T
Y
D
P
*C. 195 210 220
20.17 18.74 17.92
1.007 0.989 0.976
686,b 686.8 683!.
1126
ROBERT A. ROBINSON AND DOUGLAS A. PEAK
T
r
D
P
80 95 110 125 140
27.20 25.80 24.32 23.00 21.60
1.101 1.088 1.074 1.059 1.042
859.0 857.6 857.0 856.5 857.0
T
r
D
P
24.02 22.13 20.62
1.084 1.064 1.047
1030 1029 1027
"C.
OC.
100 120 140
Mean P = 1029, whence PA^ = 36.0
Chromium acetylacetonate. Equivalent quantities of chromium sulfate and acetylacetone were mixed in aqueous solution, and the chromium was precipitated by ammonium hydroxide. After forty-eight hours the acetylacetonate crystallized in red needles which were extracted with benzene; the benzene layer was dried and evaporated. The acetylacetonate was then purified by sublimation in vacuo; it melted at 212OC. T
r
D
P
21.24 20.20 18.92
1.072 1.059 1.042
699.5 699.2 698.8
"C.
213 225 240
1127
ALUMINUM ALKYL OXIDES AND THEIR PARACHORS
Aluminum alkyl oxides were prepared by the method of Tistschenko (11) and purified by distillation under reduced pressure. As the whole preparation always distilled over approximately a five-degree range with very small head and tail fractions, no special precautions were taken for fractionation, the material being evidently homogeneous. Parachor determinations were made on a redistilled middle fraction. Obtained in this way the oxides were viscous liquids or white non-crystalline solids with illdefined melting points, often exhibiting marked supercooling, the extreme case being the isopropoxide, m.p. 118"C., which has been supercooled to -20°C. without solidification. Analyses were made by dissolving the oxides in acid, and precipitating and weighing the alumina in the usual manner. Analysis was also attempted by evaporation of the oxide with water and ignition of the resulting alumina, but this method invariably gave high results, e.g., the butoxide gave 11.60 and 11.29 per cent A1 (calculated, 10.95 per cent). The same peculiarity is to be found in the analyses of Gladstone and Tribe (3), although no reference was made to it. The error may be associated with the difficulty of completely dehydrating the very granular alumina produced in this way, although ignition over a blow-pipe yielded no better results, or with the formation of a stable carbon-nitrogen compound of the cyanamide type. Whatever the cause, it is worth while emphasizing the error. No difficulty was encountered in the acid treatment. Aluminum ethozide distilled at 210-214°C. at 13 mm. and melted at 146-151°C. Analysis gave 16.78 and 16.70 per cent of aluminum; the calculated value is 16.63 per cent. Molecular weight in naphthalene: 631. Calculated molecular weight for [A1(OEt)31.1: 648.
I
T "C
7
I
D
I
P
.
150 165 180
13.98 13.11 12.24
0.919 0.904 0.890
Mean value of P . . . . . . . . . , . . . . . . . . . . . . . . , . . . . . , . . . . , . . . , , . .
341 .O 340.9 340.6 340.8
A redetermination on another sample gave mean value of P = 341.5. Aluminum n-propoxide distilled at 271-275°C. at 14 mm. and melted at 106-108°C. Analysis gave 13.30 per cent of aluminum; the calculated value is 13.21 per cent. Molecular weight in naphthalene: 865. Calculated molecular weight for [A1(OPr)3]4:816.
1128
ROBERT A. ROBINSON AND DOUGLAS A. PEAK
T
I
. y
I
I
D
P
OC.
110 130 150
19.18 17.92 16.66
0.957 0.938 0.921
445.9 447,4 447.5
Mean value of P . . .........................................
446.9
Aluminum isopropoxide distilled a t 151-153°C. a t 15 mm. and melted
at 118°C. Analysis gave 13.20 per cent of aluminum. Molecular weight in naphthalene: 775. Calculated molecular weight for [A1(OPr)3]4:816. P "C
.
60 80 100 120
20.28 18.84 17.04 15.68
0.944 0.926 0.904
458.6 459.1 458.3 460.5
0.881
Mean value of P . . .........................................
459.1
Aluminum n-butoxide distilled a t 274-278°C. a t 9 mm. and melted at 102-106°C. Analysis gave 10.98 per cent of aluminum; the calculated value is 10.95 per cent. Molecular weight in naphthalene: 968. Calculated molecular weight for [ A ~ ( O B U ) ~984. ] ~ : Molecular weight by boiling point in benzene: 1050. Molecular weight by boiling point in n-butyl alcohol: 1025. Electrical conductivity in n-butyl alcohol (1 g. in 50 cc.) < 4 x 10-5 mhos. T
I
Y
I
D
I
P
OC.
80 100 120 140
20.48 19.11 17.69 16.33
565.4 565.7 565.2 565.0
0.925 0,909 0.593 0.875
Mean value of P . . . .......................................
I
565.3
Aluminum isobutoxide distilled a t 248-250°C. at 11 mm. and melted a t 208-210°C. It exhibited no supercooling. Analysis gave 11.05 per cent of aluminum. Molecular weight in naphthalene: 968.
