SOLUBILITY RELATIONS O F ISOMERIC ORGANIC COMPOUNDS VI. SOLUBILITY O F T H E NITROANILINES I N VARIOUS LIQUIDS BY ARMAND R. COLLETT AND JOHN JOHNSTOX"
This investigation of the solubility of the three nitroanilines in a series of liquids was undertaken with two main objects in view. The first was to discover, if possible, whether some particular solvent would be specially advantageous in the separation of a mixture of such isomers, or in the purification by crystallization of any one of them. The second was to learn more as to the solubility relations of such closely related substances as the ortho, meta and para isomers CsH4XY. It was hoped moreover that any conclusions which could be drawn from the behavior of one series could be applied-at least as a first approximation-to other series which are not too dissimilar. The substances chosen were the nitroanilines, which are readily obtainable in a pure state and are convenient to work with. Although these compounds are important dye intermediates, little reliable information on their solubility is available; for, with the exception of the work of Bogojavlenski et a1.l on their solubility in benzene, nitrobenzene, and dibromoacetylene, and the more recent work of Sidgwick and Rubie2inbenzene and in water, only questionable data a t 20' and 25' are available. The solvents investigated were selected as typical substances in common use as solvents.
Materials I n purifying the nitroanilines by recrystallization we found the process to be more effective with two solvents used separately than with a single solvent; it is questionable however, as we shall see, in how far the use of a second solvent is in general beneficial in the separation of one isomer from the other. Ortho- Nitroaniline was crystallized from 9 57', alcohol and then repeatedly from hot water, from which it came out in fine orange-colored needles. The melting temperature as determined from the flat on the time-temperature curveS is 69.3', although by the ordinary capillary tube method it appears to be 72'. Meta- Xitroaniline was crystallized twice from alcohol and then from water; this yielded lemon-yellow blade-like crystals, the melting temperature of which is 111.8'. Para- K i t r o a m h e , crystallized several times from water, yielded, on solution in hot alcohol, a reddish residue; so it was recrystallized from alcohol and subsequently from water. The final product consisted of light yellow needles the melting temperature of which is 147.5'. *From the dissertation presented t o the Graduate School of Yale University, in June, 1923, in candidacy for the degree of Doctor of Philosophy. Bogojavlenski, Bogoliubov and Vinogradov: through Chem. Abs. 1, 2875 (1907). 2 Sidgwick and Rubie: J. Chem. SOC.,119, 1013 (1921). 8 Cf. Kohman: J. Phys. Chem., 29, 1048 (1925).
SOLUBILITIES O F ISOMERIC ORGANIC COMPOUNDS
71
Water: all the distilled water used was boiled to expel dissolved air. Alcohol: 9 jqb alcohol was refluxed with lime, distilled, dried with sodium, and redistilled twice; the major fraction boiled a t 78.4' and 762 mm. Benzene : Pre-war thiophene-free benzene was dried over sodium and distilled; redistillation yielded a major fraction boiling at 80.3-80.4' and 762 mm.' Chloroform (C. P.) was dried for several days over calcium chloride and then distilled; it boiled at 61.2-61.4' and 768 mm. Carbon tetrachloride (C. P.) was treated similarly; it boiled a t 76.5-76.55' and 765 mm. Ether (U. S . P.) was washed several times with sodium carbonate solution, subsequently with water until the aqueous layer proved to be neutral. After being dried for a week over calcium chloride, it was distilled twice; the product boiled a t 34.5-34.8' and 760 mm. Ethyl acetate (U. S . P.) was treated similarly; it boiled a t 77.0-77.3' and 760 mm. Acetone (C. P.) was dried over calcium chloride and twice distilled; the fraction used boiled at 56.1-56.2' and 756 mm. Every effort was made to prevent moisture from reaching the solvents; the distjllation apparatus was thoroughly dried, and the solvents were preserved in flasks stoppered with a calcium chloride tube.
Method Most of our determinations have been made by a slight modification of the synthetic method which has been used by a number of previous investigators2; a few were made by analysis of the solution saturated at a fixed temperature, as a check on the accuracy of the synthetic method. The essence of this method is to ascertain the temperature at which the last crystal just disappears, when a known mixture of solute and solvent is slowly heated. The chief objections to this procedure, as usually carried out, are that the observed saturation temperature may be in error because the rate of change of temperature is not slow enough to insure the attainment of equilibrium, of composition or of temperature, and that the composition of the solution may change owing to volatilization of the components. The procedure finally adopted, after a large number of preliminary experiments (not detailed here) is substantially free from these objections, and yields results which compare favorably with those obtained by analysis of a solution in contact with crystals of a definite temperature; it is as follows. Into a small bulb, of 2-4 cc. capacity, blown on a tube about 6 mm. diameter, was introduced a known weight of the substance; the tube was drawn down t o capillary size at a distance of 3-4 cm. from the bulb, an appropriate quantity of solvent introduced, the tube sealed off and 'Richards and Barry give 89.2' a t 760 mm: J. Am. Chem. SOC.,37, 996 (1915). ZAlexeiev: Ann. Physik, 28, 327 (1886); Schroder: Z. physik. Chem., 11, 449.(1893); Bogojavlenski, Bogoliubov and Vinogradov: through Chem. Abs. 1, 2874 (1907) ; Sidgmick et al: J. Chem. SOC.,107, 1202 (1915); 119, 979, 1001, 1013 (1921); 121, 1844, 1853, 2256, 2263, 2536 (1922); 123, 2813, 2819, (1923); Kendall et al: J. Am. Chem. SOC.43, 979 (1921); 45, 958 (1923); Carrick: J. Phys. Chem., 25, 648 (1921).
72
ARMAND R. COLLETT AND JOHN JOHNSTON
weighed so as to secure the weight of the solvent. The 'cubes were not completely filled, a space, usually about I cc., being left to permit of stirring, which was accomplished by attaching the tube horizontally to a heavy wire which had been soldered to the (vertical) hammer of an ordinary electric bell; this proved to be an effective method of agitation, and was used throughout the series of observations. The tube was now surrounded by a three-liter beaker containing either water (up to 80') or paraffin (beyond 80") which was kept thoroughly stirred by a stream of air; and a calibrated Ihermometer, graduated in tenths, was immersed SO that its bulb was close to the tube, a second thermometer being used to enable the proper stem correction to be made. The flame was adjusted so thac the rate of heating of the bath was about 0.1"per minute, and the temperature noted at which the last crystal disappeared; the bath was now allowed to cool slowly until about half the solute had crystallized, whereupon it was again raised in temperature and maintiained for half an hour a t about I' below that first recorded. Finally the bath temperature was rajhed at about 0.05' per minute, and the temperature at which the last crystal now disappeared was taken to correspond to saturation for the particular mixture weighed into the tube. Determinations made in this way check within about 0.2'; whereas, if one may judge from the data, the corresponding divergence in many of the previous results obtained by this general mode of experiment, has been I' or even more. A correction (which in all cases was small) was made for the change in composition caused by the presence of some of the solvent in the vapor space; it was computed from the vapor pressure of the pure solvent, and its mol fraction in the solution on the basis that Raoult's law holds. A similar calculation showed that the corresponding correction for the nitroanilines is negligible. In the analytical determinations, solvent with excess of solute was placed in glass-st
