264 ILLINOIS STATE ACADEMY OF SCIENCE 



ed. In the 4th column of this table are given the latent 

 heats of fusion as calculated from the ideal slopes of the 

 log N vs. 1/T curves. Finally, in column 6 are given the 

 quotients obtained by dividing the calculated molecular 

 latent heat of fusion expressed in small calories by the 

 absolute melting point of the substance. This, according 

 to Walden,^ should equal about 13.5. It will be observed 

 from table 2 that the constant, 13.5, applies very well to 

 most halides and nitro compounds and to many other 

 isolated compounds, especially to those substances which 

 have relative high molecular weights and moderately 

 high melting points. In general, it may be said that the 

 Walden constant for the more highly polar substances 

 such as the hydroxide, ketones, amines, etc., has a value 

 somewhat smaller than the normal. In. any case, it 

 should be jDossible, by reference to table 2, to calculate 

 an approximate latent heat of fusion and hence the ideal 

 slope for any substance whose structure and melting 

 point are known. Presuming the structure to be known, 

 it is then possible to locate approximately the position 

 of the substance in the chart, Fig. 1. With this informa- 

 tion it is then possible to calculate the solubility of the 

 substance in question in all of the substances shown in 

 the table which do not form molecular complexes or solid 

 solutions. Thus it is seen that a fair idea may be had of 

 the solubility relations of any given substance from only 

 the melting point and structure of the solute. 



The application of the principles outlined above should 

 aid materially in the choice of solvent for crystallizations 

 and also in the choice of solvent for use in molecular 

 weight determinations. 



TABLE 2. 

 The Latent Heat of Fusion of Organic Compounds. 



HYDROCAEBONS. 



Ideal 



M. Pt. Abs. Slope Si 4.58 L calc. 



Substance (Tm) (Si) (Lcalc.) Lobs. Tm 



Benzole 278.5 510 2330 2350 8.36 



Para xylole 287.4 840 3840 4170 13.4 



Diphenyl methane 299.3 810 3700 12.4 



Diphenyl 343.2 960 4390 4390 12.8 



Naphthalene 353.1 970 4440 4550 12.6 



Triphenyl methane 366.0 970 4440 12.6 