.
1129
ALUMINUM ALKYL OXIDES AND THEIR PARACHORS
T
Y
D
P
0.827 0.819
551.7 553.3
'C.
11.84 11.52
209 217
,
Mean value of P ...........................................
1
552.5
Owing to the high melting point and viscosity and the consequent difficulty of making parachor determinations, great reliance cannot be placed on this result. Aluminum sec-butoxide distilled a t 174-176°C. at 5 mm. It did not solidify even after standing for two months. Analysis gave 10.94 per cent of aluminum. Molecular weight by boiling point in benzene: 994.
I
T
I,
Y
D
I
P
'C.
50 70 90 110
23.19 21.49 19.96 18.10
0,937 0.917 0.894 0.868
576.4 577.5 581.7 584.8
Mean value of P .........................................
580.1
Antimony ethoxide was prepared by the interaction of antimony trichloride and sodium ethoxide in absolute alcohol. It distilled smoothly a t 99.5"C. at 13 mm. or 95OC. at 11 mm., without any sign of ebullition, to a colorless liquid. Molecular weight in naphthalene : 302. Calculated molecular weight for Sb(0Et)o: 257. T
Y
D
P
28.36 26.58 24.77 23.16 17.66 16.18 14.53 12.87
1.524 1.490 1.455 1.420 1.328 1.295 1.262 1.229
389.0 391.2 393.6 396.7 396.4 397.4 397.3 395.9
.
"C
17 37 57 77 130 150 170 190
Mean value of P (77-190°C.).
...............................
I
396.7
1130
ROBERT A. ROBINSON AND DOUGLAS A. PEAK DISCUSSION
The unsuitability of the alkyl oxides as a means of determining the parachor of aluminum (vide infra) necessitates the tentative acceptance of the value derived from chelated compounds. This has now been confirmed by measurements on the acetylacetonate and new measurements on the ,ethyl acetoacetate and the diethylmalonate, from which, assigning singlet linkage formulations to these compounds, values of 43.9, 41.4, and 36.0, respectively, are obtained for the parachor of aluminum. The latter value is probably somewhat erroneous because it is derived from a large molecular parachor, but taking the mean of these three values together with the three values of the parachor given by Sugden (9), a figure of 39.5 results for the parachor of aluminum. This value may be checked by taking the parachor of chromium as 53.7 from the data for chromyl chloride (2) and comparing the acetylacetonates of chromium and aluminum which are found to differ in their parachors by TABLE 1 Comparison between observed parachors of alkyl oxides and those calculated for the formula, Al(0R)a OXIDE
Ethoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-Propoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isopropoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-Butoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Isobutoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . sec-Butoxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
l p
P
(OBSERVED)
(CALCULATED)
340.8 446,6 459.1 565.3 552.5 580.1
384.8 501.8 501.8 618.8 618.8 618.8
ANOMALY
-44.0 -55.2 -42.7 -53.5 -66.3 -38.7
16.3 units. It then follows that the parachor of aluminum is 37.4. This fixes the order of the atomic parachor independent of the validity of the singlet linkage formulation of the acetylacetonate. Assuming the value of 39.5 for the parachor of aluminum obtained with the aid of singlet linkage formulations, a comparison can be made between the observed parachors of the alkyl oxides and those calculated for the simple formula, Al(OR)S, as shown in table 1. The hypothesis that the liquids have an angle of contact which would reduce the apparent surface tension can be shown to be an inadequate explanation, since an angle of contact of at least 60" would be necessary to account for the anomalies and such an angle would have been easily detected in the microscope. I n every case the meniscus was normal. Molecular weight determinations from the freezing point of naphthalene solutions and the boiling point of benzene and n-butyl alcohol solutions showed that the molecules are polymerized fourfold in solution, and since
ALUMINUM ALKYL OXIDES A N D THEIR PARACHORS
1131
the parachor was constant over a considerable temperature range (except in the case of the sec-butoxide), it is probable that any polymerization which occurs in the pure liquid is not affected by temperature. Application of the Ranisay-Shields equation to the surface tension data favors the simple formula A1(OR)3, but the validity of this equation may be questioned. It was hoped to obtain conclusive evidence on this point by a vapor density determination under reduced pressure with the apparatus previously described ( 5 ) , but, although excellent results could be obtained with other compounds, some decomposition always occurred with these alkyl oxides, This is remarkable in view of the ease with which the compounds could be distilled without decomposition, but all attempts to vaporize them in the Victor Meyer tube resulted in decomposition, although vaporization was carried out under a variety of conditions. The results, however, are not entirely without significance because in whatever way decomposition occurs, e.g.,
+ 3Etz0 2Al(OEt)s + A1203 + 3H20 + 6C2H4 ZAl(0Et)a 4 A1203
or
