T H E SYSTEM ANILINE-FORMIC ACID-WATER JAMES R. POUND AND ALLAN M . WILSON The School of Mines, Ballarat, Victoria, Australia Received September 6, 103.4
A study of mixtures of aniline, formic acid, and water was made by the methods commonly used for heterogeneous systems in equilibrium, and the results were summarized in a triangular graph in the usual manner. The region in which the components separated into two liquids was mapped out, and the distribution of the formic acid between the two liquid layers was found. The region in which crystals were deposited was also mapped out, CH202. This and the crystals were proved to be aniline formate, C ~ H T N work was done a t 15°C. However, these liquid mixtures and also the solid crystals were not in equilibrium; they changed continuously, producing formanilide and water. Thus crystals first formed in a given system of the components redissolved; and liquid mixtures at one time homogeneous, in time separated into two layers. Moreover the crystals of aniline formate were unstable in two directions, dissociating into aniline and formic acid on the one hand and giving formanilide and water on the other. Goldschmidt and Wachs (3) studied the rate of formation of anilides, proving it to be in general bimolecular, while Davis and Rixon (2) studied water, in a soluthe equilibrium, formic acid + aniline e formanilide tion of aqueous pyridine a t lOO"C., but they do not mention aniline formate. This equilibrium was confirmed in the course of the present work. Also a study of some properties (density, etc.) of certain mixtures of these three components was made by Pound and Russell (5). The aniline was purified by distillation. The formic acid-mater solutions, containing from 5 to 87 per cent formic acid, were made from the best commercial acids, 40 per cent and 87 per cent; from the latter acid, by treatment with the calculated amount of phosphorus pentoxide and by subsequent distillation i n vacuo, there was obtained the 99 per cent acid, (see ref. 4). The foimic acid content of these solutions and of the mixtures was found by titration with standard baryta water, using phenolphthalein; with this process the presence of aniline or of formanilide did not interfere. Such direct titrations of the mixtures gave the free formic acid and the acid present as aniline formate, the latter evidently dissociating in the process; such formic acid will be referred to as "free acid." By boiling solutions
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+
709
710
JAMES R. POUND AND ALLAN M. WILSON
containing formanilide with an excess of caustic soda and then cooling and titrating with acid, the “total acid” was obtained, i.e., formic acid free and as aniline formate and as formanilide. The difference, “total acid” - (‘free acid,” or ‘‘fixed acid,” gave the formic acid fixed as formanilide. “Fixed acid” was also obtained in the solution after the neutralization of the “free acid” by boiling with excess caustic soda, etc., and this procedure gave the same results as the former. Aniline was determined in the mixtures by titrating the warm acidified solutions containing potassium bromide with potassium bromate solution, using starch-potassium iodide as outside indicator; the bromate solution was standardized against pure aniline. This method gave the total aniline in the mixture, i.e., aniline free and as formate and as formanilide. TABLE 1 FORMIC ACID:USED
Aniline
Formic acid
Water
per cent
per cent
per c a t
per cent
5.164 9.73 12.91 16.17 20.29 22.79
13.55 21.7 34.65 50.1 67.05 78.7 95.09 3.61
4.45 7.6 8.45 8.1 6.7 4.85
82.0 70.7 56.9 41.8 26.25 16.45 4.91 96.39
-
The percentages (weight) of aniline, formic acid, and water will be referred t o as per cent aniline, per cent formic acid, and per cent water. Per cent formic acid without qualification means per cent of “free acid” (as defined above). I. DETERMINATION OF THE UNOBTAINABLE LIQUID MIXTURES, I.E., OF THl3 LINE RSSC
This was got by the direct titration of the following formic acid solutions by aniline. The critical mixture, shown by characteristic opalescence, could be found to within a drop of aniline, the total amount of which varied from 5 to 40 cc. (see table 1). The critical mixtures of aniline and water were taken from Applebey and Davies (l),and from other workers (see ref. 6). The above critical mixtures are marked 0 in the graph. 11. THE DISTRIBUTION OF THE FORMIC ACID
The distribution of the formic acid between the aniline and the aqueous layers gave the following results:
THE SYSTEM ANILINE-FORMIC
7 11
ACID-WATER
per cent f o r m i c per cent f o r m i c acid in acid in a n i l i n e layer aqueous layer
From 5.164 per cent formic a c i d . , ...................... From 9.73 per cent formic acid.. ...................... From 20.29 per cent formic acid., ......................
0.65 1.45 3.75
4.0 5.75 7.1
The three resulting tie lines are drawn in the graph.
