STUDIES ON PHOSPHORIC ACID* IV. Oxonium Salts of Ortho Phosphoric Acid with Certain Organic Compounds BY G. BROOKS KISG' ASD JAMES H. WALTOS
The purpose of this investigation was to study the formation of oxonium salts of phosphoric acid with various organic compounds, further, to make a comparison of phosphoric and sulfuric acids with regard to their ability to form addition compounds with organic substances. The present work was suggested by an investigation in this laboratory by Kalton and Kepfer? in which the solubility of several organic acids in solutions of ortho phosphoric acid was studied. Using solutions of phosphoric acid of various concentrations they found indications of compound formation w-ith the following organic acids: oxalic, succinnic, malic, citric and iso-valeric. The solubility of phenol was also determined and a compound indicated. X o attempt was made to determine the composition of these compounds. It was of interest to investigate the formation of t'hese substances by means of a phase rule study of the freezing point curves of mixtures of the pure components. ,4phase rule study of phosphoric acid with other compounds is mentioned only a few times in the literature and only one case was found where an organic compound was used as one of the components. Rabinow and Jakubsohn3 determined the freezing point curves of mixtures of phosphoric acid with ethyl ether and found that two compounds were formed; (C2H5)20. 6H3P04and (C?HB)2O. 4H3P04. However, numerous compounds of phosphoric acid and organic compounds have been isolated by X l a g e ~AllendorfJ5 ,~ Raikow6 and others. Kendall and Carpenter' studied the addition compounds of sulfuric acid with organic substances by means of freezing point curves, with a view of getting some insight into the probable mechanism of various sulfonation reactions. Kendall concluded that the greater the difference in acidic strength between two substances, the greater the tendency for addition compound formation. Kendall regards such addition compounds as true oxonium salts, of the following type: *Contribution from the Laboratory of General Chemistry of the University of Wisconsin. The material presented here is a portion of that to be used by G. B. King in his dissertation to be presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy at the University of Wisconsin. 2 J. Phys. Chem., 34, 543 (1930). Z. anorg. Chem., 129,5 5 (1923). Ber., 35, 2313-231 j (1902). 6 Ber., 31, 1298-1301 (1898). 8 Chem. Ztg., 24, 367 (1900); 25, 1134 (1901). ' J. Am. Chem. SOC., 36, 2498 (1914).
G . BROOKS KING A N D JAMES H. WALTON
R-COOH
+ HX = R-C=O I OH
In the present work a similar study has been made with ortho phosphoric acid and organic substances.
Experimental Pure ortho phosphoric acid was prepared and dried according to the Ross and Jones* method, modified by Walton and Keber.9 The acid so prepared gave a melting point of 42.5’ and 42.6’, taking as the melting point the temperature a t which the last crystal disappeared when the temperature was slowly raised. By supercooling some of the fused acid to 40’ and then seeding, the acid crystallized and the temperat,ure rose to 42.3’. As this method was used in taking freezing points the above value, 42.3’, was taken as the freezing point of pure ortho phosphoric acid and this value was used in plotting the curves. &lost of the organic compounds used in this investigation were those prepared by Eastman, and in only a few cases was any special purification necessary. The method of experimentation in the present work is essentially that used by Krndall and his collaborators in their study of freezing point curves. However a freezing point was taken rather than a melting point, as the latter proved unsatisfactory in this work due to the very viscous nature of the phosphoric acid. A Beckmann freezing point apparatus was employed consisting of a I ” X 8” hard glass test tube fitted with a rubber stopper, thermometer, stirrer, and a tube for the passage of dry air. A tube with a side arm was employed when the material to be added was a solid. This freezing tube was fitted into a larger test tube which served as a jacket. Phosphoric acid was weighed out as rapidly as possible in a stoppered weighing bottle and introduced into the freezing tube. Dry air was passed over the mixture in the freezing tube a t all times except in the case of benzaldehyde when dry hydrogen was used. The organic substance, if a solid, was introduced into the tube in the form of pills; if a liquid it was weighed out in a weighing pipette and introduced directly into the top of the freezing tube. The solution after being thoroughly stirred was cooled slowly until the solid phase began to separate; in most cases it was necessary, however, to seed the solution. As the solid phase began t o separate the temperature rose slightly and the highest temperature was taken as the freezing point of that particular mixture. An approximate freezing point was first run in each case SO as to regulate the bath to an appropriate temperature. Freezing points taken by this method must necessarily be slightly low because of supercooling. Because supercooling was rery marked experimental difficulties were very great in the examination of these systems especially near the eutectic on 8 Q
J. Am. Chem. Soc., 47, Unpublished results.
2165
(1925).
