T H E MOLECULAR ASSOCIATION OF FURFURAL BY FREDERICK H. GETMAN
Both the physical and the chemical properties of furfural are such as to suggest that the molecules of the substance are more or less associated. A search of the literature, however, has failed to reveal the existence of trustworthy data upon which to base any calculations of the degree to which the molecules of furfural, either per se or in solution, have undergone association. In order to secure the necessary data for such calculations the following investigation was undertaken. A number of determinations were made of the lowering of the freezing point of water and benzene produced by the addition of varying amounts of furfural to each of these two solvents. The ordinary Beckmann apparatus was used, the thermometer having recently been calibrated by the C. S. Bureau of Standards. At least three readings, agreeing to within O . O O I ~ , were secured for each solution; the usual thermometric corrections were applied and also precautions were taken to prevent too great supercooling. Great care was exercised to maintain the temperature of the freezing bath only slightly below the freezing temperature of the solution under investigation. The freezing point of the pure solvent was determined either immediately before, or immediately after, each series of measurements, and throughout the entire investigation the thermometer was maintained a t a temperature not far removed from the freezing temperature of the pure solvent. As a result of the latter precaution, the zero of the thermometer showed a maximum variation of only o.o03O, and that was observed in the early stages of the work, probably before the instrument was thoroughly “seasoned”. The furfural employed was purified in the manner described in a previous paper’, while the benzene was prepared from a sample, known to be free from thiophene, by crystallization and subsequent distillation of the molten crystals over freshly cut sodium. The aqueous solutions were prepared with so-called “conductivity water”. Each solution was made up by direct weighing and its concentration expressed in terms of mols per 1000grams of solvent. Biltz2having emphasized the importance of determining the value of molecular weights at infinite dilution by means of graphic extrapolation, where the osmotic laws are strictly valid, we have followed the procedure herewith outlined. After having determined the freezing point depressions produced by such concentrations of furfural as could be accurately measured by means of the thermometer, the values of the various depressions were plotted against the corresponding concentrations, and a smooth curve was drawn through the points thus obtained. This curve obviously passes through the origin. From this curve, the values of the freezing point depressions, a t rounded J. Phys. Chem. 28, Z I Z (1924) Bilta: “Practical Methods for determining Molecular Weights,” p. I 19.
396
FREDERICK H . GETMAN
concentrations, were read off and the corresponding values of the molecular depression, A t / N , were calculated. I n addition to determining the freezing point depressions produced by freshly distilled furfural, it seemed of interest t o make a similar series of determinations with furfural which had been exposed to the action of sunlight for about three months after distillation and, in consequence, had acquired a dark brown color. The experimental results are recorded in the following tables (I to IV) in which N denotes the concentration in mols per 1000 grams of solvent and A t the corrected depression of the freezing point.
TABLE I Furfural (freshly distilled) in Water
x
At/N
N
I .87
0.3
1.89 1-91 I .89
0.4
0.I
0.0187 0.056 0.095 0.189
0.2
0.370
I .85
0.01
0.03
0.05
At
0.5
0.6
At
0.546 0.703 0.845 0.982
At/N
.82 1.76 I .69 1.64 I
TABLEI1 Furfural (freshly distilled) in Benzene N
At
At/N
N
0.01
0.052
5.20
0.3
0.02
5.20
0.4
0.05
0.104 0.264 0.530
5.28 5.30
0.5
0.I 0.2
1.034
5.17
0.6 0.7
At
1.497 1.932 2.340 2.724 3.IO1
At/N
4.99 4.83 4.68 4.54 4.43
TABLE 111 Furfural (colored) in Water N
At
0.02
0.019 0.039
0.05
0.IO1
0.1
0.198 0.380
0.01
0.2
At/N
N
I .go 1.95 2 .oo 1.98 I .90
0.3
0.4 0.5
0.6
At
0,542 0.690 0.828 0.962
At/N I .81
1.73 I.6j I .60
TABLE IV 'Furfural (colored) in Benzene N
At
At/N
0.01
0.052
5.20
0.02
0.105
0.05 0.I
0.267 0.536
0.2
1.075
5.25 5.34 5.36 5.38
N 0.3
1 ' 533
5.11
0.4
I .926
0.5 0.6
2.310
0.7
3.070
4.82 4.62 4.50 4.39
At
2.700
At/N
397
MOLECULAR ASSOCIATIOX O F FURFURAL