DPG The curve DPG, bounding the region in which crystals separated, was determined by three methods: a. Mixtures were made up from known weights of aniline and of formic acid solutions, and the resulting crystals and solutions were agitated in a thermostat a t 15°C. for 3 hour or more, and then separated and analyzed for “free acid” and aniline. The crystals were needle-shaped and long, especially if formed slowly or on the sides of the vessel above the main 111. DETERMINATION O F THE CURVE
TABLE 2
II
COMPOSITION OF SOLUTION
COMPOSITION OF CRYSTALS
I
Aniline
Formic acid
Water
Aniline
Formic acid
per cent
per cent
per cent
per rent
per cent
per cent
Water
79.6 64.8 51.9 38.5 36.8 35.1 38.1
7.2 11.3 12.5 18.4 24.0 33.1 41.1
13.2 23.9 35.6 43.1 39.2 31.8 20.8
68.7 66.9 63.2 64.0 61.1 60.6 62.7
29.5 30.4 29.9 31.8 31.4 32.7 33.6
1.8 2.7 6.9 4.2 7.5 6.7 3.7
mixture. Rapid mixing gave small and entangled crystals and a semisolid mass which disentangled on agitation. On long agitation squat and compact crystals sometimes took the place of the needles. Both these kinds of crystals were aniline formate (see later). On further agitation the crystals in some experiments disappeared altogether. The method of residues was used thus to give the points on the curve DPG and the tie lines leading to the point H , representing aniline formate. The results were only approximate: long agitation meant that much formanilide was formed, which of course vitiated the result, and short agitation meant that the solution and crystals had not perhaps reached equilibrium a t 15°C. Mixtures of aniline with the more concentrated formic acids were the most affected by the former complication, but all of the results were somewhat so affected. Hence especially the percentage of free acid in the solutions was lower than it should have been. A few typical results were those given in table 2. These points are marked 0 in the graph.
712
J A M E S R. POUND AND ALLAN M. WILSON
b. Various formic acid solutions were titrated by aniline, a t 15"C., until crystals permanently remained in the mixtures. This method gave points along the curve PG. These crystals were squat. The points obtained in this manner probably tend to lie below the true solubility curve, i.e., crystals appeared momentarily as the aniline was run in and subsequently did not dissolve within a short time. When the 99 per cent formic acid was thus titrated with aniline, crystals remained when the mixture contained 34.4 per cent aniline,-obviously low. By taking aniline formate and finding the amount of 99 per cent acid which would just dissolve it, the point G was found a t 37 per cent aniline,-a value which is still probably low. Thus 10 cc. (11.87 g.) of 76.08 per cent formic acid required 6.86 cc. ANILINE
FORM~CACID
WATER
FIG.1. THESYSTEM ANILINE-FORMIC ACID-WATER
(7.07 g.) of aniline, and the mixture then contained 37.3 per cent of aniline.
It was found that for crystals to persist in the mixtures the following percentages of aniline were required: 48.9 per 61.44 per 76.08 per 87.23 per
cent cent cent cent
formic acid solution formic acid solution formic acid solution formic ,acid solution
required required required required
33.2 per 35.9 per 37.3 per 38.2 per
cent cent cent cent
aniline aniline aniline aniline
These points are marked X in the graph. c. Aniline was titrated with various formic acid solutions a t 15°C. until crystals persisted in the mixtures. Thus points along the curve D P were obtained. The crystals formed were generally needle-shaped. Thus 28.7 g. of aniline take 1.00 g. of 48.9 per cent formic acid solution, and the per cent of aniline in the mixture is thus 96.6. The mean results indicate that for crystals to persist in the mixtures the following percentages of aniline were required :
