Jones. This compound is so unstable, so active, that it polymerizes
with explosive violence at temperatures slightly
above that at which liquid air boils. Such illustrations afford
clear proof that, as before mentioned, valency is a reciprocal
function—that it is impossible to regard the units of affinity of
the atoms of different elements as of equivalent value and capable
of satisfying each other mutually.
There is no reason to suppose that an uneven number of affinities can be active in the carbon atom; in devising structural formulae, it is therefore always considered necessary to account for the disposition of the four units of affinity, the four valencies, of the carbon atom. In 1900 some excitement was aroused by the discovery by Gomberg of a remarkable hydrocarbon formed by the withdrawal of the chlorine atom from chlorotriphenylmethane, C(C6H5)3Cl: at first it was contended that this was a compound of triad carbon, triphenylmethyl; it is now generally admitted, however, that such cannot well be the case and that one of the phenyl groups becomes altered in structure and converted into a dyad radicle (see Triphenylmethane).
The homologies of methane—the hydrocarbons of the paraffin or CnH2n+2 series, in which the carbon atoms are associated by single affinities, their remaining affinities being engaged by hydrogen atoms—behave chemically as saturated compounds and are apparently incapable of entering into combination with other molecules. But it is important to guard against the assumption that they are actually saturated in any absolute sense. Even gases such as helium and argon, destitute as they appear to be of all chemical activity, must be credited with the possession of some measure of affinity—as they can be liquefied; moreover, as Sir James Dewar has shown, when helium is liquefied in contact with charcoal a not inconsiderable amount of heat is liberated beyond that given out in the mere liquefaction of the gas. The argument may be extended to hydrogen and the paraffins and it may even be supposed that the amount of residual affinity increases gradually as the series is ascended this would account for the fact that their activity, the readiness with which they are attacked, increases slightly as the series is ascended. In any case, it cannot well be supposed that carbon and hydrogen mutually satisfy each other even in the paraffins.
The manner in which the valencies of the carbon atom are disposed of in the case of unsaturated hydrocarbons—that is to say, those containing a lower proportion of hydrogen than is indicated by the formula CnH2n+2—has given rise to much discussion, the subject being one which affords an opportunity for great difference of opinion. In ethylene, C2H4, each carbon atom is attached to only two hydrogen atoms, as two affinities of each atom are therefore free to enter reciprocally into combination. These atoms certainly do not combine twice over in the way in which the two atoms of carbon in ethane, H3C·CH3, enter into combination—if they did, ethylene should be a saturated compound, whereas actually it behaves as an eminently unsaturated substance. It was contended by Julius Thomsen, on the basis of determinations of the heat of combustion of the hydrocarbons, that the two carbon atoms in ethylene are less firmly united in ethylene than are those in ethane; moreover, that in acetylene, C2H2, in which there are three affinities at the disposal of 'each of the two carbon atoms, the union is even less firm than in ethylene. The argument on which these conclusions are founded has been called in question and the data are clearly insufficient to justify their acceptance; moreover, the stability of acetylene at high temperatures, also the readiness with which ethylene is often formed and with which ethenoid compounds revert to the parafiin type may be cited as arguments against them.
In dealing with such a problem, it is necessary to take into account the evidence we have that valency is a directed function. The tetrahedron is now accepted as the most suitable model of the carbon atom to be visualized whenever carbon is thought of; moreover, it is held that the directions in which valency acts are appropriately pictured if they are regarded as proceeding from the centre of mass to the four solid angles of the tetrahedron. In such a case, two affinities proceeding from each of two carbon atoms do not meet and overlap but cross, each pair at a considerable angle through which they must be deflected to bring them into contact. Von Baeyer has suggested that this angle, 12(109° 28′), is the measure of the strain imposed upon the affinities and that the existence of this strain affords an explanation of the readiness with which ethylene lapses into a derivative of ethane when suitable opportunity is given to combine with some other substance. Another way of looking at the matter is to suppose that the affinities do not, as it were, overlap but merely cross each other and that the angle of approach referred to is a direct measure of the degree of unsaturatedness: such a view is more in accordance with Thomsen’s contention. In any case, the ethenoid condition of unsaturatedness at the junction of two carbon atoms is a centre at which altogether peculiar properties, chemical and physical, are developed—the most noteworthy being the enhanced refractive power. The ethenoid symbol C=C is therefore of peculiar significance. It is a remarkable fact that the properties of ring systems generally are in accordance with the above hypothesis—the degree of unsaturatedness diminishing as “the angle of approach” is diminished, the more nearly the affinities can be pictured as overlapping.
The most stable arrangement of the carbon affinities would appear to be that in benzene and compounds of the benzene type—whatever that may be. The determination of the “structure” of this hydrocarbon has given rise to a large amount of paper warfare. Two tendencies may be said to have been brought together in the course of this discussion: on the one hand, the desire to arrive at a determination of the actual structure; on the other, the desire to devise formulae which shall be faithful expressions of functional behaviour and broadly indicative of the structural relationship of the constituent elements. Thexlatter is perhaps the tendency which is now in the ascendant: we are beginning to realise, particularly in the case of carbon compounds, that formulae are primarily expressive of behaviour-being based on the observation of behaviour. Thus in the case of all parafiinoid compounds, the symbol C–C has a distinctive meaning, as indicating saturation; in the case of ethenoid compounds, the symbol C=C has an equally distinctive meaning, indicating a particular degree of unsaturatedness.
From this point of view, therefore, the benzene symbol originally proposed by Kekulé is misleading, inasmuch as it indicates that the hydrocarbon contains three ethenoid junctions; it should therefore be an eminently unsaturated compound, which is not the case. On this account the centric formula is to be preferred as an expression of the properties of the compound. The non-metallic elements other than carbon all form volatile hydrides and methides from which their fundamental valencies can be deduced without difficulty. Chlorine, oxygen, nitrogen and silicon may be regarded as typical of the four classes into which the non-metals fall. But the number of hydrogen and methyl radicles which the atom carries cannot be taken as the measure of absolute valency in the case of elements of the chlorine, oxygen and nitrogen classes. The hydrides of the elements of these classes must all be regarded as more or less unsaturated compounds, the fact that gases such as hydrogen chloride and ammonia are intensely soluble in water being clearly a proof that their molecules are greatly attracted by and have great attraction for water molecules; it is remarkable, however, that although hydrogen chloride and ammonia are easily soluble in water and also combine readily with one another, they are gases which are by no means easily condensed—in other words, the molecules in each gas have little tendency to associate among themselves. It may also be pointed out that, to account for the properties of liquid water, it is necessary to suppose that the simple molecules represented by the symbol H2O have a very considerable mutual afhnity and that water consists largely of complex molecules.[1] Taking into account
- ↑ On this account it is desirable to confine the term water to the liquid and to distinguish the simple molecule represented by the symbol H2O by a separate name—that proposed is Hydrogen. Liquid water is probably a mixture of several polyhydrones together with more or less hvdrone.