I747
STUDIES ON PHOSPHORIC ACID
the phosphoric acid side of the system; moreover the mixture would sometimes solidify to a jelly-like mass with no definite point of fusion and it was necessary to remelt and repeat the operation until crystallization took place. This latter difficulty was encountered when supercooling was most marked. In some systems where the freezing point of the mixture was low the phosphoric acid became very viscous and stirring produced minute air bubbles which rendered the solution turbid, making it impossible to detect the formation of minute crystals. I n such cases the solution was seeded, cooled, and the lag in temperature, which was always followed by a slight rise, carefully noted. This gave a means of obtaining freezing points in the region of the eutectic with a fair degree of accuracy, As the percentage of phosphoric acid was decreased the freezing point was more definite after the eutectic was passed and the supercooling less marked. Points in the region of the maxima on all curves were very definite but the points in the region of the eutectic are not accurate to more than a degree. I n some cases the solutions became so deeply colored that an examination was impossible. If a compound with a congruent melting point is formed in a two component system, a maximum appears in the curve, and from this maximum the melting point and composition of the compound can be obtained. Tabulation of Results I. ACIDS. I. Acetic acid: the compound CH3COOH H 3 P 0 4was obtained; stable at its maximum; melting point 33.8". The compound is white and crystallizes very readily. Some of the crystals were separated from the mixture and gave a melting point slightly lower than the one recorded above. See Fig. I, Curve 11. Solid phase H3P04 ScHSP04 roo 90.8 8 5 . 9 84.9
F.T.*
42.3 2 7 . 0 'Freezing temperature.
14.8
14.0
Solid phase CH3COOH. H3PO4 % H H ~ P O 8~0 . 0
F. T. YiH3P04
F.T.
18.9 63.6 33.8
77.4 21.7 57.4 31.6
74.8 25.6 56.7 32.7
73.1 27.9 51.0 30.3
72.j
29.6 42.6 23.0
67.j 32.7 38.0 14.5
64.4 32.7
2. Propionic acid: no compound was indicated in this system in the region studied. The runs were not carried below -3'. Solid phase H,PO, % HaPo1 I O O 90.6 8 4 . 5 8 2 . 4 78.1 74.6 73.0 7 0 . 4 F. T. 42.3 33.2 26.3 20.3 1 2 . 7 6.2 2.0 -z.(*I')
3. n-Butyric aczd: no compound was indicated in the region studied.
7c H3P01
F.T.
100
42.3
87.7 34.2
Solid phase H3P01 77.8 70.0 25.8 16.j
G . BROOKS KING AND J.4MES H. WALTON
I748
On adding more butyric acid the mixture froze around oo solidifying to a jelly without any definite point of fusion. The runs were discontinued at this point. 4. n-Caproic acid: no compound was found in this system. A mixture of the two components was light brown in color. Solid phase % H3PO4 9 3 . 8 8 8 . 6 8 0 . 0 7 2 . j 6 6 . 4 6 1 . 5 5 6 . 7 53.9
F. T.
40.9
39.9
38.2
35.8
32.8
30.4
23(*1')
24.0
5 . Benzoic acid: no compound was obtained from this system. After the addition of about fifteen per cent benzoic acid, separation into two layers took place and the temperature of fusion remained constant. Also the acid sublimed so readily that any degree of accuracy was impossible. Solid phase C6HsCOOH %, &PO4 99.1 98.1 97.1 96.3 95.4 94.3 92.6 90.0
F. T. %"SPO(
F. T.
73.0 84.9 115.5
44.0
87.2 113.8
84.5 82.6 115.8
92.7 20.5 116.4
98.0 17.2
116.5
101.3 106.0 15.4 6.0 116.9 1 1 9 . 0
112.0 0.0
122.3
6. Pyruvic acid: the compound CH3COCOOH was obtained; stable a t its maximum; melting point 36.4'. Fig. I , Curve 111. Solid phase & P o ( % Hap01 100 87.6 82.9 77.1
F. T.
42.3
28.0
21.5
16.6
Solid phase CH3COCOOH. H3PO4 %"3P04
F. T. %HaPo4
F. T.
70.4 69.1 24.( +1')26.4 52.9 45.7 35.8 35.2
62.7 33.6 38.9 33.7
61.7 34.4 30.0 30.6
54.3 36.4 24.3 26.5
7. Several other acids were used as one component with phosphoric acid. Succinnic acid decomposes slowly when heated with phosphoric acid as is evidenced by a slow effervescence. Tartaric acid deepens to a dark brown color with phosphoric acid. Cinnamic acid reacts vigorously with decomposition. 0-Toluic acid is insoluble in phosphoric acid. Oxalic acid decomposes rapidly when heated with phosphoric acid. Xalic acid is also decomposed readily. 8. Monochloro-acetic acid: no compound was indicated in this system. Fig. I , Curve IV. Solid phase H 3 P 0 4 ?( H3PO4 IOO 96.0 86.5 76.9
F. T.
42.3
38.8
32.1
26.6
Solid phase CH2ClCOOH
7~HaPo4 F. T. % H3P04 F. T.