THE SYSTEM ANILINE-FORMIC
30.5 per 35.67 per 43.8 per 48.9 per 76.08 per
cent cent cent cent cent
formic formic formic formic formic
acid acid acid acid acid
713
ACID-WATER
solution required solution required solution required solution required solution required
54.2 per 65.1 per 86.9 per 9F.1 per 97.9 per
cent cent cent cent cent
aniline aniline aniline aniline aniline
These points too are marked X in the graph. IV. CHANGES I N THE LIQUID MIXTURES ON KEEPING
Mixtures containing appreciable formic acid and water showed a pink color on addition of the aniline; this increased in intensity for a few minutes and then decreased after ten minutes or more. Mixtures of the three components were kept for many days a t room temperatures, about 10°C., some in stoppered flasks in desiccators, others in sealed tubes; they were analyzed from time to time. In all of them the “free formic acid” plus the “fixed formic acid” equalled the original “free formic acid.” The ‘(free acid” decreased roughly in accordance with the bimolecular action, i.e., the formic acid fixed as formanilide in a given time was roughly proportional to the product of the aniline and formic acid percentages (concentrations). Moreover the speed of this action rapidly increased as the water-content decreased; a lowering of the water by about 14 per cent trebled the speed of action, other conditions being equal. Also a state of equilibrium was obtained after ten days or so, if the solution remained homogeneous; no satisfactory equilibrium constant was obtained however. A few typical results are given in table 3. The formation of formanilide may be taken as equivalent to adding more aniline to the mixtures. Thus a mixture originally containing p per cent aniline, q per cent formic acid, and r per cent water, when the per cent of free formic acid falls to (q - x), becomes ( p - 2.02 x) per cent aniline, (q - x) per cent formic acid, (2.63 x) per cent anilide, and (T 0.39 x) per cent water, which behaves as ( p 0.61 x) per cent aniline, (q - x) per cent formic acid, and ( r 0.39 x) per cent water, or the mixture moves to states along lines parallel to PS and QS,i.e., parallel to the line from the formic acid corner to the (61 per cent aniline, 39 per cent water) mixture, FL. Thus mixture P, No. 27, on the eighth day when it had just separated, contained only 8.6 per cent (‘freeacid,” and thus the “fixed acid” was 10.6 per cent, which was equivalent to 21.4 per cent of fixed aniline, as 27.9 per cent of anilide, and to 4.15 per cent of extra water; and the mixture then contained 8.6 per cent of (‘free acid,” 15.8 per cent of free aniline (the dissociable aniline formate being included in these figures), with 27.9 per cent of anilide and 47.7 per cent of water; this mixture behaved as one with 8.6 per cent formic acid, 43.7 per cent aniline, and 47.7 per cent water, and therefore separated into two layers.
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+
+
714
JAMES R. POUND AND ALLAN M. WILSON
TABLE 3 MIXTURE NO.
3
6
I I
FORMIC ACID OBTAINED AFTER TIME GIVEN
Aniline
Formic acid
Time
Formic acid
per cent
per cent
daw
per cent
30.9
39.15
35 61 71
19.95
25.65
36.35
Water
40 68
+
24
26.75
9.15
25.3 25.4
2 4 9 27 34 *42 63 74
35.26 29.93 27.2 24.8 24.21 24.17 24.18 24.18 24.28 24.27
4.69
2 8 16 26
4.20 4.13 4.02 3.92 9.96 8.64 8.16 Separated 12.9 9.44 Separated
H
10
31 .O
30.9
35.82
25
19.8
10.36
2 8 11 14
26
28.0
14.6
2 8 9
QS 27
37.2
31
78.65
32
39.7
19.2
5.35
52.6
2 8
14,25 8.58, separated PS
2
4.11, separated
i
46.5 36.9 34.4 34.1 34.12
2 9 17 30
THE SYSTEM ANILINE-FORMIC
715
ACID-WATER
TABLE 3-Concluded FORMIC ACID OBTAINFlD AFTER TIME
MIXTURE
GIVEN
NO.
35
Aniline
Formic acid
Time
Formic acid
per cent
per cent
daya
per cent
49.0
2 9 17 30
44.8 41.5 41 .O 40 .O
i
68.5 65.5 63.1 62.7 62.66
20.3
30.7
(
4.