the number of molecules produced by decomposition must be larger than the number of alkyl oxide molecules even if these are unpolymerized. Thus decomposition would lom7er the apparent molecular weight. The observed molecular weights were with one exception larger than those calculated from the simple formula, and in the single case in which agreement was obtained it was apparent that considerable decomposition had occurred. Vapor density determinations therefore show that polymerization occurs, but afford no estimate of the degree of polymerization. The balance of evidence is in favor of the formula [Al(OR),](, in which case the compounds are analogous to the thallous alkyl oxides which Sidgwick and Sutton (7) have shown to be tetramolecular in alcohol and benzene solution but which, surprisingly, give a normal value for the parachor of thallium (9). The parachor anomaly of the aluminum alkyl oxides is paralleled to a lesser extent in antimony ethoxide, which presents the phenomenon of an increasing parachor at low temperatures (presumably due to slight polymerization), followed by a constant parachor a t higher temperatures. This constant parachor, which indicates complete depolymerization, falls short of that calculated from the known parachor of antimony by 14 units. Germanium ethoxide also exhibits a deficiency of seven units (6). Methyl and ethyl orthosilicates have parachors increasing with temperature (lo), differences of approximately 5.3 and 6.3 units being observed over a 54°C. range. The boron esters are normal (1). The formula [Al(OR)3]4would bring these compounds into line with the “alkoxo salts” of Meerwein and Bersin (4), if [A1(OR)3]4is written
1132
ROBERT A. ROBINSON AND DOUGLAS A. PEAK
AI [Al(OR)&, but a polar formulation is improbable because the aluminum alkyl oxides, like the “alkexo salts,’’have the non-polar properties associated with covalent links, e.g., they melt and distil at low temperatures and the n-butoxide has a very small conductivity in n-butyl alcohol solution. By analogy with the ring structure assigned by Sidgwick and Sutton (7) to thallous ethoxide,faluminum ethoxide may be written as in formula I,
..
R R O R 0 R:O.AI-O.Al.O:R R:O O:R R : 0 . AI 0 A l e 0 : R 0 R O R R
..
..
I but because of the ring structure in I the parachor anomaly must be raised to 45.8, and a negative anomaly of this magnitude can only be accounted for by four singlet linkages associated with each aluminum atom. A model constructed with the four aluminum bonds arranged tetrahedrally and the A1-0-A1 bonds making an angle of 110” shows that a ring structure of this type is practically strainless, although the eight atoms in the ring do not all lie in the same plane. However, although this formulation may be supported on steric grounds, it seems highly improbable that stability can be attained with only four electrons in the valency shell of each aluminum atom and five unshared electrons to each oxygen atom. A further difficulty is encountered in the fact that larger negative anomalies are found in the higher alkyl oxides of aluminum. This is not peculiar to this series of homologues, Thus the parachor of beryllium propionylacetonate is higher than that of the acetylacetonate by 68.6 (calculated, 78), while the difference between the parachors of acetyl- and propionylacetone is 34.3 (calculated, 39), and that between sulfonal and trional is only 28.3 units. SUMMARY
1. Aluminum alkyl oxides have been prepared from ethyl, n-propyl, isopropyl, n-butyl, isobutyl, and secondary-butyl alcohols; also prepared were the acetylacetonates of chromium and aluminum, alumino ethyl acetoacetate, alumino diethylmalonate, and antimony ethoxide. 2. Molecular weight determinations indicate a fourfold polymerization of the alkyl oxides of aluminum, which is supported qualitatively by vapor density measurements.
ALUMINUM ALKYL OXIDES AND THEIR PARACHORS
1133
3. Antimony ethoxide with a normal molecular weight exhibits a parachor deficiency of 14 units, deficiencies also being found in the ethoxides of silicon and germanium. The parachor deficiency found in the aluminum alkyl oxides is of a larger magnitude and varies with the nature of the alkyl group. The only reasonable formulation of these compounds which will account for these deficiencies by means of singlet linkages consists of an eight-membered ring with all the oxygen atoms attached to the aluminum atoms by singlet linkages. In this formulation the electron octets of all the atoms are filled with the exception of those of the aluminum atoms, which only possess four electrons each. We wish to acknowledge the valuable assistance rendered by Mr. W. S. Rapson, M.Sc., in the preparation of many of the compounds used in this investigation. REFERENCES (1) ETRIDQE AND SUQDEN: J. Chem. SOC.1928, 989. (2) FREIMAN AND SUQDEN:J. Chem. SOC. 1928, 263. (3) GLADSTONE AND TRIBE:J. Chem. SOC.39, 1 (1881). (4) MEERWEIN AND BERSIN:Ann. 476, 113 (1929). (5) PEAKAND ROBINSON:J. Phys. Chem. 38, 941 (1934). (6) SIDQWICK AND LAUBENQAYER: J. Am. Chem. SOC.64, 948 (1932). (7) SIDQWICK AND SUTTON: J. Chem. 800.1930, 1461. (8) SUQDEN:J. Chem. SOC. 126, 1167 (1924). (9) SKJQDEN: J. Chem. SOC.1929, 316. (10) SUQDEN AND WILKINS:J. Chem. SOC.1931, 126. (11) TISTSCHENKO: J. Russ. Phys. Chem. SOC.31, 694, 784 (1899). (12) WISLICENUS AND KAKJFMANN: Ber. 28, 1323 (1895).
