THE AUTOXIDATION OF STANNOUS CHLORIDE. I V THEEFFECT OF SOME NON-AQUEOUS SOLVENTS~ ROBERT C. HARING AND JAMES H. WALTON Department of Chemistry, University of Wisconsin, Madison, W i s c o n s i n Received J u l y 28, 1933
Previous work by the authors (4) has shown that the autoxidation of stannous chloride is a thermal and photochemical chain reaction, showing peroxide formation, induced oxidation of other molecular species present in the solution, and great sensitivity to positive and negative catalysts. The complex HSnC13 or H2SnC14was described as playing an important rdle in the oxidation of the stannous chloride. Because of the interference of hydrolysis in the study of the reaction in aqueous solution, it was desired to determine the effect of acid concentration in some organic solvent. After many preliminary experiments, some of which are reported in paper I1 of this series (4), benzyl alcohol was finally selected as a suitable solvent. AUTOXIDATION O F STANNOUS CHLORIDE I N BENZYL ALCOHOL
Benzyl alcohol was purified by vacuum distillation. The product boiled at 98-99°C. (uncorrected) at 17 mm. pressure, a value consistent with those recorded in the literature. Five different batches were prepared and found to give reproducible results. Dry hydrogen chloride was passed into the alcohol, and the solution was used immediately in order to prevent any invalidation of the results by the formation of benzyl chloride through esterification. The autoxidation in benzyl alcohol was characterized by a short period of rapid absorption of oxygen, followed by a period of slow oxygen consumption, progressing toward complete oxidation. The initial period lasted from 5 to 8 minutes, and in order to express all data on a comparable basis, an arbitrary period of 10 minutes was taken, and the data expressed as the number of milliatoms of oxygen consumed in that time. This value, plotted against the normality of the hydrochloric acid, gives a straight line nearly to the point where the hydrochloric acid is equivalent to the stannous chloride, and then approaches a constant value. This effect was 1 This research was financed by a grant from the Research Committee of the University of Wisconsin, Dean C. S. Slichter, Chairman.
1-53
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ROBERT C. HARING AND JAMES H. WALTON
studied a t two concentrations of stannous chloride, and the data are given in table 1 and figure 1. The effect of varying the concentration of the stannous chloride at a constant hydrochloric acid concentration was also studied a t one acid concentration. These data are presented in table 2 and figure 2. I n both studies it will be noticed that the amount of oxygen consumed in the 10-minute period approaches a constant value as the variable concentration increases. The data for the variable stannous chloride concentration show this in the more pronounced way, since complete oxidation was not approached in the 10-minute absorption period in these runs. TABLE 1 Effect of concentration o j hydrochloric acid o n the autoxidation o j stannous chloride in benzgl alcohol (A) BTANNOUS CHLORIDE CONCENTRATION =
1.018
MILLIMOLES I N
26
CC
Concentration of hydrochloric acid
Oxygen used in 10 minutes
N
VL'dliUtOm8
None 0.016 0.020 0.034 0.041 0.049 0.059 0.063 0.071 0.082 0.095 0.101 0.221 0.226 0.233 0.368
0.063 0.235 0.296 0.463 0.590 0.700 0.713 0.787 0.924 1.02 1.13 1.20 1.18 1.20 1.23 1.17
I/
I/
( B ) S T A N N O U S CHLORIDE CONCENTRATION = 2.038 MILLIMOLES I N 25 cc.
Concentration of hydrochloric acid
Oxygen used in 10 minutes
N
miztiatom8
0.037 0.074 0.114 0.127 0.159 0.189 0.236 0.263 0.329
0.596 1.11 1.64 1.86 2.09 2.21 2.32 2.37 2.40
It was found that induced oxidation of the benzyl alcohol occurs. At a stannous chloride concentration of 1.019 millimoles in 25 cc. of benzyl alcohol, and hydrochloric acid concentration varying from 0.10 N t o 0.35 N , 1.21 milliatoms of oxygen were used, on the average. Since one milliatom of oxygen is equivalent t o one millimole of stannous chloride, this was an excess of 18.6 per cent. Two runs with twice the amount of stannous chloride and an acid concentration of 0.25 N to 0.35 N gave practically the same value, 20.9 per cent. The fact that the curves of figures 1 and 2 approach constant values,
155
AUTOXIDATION OF STANNOUS CHLORIDE. IV
Normality of hydrochloric acid
FIG. 1. THE EFFECTOF CONCENTRATION OF HYDROCHLORIC ACID ON
THE
AUTOXIDATION OF STANNOUS CHLORIDE IN BENZYL ALCOHOL A, stannous chloride concentration = 1.019 millimoles in 25 cc.; B, stannoue chloride concentration = 2.038 millimoles in 25 cc.