39
18.9
70.7
11 16 25
Undoubtedly this hypothesis was in general accordance with the facts observed. Thus mixtures like Nos. 3,6,10,32,35, and 39 came to equilibrium and never separated into layers; mixtures near the line BSSC, like Nos. 31, 25, 26, 27, soon separated, and a t points S as calculated above. Mixtures within the area DPG formed crystals, which dissolved in time, and these mixtures then separated in a similar manner to the former ones. Mixture No. 24 shows the influence of water, and mixture No. 31 that of aniline, on the rate of change, on the equilibrium, and on the final separation into two layers. V. THE CRYSTALS, ANILINE FORMATE, AND THEIR CHANGES ON KEEPING
The crystals of aniline formate were not readily soluble in benzene, toluene, carbon tetrachloride, or petrol; they readily dissolved in water, methyl and ethyl alcohols, acetone, ethyl acetate, chloroform containing 2 per cent alcohol (B.P. chloroform), and in ether. They recrystallized as the ether evaporated. The crystals when kept beneath petrol soon became brown, and simple washing with petrol made them brownish. The white crystals remained unchanged beneath toluene for a few days, but then changed rapidly and soon a brown liquid, aniline or formanilide, was left. When kept in the air or dried in a current of air, the crystals acquired a pinkish or yellowish or brownish tinge. Many samples of these crystals were made and were washed with one or other of the first four solvents, dried in desiccators over sulfuric acid or calcium chloride, and examined from time to time. Two white samples of the acicular crystals, analyzed a t once, both gave 33.2 per cent formic acid and 66.1 per cent aniline, thus per cent of moisture by difference was 0.7; another sample gave 33.8 per cent formic acid, but smelt of free formic acid. Theoretically aniline formate contains 33.1 per cent formic acid and 66.9
716
JAMES R. POUND AND ALLAN M. WILSON
per cent aniline. When dry crystals of the aniline formate were kept over sulfuric acid, the smell of formic acid could be detected after some hours and aniline could be detected in the sulfuric acid after some days. The dissociation into aniline and formic acid is therefore appreciable at room tempera tures. The crystals were usually in long needles, but (see above) occasionally there were obtained more compact forms (squat monoclinic prisms) ; these had the same composition as the needle-shaped crystals. Microscopic examination of freshly formed samples of the crystals indicated the presence of some compact crystals along with the main mass of acicular crystals. These last apparently changed the more quickly (surface effect?), leaving
Ill 1 1
3
2
4
[ 5
r: 1:
/ ; 6
7
FIG.2. CRYSTALS OF ANILINE FORMATE
the compact forms. A second crop of crystals formed from a mixture of formic acid and aniline also tended to consist of compact forms. The aut,hors owe thanks to Mr. H. Yates, who examined and sketched these crystals and who supplied the following notes on the accompanying sketches (figure 2) : “In the graph the figures 1,2, and 3 are various shapes of acicular crystals, showing three types of crystal-face, of which two are developed in 1, and all three in 2, while in 3 the vertical faces are fully developed but there is only one of each type of inclined face. Crystal 1 was actually seen to grow into form 2 by developing of the third type of face. The angle between the two faces in 1 is 52’. “Figures 4 and 5 represent tabular crystals lying on the clino-pinacoid plane, From these shapes the interfacial angles are: ac = 57’20’; ad = 59’; cd = 63’40’.
THE SYSTEM ANILINE-FORMIC
’
ACID-WATER
717
“Figure 6 is an actual sketch of a typical prismatic crystal under the microscope. “Figure 7 is the same crystal correctly oriented, showing its monoclinic symmetry and the positions of the crystallographic axes.” The melting point of the crystals was 62°C. (maximum value), with average values about 60°C. The values varied as might be expected; also a remelting was always a t a lower temperature than the original. The crystals on keeping gradually became sticky; then very small crystals above or apart from the main lot became yellowish and liquefied; in a sealed vessel after several days a film of liquid (droplets) could be seen between the crystals on the walls of the vessel. Then the stickiness increased and the sample became yellowish or brownish, and finally free liquid (solution) accumulated, and in a couple of days more the crystals had disappeared. These changes took in all about seventeen days in a closed vessel at room temperatures (about 10°C.). The petrol-washed samples changed very quickly. One such sample after nine days over calcium chloride had formed a yellowish liquid, and after twenty-four days a brown liquid; after sixty days new crystals had formed. The liquid was drained from these crystals, which were then washed and recrystallized from xylene; they gave a melting point of 43”C., indicating that they were (impure) formanilide (map. = 46°C.). The crystals kept in contact with the mother liquor changed the fastest of all; they took but two or three days to disappear. Many samples of crystals were kept in sealed vessels or over various drying agents and other reagents, but the