[23] Such carriers or media for the transference of chlorine and the halogens in general were long known to exist in iodine and antimonious chloride, and have been most fully studied by Gustavson and Friedel, of the Petroffsky Academy—the former with respect to aluminium bromide, and the latter with respect to aluminium chloride. Gustavson showed that if a trace of metallic aluminium be dissolved in bromine (it floats on bromine, and when combination takes place much heat and light are evolved), the latter becomes endowed with the property of entering into metalepsis, which it is not able to do of its own accord. When pure, for instance, it acts very slowly on benzene, C6H6, but in the presence of a trace of aluminium bromide the reaction proceeds violently and easily, so that each drop of the hydrocarbon gives a mass of hydrobromic acid, and of the product of metalepsis. Gustavson showed that the modus operandi of this instructive reaction is based on the property of aluminium bromide to enter into combination with hydrocarbons and their derivatives. The details of this and all researches concerning the metalepsis of the hydrocarbons must be looked for in works on organic chemistry.
[24] As small admixtures of iodine, aluminium bromide, &c., aid the metalepsis of large quantities of a substance, just as nitric oxide aids the reaction of sulphurous anhydride on oxygen and water, so the principle is essentially the same in both cases. Effects of this kind (which should also be explained by a chemical reaction proceeding at the surfaces) only differ from true contact phenomena in that the latter are produced by solid bodies and are accomplished at their surfaces, whilst in the former all is in solution. Probably the action of iodine is founded on the formation of iodine chloride, which reacts more easily than chlorine.
[25] Metalepsis belongs to the number of delicate reactions—if it may be so expressed—as compared with the energetic reaction of combustion. Many cases of substitution are of this kind. Reactions of metalepsis are accompanied by an evolution of heat, but in a less quantity than that evolved in the formation of the resulting quantity of the halogen acids. Thus the reaction C2H6 + Cl2 = C2H5Cl + HCl, according to the data given by Thomsen, evolves about 20,000 heat units, whilst the formation of hydrochloric acid evolves 22,000 units.
[26] With the predominance of the representation of compound radicles (this doctrine dates from Lavoisier and Gay-Lussac) in organic chemistry, it was a very important moment in its history when it became possible to gain an insight into the structure of the radicles themselves. It was clear, for instance, that ethyl, C2H5, or the radicle of common alcohol, C2H5·OH, passes, without changing, into a number of ethyl derivatives, but its relation to the still simpler hydrocarbons was not clear, and occupied the attention of science in the ‘forties’ and ‘fifties.’ Having obtained ethyl hydride, C2H5H = C2H6, it was looked on as containing the same ethyl, just as methyl hydride, CH4 = CH3H, was considered as existing in methane. Having obtained free methyl, CH3CH3 = C2H6, from it, it was considered as a derivative of methyl alcohol, CH3OH, and as only isomeric with ethyl hydride. By means of the products of metalepsis it was proved that this is not a case of isomerism but of strict identity, and it therefore became clear that ethyl is methylated methyl, C2H5 = CH2CH3. In its time a still greater impetus was given by the study of the reactions of monochloracetic acid, CH2Cl·COOH, or CO(CH2Cl)(OH). It appeared that metalepsical chlorine, like the chlorine of chloranhydrides—for instance, of methyl chloride, CH3Cl, or ethyl chloride, C2H5Cl—is capable of substitution; for example, glycollic acid, CH2(OH)(CO2H), or CO(CH2·OH)(OH), was obtained from it, and it appeared that the OH in the group CH2(OH) reacted like that in alcohols, and it became clear, therefore, that it was necessary to examine the radicles themselves by analysing them from the point of view of the bonds connecting the constituent atoms. Whence arose the present doctrine of the structure of the carbon compounds. (See Chapter VIII., Note [42].)
[27] By including many instances of the action of chlorine under metalepsis we not only explain the indirect formation of CCl4, NCl3, and Cl2O by one method, but we also arrive at the fact that the reactions of the metalepsis of the hydrocarbons lose that exclusiveness which was often ascribed to them. Also by subjecting the chemical representations to the law of substitution we may foretell metalepsis as a particular case of a general law.