It will be observed that the molecular depression of the freezing point recorded ineach of the foregoing tables, increases to a maximumvalue and then steadily decreases as the concentration of the solutions increases. If furfural were perfectly normal in its behavior, however, the value of the molecular lowering of the freezing point should be I -86 in aqueous solutions and 5 . I 2 in benzene solutions. It follows, therefore, that only in extremely dilute solutions of water and benzene does furfural have an approximately normal molecular weight. The abnormal values of At,”, observed in the more concentrated solutions, are probably due to the tendency of furfural to undergo both solvation and association. In the more dilute solutions the tendency of the molecules of solute to become solvated predominates and, in consequence, the values of A t / N are greater than the normal molecular lowering. As the concentration increases, the tendency of the molecules of solute to become associated increases to such an extent that ultimately the effect of association completely masks the effect of solvation and the values of A t / N become less than the normal value. Hence, when the two processes of solvation and association are coexistent, it is evident that the ratio of the calculated to the normal molecular weight of the solute does not afford an accurate measure of its degree of association in solution. Nevertheless, we may regard the ratio of the normal value of A t / K to its abnormal value in any one of the more concentrated solutions ns affording a rough approximation to the value of x, the factor of association of the solute. Thus, if we calculate the values of this ratio in 0.5 N solutions we obtain the results given in Table V.
TABLE V In Water z for freshly distilled
C5H402
z for colored C5H402
1.11 I.
14
In Benzene 1.11 I.
13
It will be observed that the degree of association, in solution, of the freshly distilled substance is slightly less than that of the sample of furfural which had been exposed to the action of light. A similar difference will be noted in the behavior of these two solutes in the more dilute solutions where the influence of solvation is apparent. I n Tables I to I V it will be noted that freshly distilled furfural has undergone solvation to a smaller extent than has the colored modification. As to the molecular complexity of furfural per se, there is very little available data upon which to base any calculations of the factor of association. From Trouton’s well-known relation, Ml,/T = k, where M denotes the molecular weight, l, the heat of vaporization1 and T the absolute boiling point of the substance, we find the following value for the constant, k
k=
9 6 . 0 3 X 107.91 = 23.8 161.7-t-273
The value of 1” here used was determined by Prof. J. H. Mathews who very kindly supplied the author with his data on furfural.
3 98
FREDERICK H. GETMAN
The mean value of k for normal liquids being 20.7, we conclude that furfural is slightly dissociated. From the modified form of the Trouton relation proposed by Kernst and Binghaml, Ml,/T = 1 7 0.011T,
+
in which the symbols have the same significance as in the Trouton relation, we find R/Il,/T = 23.8 while 1 7 O.OIIT = 21.8. The difference between the two sides of the equation being assumed to afford an approximate measure of the degree of association, x, it will be seen that the value of this factor is 2 . Again we may make use of Longinescu’s relation2,
+
n =
(L)2 IOO d
in which n is the number of atoms per molecule of liquid, d its density and T its absolute boiling point. Substituting in this equation we have
The formula, CbH402, shows that a molecule of furfural contains 1 1 atoms, hence the degree of association is I -3. While the foregoing calculations cannot be regarded as more than rough approximations, the results obtained by the three different methods agree in indicating that the molecules of furfural are more or less associated in the liquid state. The results of this investigation may be briefly summarized as follows: ( I ) The depression of the freezing points of water and benzene produced by the addition of varying amounts of furfural have been studied. It has been found that only in extremely dilute solutions does furfural have a normal molecular weight. The abnormalities in the values of the molecular depression of the freezing point are ascribed to the tendency of the solute to undergo both solvation and association, the former predominating in dilute solutions and the latter in concentrated solutions. ( 2 ) A similar series of measurements was carried out with a sample of furfural which had been exposed to the action of sunlight for about three months after distillation. While the results were similar to those obtained with the freshly distilled liquid the latter was found to be slightly less associated than that which had been exposed to the action of light. (3) The approximate degree of association of pure liquid furfural has been calculated by three different formulae and found to be slightly greater than unity. Hillside Laboratory, Stamford, Conn. Nernst and Bingham: J. Am. Chem. SOC., 28, 730 (1906). Longinescu: J. Chim. phys. 1 , 296, 391 (1903).