66.0 38.4 43.4 50.8
61.5 41.1 43.2 50.6
60.0 43.4 35.3 52.2
58.2
44.3 31.6 54.2
54.0 46.6 26.0
55.0
51.6 47.4
46.4 49.7
STUDIES ON PHOSPHORIC ACID
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9. Phenylacefic acid: no compound with phosphoric acid was indicated in a phase rule study of this system. Solid phase CeHjCHzCOOH % H3POd 87.0 79.0 68.1 64.9 60.0 53.0 44.7 27.8 F. T. 45.1 50.7 j6.2 58.1 60.0 62.0 64.2 68.3 $7~Hd'01 17,3 3.6 0.0
E'. T.
70.2
72.0
76.7
I. Benzaldehyde and H,POd. Subtract j o from temp. scale. 11. Acetic acid and H3POa. Subtract 35 from temp. scale. 111. Pyruvic acid and H3PO+ Subtract 2 0 from temp. scale. IF'. Monochloracetic acid and HaPo,. Add 20 to temp. scale. V. Guaiacol and HaPOa. Add 5 to temperature scale.
11. ALDEHYDES. I. Benzaldehyde: the compound C ~ H E C H O . H3P04 was obtained; stable a t its maximum. Melting point 43.0'. The solution of the two components was a light orange in color. Dry hydrogen was passed through the freezing tube a t all times to prevent oxidation of the aldehyde. Crystallization was brought about with some difficulty, it being necessary to first freeze the mixture and then preserve a crystal in the upper part of the tube for later crystallization. The freezing point of a mixture which was allowed to stand over
G. BROOKS KING A N D JAMES H. W A L T O N
I750
several days vaned from that of the original, indicating a probable condensation. RaikowG reports the isolation of this compound. Fig. I, Curve I.
F.T.
42.3
Solid phase H 3 P 0 4 86.0 83.8 76.; i6.0 34.2 32.0 24.9 23.7
7~ H3P04
71.3 24.0 44.7 42.0
Solid phase CsHdCHO* &Po4 66.2 65.4 62.j 5 5 . 5 31.4 33.6 36.0 4 0 . 7 40.8 35.0 30.7 40.2 36.3 34.1
YcH3PO,
F.T. c/c H3PO4 F. T.
IOO
54.9 42.6
48.0 47.4 43.0 42.2
2. Furjural which had been freshly distilled, when added to phosphoric acid turned a very dark red, almost black color, rendering an examination of this system impossible.
3. Anisaldehyde: the compound CH30C6H4CH0 was obtained, stable a t its maximum; melting point 83.6". The mixture of the two components gave a red colored solution. Solid phase H 3 P 0 4 92.3 90.5 84.7 F. T. 42.3 36.7 34.5 27.5 Solid phase CH30C6H4CH0. H3P04 yCH 3 P 0 4 8 0 . 0 7 7 . 0 76.0 7 4 . 0 72.9 68.0 66. 26( +I") 40.3 47.6 5 z ( +I") 52.6 58( A Z O ) 63( *IO) F.T. 46.1 43.6 40.0 GH3P04 59.2 56.1 j2.0 49.5 ~I(*I') 74.8 80.j 81.7 83.4 83.4 83.6 F.T. 17.6 % H 3 P 0 4 33.0 32.5 30.0 2 2 . 0 F.T. 81.2 81.5 80.9 76.9 73.7 %HaPo4
IO0
39.0 83.2
111. KETOSES. I. Benzalacetone: an examination of this system was impossible as the two components gave a solution almost black in color. 2. Acetophenone: the compound C6H5COCH3 HJ'O, was obtained; stable a t its maximum; melting point 87.9'. Considerable difficulty was encountered on the phosphoric acid side, due to the fact that the solution became a deep green in color and it mas difficult to obtain an accurate freezing point. The portion of the curve on the phosphoric acid side before the eutectic was reached is a t best only approximate. I