[28] This may be taken advantage of in the preparation of nitrogen. If a large excess of chlorine water be poured into a beaker, and a small quantity of a solution of ammonia be added, then, after shaking, nitrogen is evolved. If chlorine act on a dilute solution of ammonia, the volume of nitrogen does not correspond with the volume of the chlorine taken, because ammonium hypochlorite is formed. If ammonia gas be passed through a fine orifice into a vessel containing chlorine, the reaction of the formation of nitrogen is accompanied by the emission of light and the appearance of a cloud of sal-ammoniac. In all these instances an excess of chlorine must be present.
[29] The hydrochloric acid formed combines with ammonia, and therefore the final result is 4NH3 + 3Cl2 = NCl3 + 3NH4Cl. For this reason, more ammonia must enter into the reaction, but the metalepsical reaction in reality only takes place with an excess of ammonia or its salt. If bubbles of chlorine be passed through a fine tube into a vessel containing ammonia gas, each bubble gives rise to an explosion. If, however, chlorine be passed into a solution of ammonia, the reaction at first brings about the formation of nitrogen, because chloride of nitrogen acts on ammonia like chlorine. But when sal-ammoniac has begun to form, then the reaction directs itself towards the formation of chloride of nitrogen. The first action of chlorine on a solution of sal-ammoniac always causes the formation of chloride of nitrogen, which then reacts on ammonia thus: NCl3 + 4NH3 = N2 + 3NH4Cl. Therefore, so long as the liquid is alkaline from the presence of ammonia the chief product will be nitrogen. The reaction NH4Cl + 3Cl2 = NCl3 + 4HCl is reversible; with a dilute solution it proceeds in the above-described direction (perhaps owing to the affinity of the hydrochloric acid for the excess of water), but with a strong solution of hydrochloric acid it takes the opposite direction (probably by virtue of the affinity of hydrochloric acid for ammonia). Therefore there must exist a very interesting case of equilibrium between ammonia, hydrochloric acid, chlorine, water, and chloride of nitrogen which has not yet been investigated. The reaction NCl3 + 4HCl = NH4Cl + 3Cl2 enabled Deville and Hautefeuille to determine the composition of chloride of nitrogen. When slowly decomposed by water, chloride of nitrogen gives, like a chloranhydride, nitrous acid or its anhydride, 2NCl3 + 3H2O = N2O3 + 6HCl. From these observations it is evident that chloride of nitrogen presents great chemical interest, which is strengthened by its analogy with trichloride of phosphorus. The researches of F. F. Selivanoff (1891–94) prove that NCl3 may be regarded as an ammonium derivative of hypochlorous acid. Chloride of nitrogen is decomposed by dilute sulphuric acid in the following manner: NCl3 + 3H2O + H2SO4 = NH4HSO4 + 3HClO. This reaction is reversible and is only complete when some substance, combining with HClO (for instance, succinimide) or decomposing it, is added to the liquid. This is easily understood from the fact that hypochlorous acid itself, HClO, may, according to the view held in this book, be regarded as the product of the metalepsis of water, and consequently bears the same relation to NCl3 as H2O does to NH3, or as RHO to RNH2, R2NH, and R3N—that is to say, NCl3 corresponds as an ammonium derivative to ClOH and Cl2 in exactly the same manner as NR3 corresponds to ROH and R2. The connection of NCl3 and other similar explosive chloro-nitrogen compounds (called chloryl compounds by Selivanoff; for example, the C2H5NCl2 of Wurtz is chloryl ethylamine), such as NRCl2 (as NC2H5Cl2), and NR2Cl (for instance, N(CH3CO)HCl, chlorylacetamide, and N(C2H5)2Cl, chloryl diethylamine) with HClO is evident from the fact that under certain circumstances these compounds give hypochlorous acid, with water, for instance, NR2Cl + H2O = NR2H + HClO, and frequently act (like NCl3 and HClO, or Cl2) in an oxidising and chloridising manner. We may take chloryl succinimide, C2H4(CO)2NCl for example. It was obtained by Bender by the action of HClO upon succinimide, C2H4(CO)2NH, and is decomposed by water with the re-formation of amide and HClO (the reaction is reversible). Selivanoff obtained, investigated, and classified many of the compounds NR2Cl and NRCl2, where R is a residue of organic acids or alcohols, and showed their distinction from the chloranhydrides, and thus supplemented the history of chloride of nitrogen, which is the simplest of the amides containing chlorine, NR3, where R is fully substituted by chlorine.
