Nitrogen Chloride, NCl₃

This compound was first isolated by Dulong in 1811, by the action of chlorine on aqueous ammonium chloride:

NH₄Cl + 3Cl₂ = NCl₃ + 4HCl.

The preparation of this explosive substance may be safely effected by inverting a 2-litre flask filled with chlorine over a leaden capsule in a warm saturated solution of ammonium chloride. Small oily drops of the nitrogen chloride form on the sides of the flask and collect in the capsule, while absorption of the ammonium chloride takes place into the flask. Only small quantities of the compound are allowed to accumulate in the capsule, which is removed by tongs and replaced by a fresh one.

Nitrogen trichloride separates at the anode during the electrolysis of a saturated solution of ammonium chloride at 28° C.

Ammonium chloride reacts with a solution of hypochlorous acid to form nitrogen trichloride, also with sodium hypochlorite, or a suspension of bleaching powder in water (calcium hypochlorite). A convenient method is to add 300 c.c. of a solution (20 per cent.) of ammonium chloride to 3 litres of a suspension of bleaching powder in water containing 22-5 grams of active chlorine saturated with hydrogen chloride. This reaction mixture is shaken with 300 c.c. of benzene, in which the nitrogen trichloride dissolves, and this benzene solution is poured through a filter containing 20 grams of calcium chloride. The dried solution thus obtained contains about 10 per cent, of nitrogen chloride.

A modification of this method is to carry out the experiment at 0° C., omitting the hydrogen chloride and the benzene.

Anhydrous ammonia and chlorine react to give nitrogen trichloride, much of which decomposes into nitrogen and chlorine either directly or by the action of the ammonia.

Nitrogen chloride is a yellow volatile oil with a pungent smell resembling chlorine, the vapour of which attacks the eyes and mucous membrane. The density is 1.653 and boiling-point 71° C. When heated above 93° C. it decomposes explosively into its elements. It explodes violently when exposed to strong light or when brought into contact with many substances, such as ozone, phosphorus, arsenic, alkalies, and organic matter. Thus a feather dipped in turpentine and applied to a small drop of the oil detonates it immediately. Generally speaking, it is stable in the presence of metals, strong acids, resins, and sugars. Organic solvents dissolve nitrogen chloride, giving highly refracting yellow solutions which are stable in the dark, but gradually decompose when exposed to light.

Many experimenters have suffered from the explosive properties of this substance, including Dulong, Faraday, and Davy.

The heat of, formation is given as -38.477 Cals. A more recent determination gives the heat of formation of the compound in carbon tetrachloride from gaseous nitrogen and gaseous chlorine as –54.700 Cals.

Nitrogen chloride is hydrolysed by water to ammonia and hypo-chlorous acid,

NCl₃ + 3H₂O ⇔ NH₃ + 3HClO;

which is probably an explanation of the ready formation of ammonia, and also of the greater solubility of the compound in hydrochloric acid than in sulphuric acid, as the former reacts with the hypochlorous acid. Thus, when a carbon-tetrachloride solution of nitrogen chloride is shaken with moderately concentrated hydrochloric acid and the aqueous solution (freed from excess NCl₃) boiled with potassium hydroxide, a quantity of ammonia is evolved:

NCl₃ + 3H₂O ⇔ NH₃ + 3HClO,
HClO + HCl ⇔ Cl₂ + H₂O,
NH₃ + HCl ⇔ NH₄Cl.

The benzene solution of nitrogen chloride may be used as a chlorinating agent for organic compounds. Aqueous solutions of ammonium chloride decompose nitrogen chloride - the concentrated solution quickly and the dilute solution more slowly:

NH₄Cl + NCl₃ = N₂ + 4HCl.

The formula NCl₃ was first assigned by Dulong, although Davy considered that there was chlorine in excess of that required by this formula, while some later investigators concluded that hydrogen was also present. Gattermann, however, found that the amount of chlorine present closely agreed with that for NCl₃, by decomposing nitrogen trichloride with ammonia, and estimating the chlorine in the resulting ammonium chloride as silver chloride. This result has been confirmed.