changing to liquid was never stopped, though it might have been retarded by the absence of moisture. The free formic acid content diminished, and the fixed formic acid content increased continuously. In general, the (free plus fixed) formic acid in old crystals equalled the original free acid; in some samples there may have been some adsorption of water, and in other samples some preferential loss of formic acid over loss of aniline; but the loss of acid by volatilization was insignificant as compared with loss of acid by change into formanilide. Thus the crystals A (see table 4) gave a final liquid containing 7.3 per cent of free acid and 25.1 per cent of fixed acid; total = 32.4 per cent, original = 32.2 per cent. The crystals 0 gave on the fourth day 32.7 per cent of free acid and 1.0 per cent of fixed acid; total = 33.7 per cent, original = 33.8 per cent. The crystals L, from the third experiment reported in section IIIa, gave a final liquid containing 4.7 per cent of free acid and 26.9 per cent of fixed acid; total = 31.6 per cent, original = 29.9 per cent. However they were put in a desiccator overnight before sealing. After nine days crystals N were nearly all liquefied, and the sample went 16.2 per cent of free acid, 8.4 per cent of fixed acid,-indicating that much water had been picked up; the outer water contained both aniline and
’
718
JAMES R . POUND AND ALLAN M. WILSON
formic acid. Allowing for the water picked up, the rate of fixation of acid as formanilide was found to be the same as for the first six days. The free acid content of some samples of crystals kept a t room temperatures varied as shown in table 4. TABLE 4 DRIGINAL CRYBTALS WASHED WITH
/[
PIR CENT 'ORMI( ACID
AFTER DAYS
FINAL STATE
:HANGE I N PER ClNT FORMIC ACID
PER DAY
I n sealed tube Over CaClz
32.2 32.2
12 12
7 . 3 Liquid 24.1 Quite sticky
2.08 0.68
In sealed tube Over H z S 0 4
34.6 34.6
6 14
3.8 1.8
Over PZOS
34.6
14
11.7 Liquid 9 . 3 Two-thirds liquid 15.2 One-third liquid
I n sealed tube Over H$Od Over KOH
32.5 32.5 32.5
13 13 13
7 . 9 Liquid 28.8 Sticky 27.3 Sticky
1.9 0.28 0.40
0
I n air
33.8
6
0.22
N
Over water
33.8
6
M
33.8
6
Q
Over NaOH solution Over Gus04
33.8
6
R
Over HzS04
33.8
7
33.8
6
33.8
9
32.5 Slightly sticky yellowish 26.2 Very moist; white 26.4 Quite moist; white 29.6 Sticky; brownish 32.2 Slightly sticky white - Slightly sticky white 31.2 Slightly sticky white
CC14, toluene, and dry air.. . . ,
'IF
CC14and dry a i r . .
Toluene, CCh, and dry air.. . .
CONDITION OF KEEPING
PER CENT FORMI' ACID
1.4
1.25 1.2
0.7 0.23
-
P
OverrPzOb
K
I n sealed tube
33.0
15
7.2
Liquid
1.7
(Only drained from liquor). . . . L
I n sealed tube
29.9
24
4.7
Liquid
1.05
Toluene. . . . . . . . .
.
0.29
The samples A, E, F, K, and L had just liquefied completely in the time shown, This liquefaction took, on the average, seventeen days, and then the per cent of free acid had fallen to the average value of 6.2 (seven samples). The original condition of the crystals evidently had an influence
THE SYSTEM ANILINE-FORMIC
ACID-WATER
719
on their subsequent rate of change. Possibly the presence of excess of formic acid in the crystals E, C, and D accelerated their changing. Also the crystals passed to liquid and lost more free acid in sealed vessels than in the open air. The average loss in per cent of free acid, excluding samples C, D, and E, in sealed samples was 1.6 per cent per day (seven samples) ; in samples over sulfuric acid, 0.22 per cent per day (five samples) ; and in samples over calcium chloride, 0.66 per cent per day (three samples). The drier the atmosphere about the crystals the slower seemed to be the rate of change of free acid, but even with excess of phosphorus pentoxide, as in sample P, the characteristic changing to formanilide still went on. The crystals, apart from the consequences of this changing, did not appear to be deliquescent. I n conclusion, the crystals of aniline formate change according to both the reactions: Aniline
+ Formic acid (1)
Aniline formate 5 Formanilide ' (2)
+ Water
Probably change 2 is accelerated by water and by formic acid (see ref. 2); this change indeed may only occur in solution. Moreover as the products of the dissociation 1 may both provide such a solvent and are also both hygroscopic, the difficulty of stabilising the compound, aniline formate, will be-realized. SUMMARY
1. The authors have studied the behavior of mixtures of aniline, formic acid, and water a t room temperatures. 2. The region of partially miscible mixtures has been mapped out. 3. The distribution of formic acid between the aniline and the aqueous layers has been found. 4. The region in which crystals of aniline formate separated has been mapped; no other solid phase was found. 5. The change of the liquid mixtures into formanilide was studied, 6. The change of the solid crystals of aniline formate into formanilide was studied. REFERENCES (1) APPLEBEYAND DAVIES:J. Chem. SOC.127, 1842 (1925). (2) DAVISAND RIXON:J. Chem. SOC.107, 728 (1915). (3) GOLDSCHMIDT AND WACHS: Z. physik. Chem. 24, 353 (1897). (4) JONES, D. C.: J. SOC.Chem. Ind. 38, 362 (1919). (5) POUND AND RUSSELL: J. Chem. SOC. 126, 769 (1924). (6) SEIDELL:Solubilities of Inorganic and Organic Substances. D. Van Nostrand Co., New York (1917).
