. It is very explosive even at low temperatures.
Chlorazide
Chlorazide may be made by acidifying a solution containing sodium azide and hypochlorite by a weak acid such as boric. It is a colourless gas, smelling like hypochlorous acid. In alkaline solution it is rapidly hydrolysed to its generators, the reaction proceeding from left to right:
N3H + HOCl ⇔ N3Cl + H2O.
The gas easily explodes with great violence and the appearance of blue flames.
The constitution of organic compounds containing the azide ring is established by the consideration of their preparations and reactions, on the assumption that the usual valencies are exerted by the elements involved. Thus phenyl azide is made by the condensa tion of phenyl hydrazine and nitrous acid:
The analogous reaction between hydrazine itself and nitrous acid (vide supra) leads to an analogous formula for the hydrogen compound, which also agrees well with the other reactions used in its preparation. Azoimide is unique in being the only compound containing one atom of hydrogen united with more than one atom of a second element. It is so extremely improbable that the atomic weight of hydrogen should be divided by three on this account, that preferably the univalency of the group N3 - is assumed and accounted for by the cyclic formula.
The compound may easily be recognised by the highly characteristic properties and reactions mentioned above. In solution it gives precipitates with silver salts, as halides do, but may be distinguished from chloride and bromide by means of the brown precipitate of the cupric salt. Another distinctive test is an intense red colour with ferric salts, (FeN9). The evolution of nitrogen when it is oxidised by iodine, etc. (vide infra), also serves to distinguish it from halides as well as from thiocyanates.
The quantitative analysis depends on oxidation with iodine or nitrous acid. A neutral or alkaline solution is mixed with a slight excess of iodine solution and a crystal of sodium thiosulphate added, which hastens the evolution of nitrogen. In very dilute solutions eerie nitrate or sulphate should be added.
The reaction between an azide and nitrous acid is quantitative, and may be used to estimate the latter.
A measured excess of the azide is added to the acidified nitrite solution. After shaking for a few minutes the mixture is made alkaline with baryta, and is boiled to expel the gases. It is then acidified with acetic acid, and the excess of azoimide titrated with iodine and thiosulphate,
Azides may also be estimated as nitrogen after decomposition with eerie ammonium nitrate in a nitrometer.