The diagram which is obtained when isomeric transformation does not occur within measurable time at the temperature of the melting point is somewhat different from that already given in Fig. 59. In this case, the two freezing point curves AC and BC (Fig. 60) can be readily realized, as no isomeric change occurs in the liquid phase. Suppose, however, that at a higher temperature, t′, reversible isomeric transformation can take place, the composition of the liquid phase will alter until at the point x′ a condition of equilibrium is reached; and the composition of the liquid at higher temperatures will be represented by the curve x′F. Below the temperature t′ the position of the equilibrium curve is hypothetical; but as the temperature
falls the velocity of transformation diminishes, and at last becomes practically zero. The equilibrium curve can therefore be regarded as dividing into two branches x′G and x′H. At temperatures between G and t′ the α modification can undergo isomeric change leading to a point on the curve Gx′; and the β modification can undergo change leading to a point on the curve Hx′. The same condition of equilibrium is therefore not reached from each side, and we are therefore dealing not with true but with false equilibrium (p. [5]). Below the temperatures G and H, isomeric transformation does not occur in measurable time. We shall not, however, enter into a detailed discussion of the equilibria in such systems, more especially as they are not systems in true equilibrium, and as the temperature at which true equilibrium can be established with appreciable velocity alters under the influence of catalytic agents.[[283]] Examples of such systems will no doubt be found in the case of optically active substances, where both isomerides are apparently quite stable at the melting point. In the case of such substances, also, the action of catalytic agents in producing isomeric transformation (racemisation) is well known.
Transformation of the Unstable into the Stable Form.—As has already been stated, the stable modification in the neighbourhood of the melting point is that one which is in equilibrium with the liquid phase at the natural freezing point. In the case of polymorphic substances, we have seen (p. [39]) that that form which is stable in the neighbourhood of the melting point melts at the higher temperature. That was a
consequence of the fact that the two polymorphic forms on melting gave identical liquid phases. In the present case, however, the above rule does not apply, for the simple reason that the liquid phase obtained by the fusion of the one modification is not identical with that obtained by the fusion of the other. In the case of isomeric substances, therefore, the form of lower melting point may be the more stable; and where this behaviour is found it is a sign that the two forms are isomeric (or polymeric) and not polymorphic.[[284]] An example of this is found in the case of the isomeric benzaldoximes (p. [203]).
Since in Fig. 59 the α modification has been represented as the stable form, the transformation of the β into the α form will be possible at all temperatures down to the transition point. At temperatures below the eutectic point, transformation will occur without formation of a liquid phase; but at temperatures above the eutectic point liquefaction can take place. This will be more readily understood by drawing a line of constant temperature, HK, at some point between C and B. Then if the β modification is maintained for a sufficiently long time at that temperature, a certain amount of the α modification will be formed; and when the composition of the mixture has reached the point H, fusion will occur. If the temperature is maintained constant, isomeric transformation will continue to take place in the liquid phase until the equilibrium point for that temperature is reached. If this temperature is higher than the natural melting point, the mixture will remain liquid all the time; but if it is below the natural melting point, then the α modification will be deposited when the system reaches the condition represented by the point on the curve AC corresponding to the particular temperature. As isomeric transformation continues, the freezing point of the system will rise until it reaches the natural freezing point D. Similarly, if the α modification is maintained at a temperature above that of the point D, liquefaction will ultimately occur, and the system will again reach the final state represented by D.[[285]]
Examples.—Benzaldoximes. The relationships which have just been discussed from the theoretical point of view will be rendered clearer by a brief description of cases which have been experimentally investigated. The first we shall consider is that of the two isomeric benzaldoximes:[[286]]—
| Benzantialdoxime (α-modification). | Benzsynaldoxime (β-modification). |
Fig. 61 gives a graphic representation of the results obtained.