[29 bis] In preparing NCl3 every precaution must he used to guard against an explosion, and care should he taken that the NCl3 remains under a layer of water. Whenever an ammoniacal substance comes into contact with chlorine great care must be taken, because it may be a case of the formation of such products and a very dangerous explosion may ensue. The liquid product of the metalepsis of ammonia may be most safely prepared in the form of small drops by the action of a galvanic current on a slightly warm solution of sal-ammoniac; chlorine is then evolved at the positive pole, and this chlorine acting on the ammonia gradually forms the product of metalepsis which floats on the surface of the liquid (being carried up by the gas), and if a layer of turpentine be poured on to it these small drops, on coming into contact with the turpentine, give feeble explosions, which are in no way dangerous owing to the small mass of the substance formed. Drops of chloride of nitrogen may with great caution be collected for investigation in the following manner. The neck of a funnel is immersed in a basin containing mercury, and first a saturated solution of common salt is poured into the funnel, and above it a solution of sal-ammoniac in 9 parts of water. Chlorine is then slowly passed through the solutions, when drops of chloride of nitrogen fall into the salt water.
[30] Quicklime, CaO (or calcium carbonate, CaCO3), does not absorb chlorine when cold, but at a red heat, in a current of chlorine, it forms calcium chloride, with the evolution of oxygen. (This was confirmed in 1893 by Wells, at Oxford.) This reaction corresponds with the decomposing action of chlorine on methane, ammonia, and water. Slaked lime (calcium hydroxide, CaH2O2) also, when dry, does not absorb chlorine at 100°. The absorption proceeds at the ordinary temperature (below 40°). The dry mass thus obtained contains not less than three equivalents of calcium hydroxide to four equivalents of chlorine, so that its composition is [Ca(HO)2]5Cl4. In all probability a simple absorption of chlorine by the lime at first takes place in this case, as may be seen from the fact that even carbonic anhydride, when acting on the dry mass obtained as above, disengages all the chlorine from it, leaving only calcium carbonate. But if the bleaching powder be obtained by a wet method, or if it be dissolved in water (in which it is very soluble), and carbonic anhydride be passed into it, then chlorine is no longer disengaged, but chlorine oxide, Cl2O, and only half of the chlorine is converted into this oxide, while the other half remains in the liquid as calcium chloride. From this it may be inferred that calcium chloride is formed by the action of water on bleaching powder, and this is proved to be the case by the fact that small quantities of water extract a considerable amount of calcium chloride from bleaching powder. If a large quantity of water act on bleaching powder an excess of calcium hydroxide remains, a portion of which is not subjected to change. The action of the water may be expressed by the following formulæ: From the dry mass Ca3(HO)6Cl4 there is formed lime, Ca(HO)2, calcium chloride, CaCl2, and a saline substance, Ca(ClO)2. Ca3H6O6Cl4 = CaH2O2 + CaCl2O2 + CaCl2 + 2H2O. The resulting substances are not equally soluble; water first extracts the calcium chloride, which is the most soluble, then the compound Ca(ClO)2 and ultimately calcium hydroxide is left. A mixture of calcium chloride and hypochlorite passes into solution. On evaporation there remains Ca2O2Cl43H2O. The dry bleaching powder does not absorb more chlorine, but the solution is able to absorb it in considerable quantity. If the liquid be boiled, a considerable amount of chlorine monoxide is evolved. After this calcium chloride alone remains in solution, and the decomposition may be expressed as follows: CaCl2 + CaCl2O2 + 2Cl2 = 2CaCl2 + 2Cl2O. Chlorine monoxide may be prepared in this manner.
It is sometimes said that bleaching powder contains a substance, Ca(OH)2Cl2, that is calcium peroxide, CaO2, in which one atom of oxygen is replaced by (OH)2, and the other by Cl2; but, judging from what has been said above, this can only be the case in the dry state, and not in solutions.