TABLE 2 Eflect of concentration of the stannous chloride o n i t s autoxidation in benzyl alcohol Hydrochloric acid concentration = 0.078 N STANNOUS CHLORIDE CONCENTRATION
1
OXYGEN USED IN 10 MINUTES
mil2imoles i n dG cc.
MiZZiatoms
0 0.509 1.019
0 0.589 0.960*
1.528 2.038 3.057 4.076
1.18 1.18* 1.28
1.28
* Interpolated from figure 1. shows again the presence of the equilibrium between stannous chloride and hydrochloric acid, and shows that these complexes are the principal forms
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ROBERT C. HARING AND JAMES H. WALTON
in which the stannous chloride is oxidized in benzyl alcohol. As the concentration of either of the constituents of the complex is increased, holding the other constant, the concentration of the complex, and thus the rate of oxidation, increases toward a constant value.
FIQ. 2. THE EFFECTOF CONCENTRATION OF STANNOUS CHLORIDE ON AUTOXIDATIONIN BENZYLALCOHOL Hydrochloric acid concentration = 0.078 N
ITS
AUTOXIDATION OF STANNOUS CHLORIDE I N DIOXAN
A large amount of time was spent on the autoxidation of stannous chloride in dioxan (1,Pdioxane), since this solvent was expected to be very resistant to oxidation or reduction. The dioxan was purified by refluxing for 7 to 12 hours with one-tenth its volume of 1N hydrochloric acid to hydrolyze the acetal of ethylene glycol, which is always found as an impurity. Oxygen was passed through the ether during the hydrolysis to oxidize the aldehyde produced. The product was dried with solid potassium hydroxide, fractionated to remove low boiling material and the ethylene glycol, and finally was refluxed over molten sodium until no further reaction occurred, and the globules of sodium appeared bright (12 to 36 hours). The pure dioxan was then dis-
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AUTOXIDATION O F STANNOUS CHLORIDE. IV
tilled from the sodium (boiling point, 99.7-99.9"C., uncorrected, a t 740 mm.; freezing point, 11.80' i 0.0lOC.). This product is apparently of a higher degree of purity than that previously reported in the literature
0,s).
1.
Stannous chloride was found to form a molecular compound with dioxan in the ratio 1 :1. The salt was recrystallized from dioxan of high purity, and the product analyzed. The percentages of chlorine and tin were found to be 25.40 and 43.12, as compared with 25.54 and 42.75 calculated for a 1:1 ratio of stannous chloride to dioxan. This recrystallized molecular compound was used to determine the extent of complex formation between stannous chloride and hydrochloric acid in dioxan solution. This mas determined by the method of freezing point lowering, for which the constant in dioxan is 5.0 ( 5 ) . The freezing point lowering of solutions of stannous chloride and of hydrochloric acid in dioxan were determined, equal volumes of the two solutions mixed, and the freezing point lowering again determined. TABLE 3 Complex formation between stannous chloride and hydrochloric acid in dioxan BTANNOU8 CHLORIDE CONCENTRATION
HYDROCHLORIC ACID CONCENTRATION
(Before mixing)
M
0.0315 0.0315
I
1
~
M
0,1828 0.3656
1
FREEZING POINT L O W E R I N G
Obsd.
0.491" 0.901
1
1
Calcd.
( N o comdex)
0.536" 0.993
1
I
Calcd. IHSnCId
0,457' 0.914
I
1
I
Calcd. (H~snC14)
0.378' 0.835
Complex formation is definitely shown in the data of table 3, which gives the experimental lowering compared with the theoretical lowering calculated (a) with no complex formation, (b) with all the stannous chloride present as HSnC13, and (c) with all the stannous chloride present as H2SnC14. High accuracy is not claimed for these results because of the extreme difficulty of preventing oxidation of the stannous chloride in acid concentrations as high as these. These determinations were made under nitrogen, and the solutions kept under nitrogen during as much of the handling as was possible, but a filtration was necessary, and a small amount of oxidation is very probable. The autoxidation of stannous chloride in dioxan was found to resemble the oxidation in benzyl alcohol. There is a period of rapid absorption followed by a slow period in some runs. The oxidation is faster than in benzyl alcohol however, the initial rapid period being complete in 14 minutes. In the purest samples, freshly distilled, the subsequent slow absorp-
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ROBERT C. HARING AND JAMES
H.
WALTON
tion was not present, but these samples of dioxan developed the slow absorption period after standing for several days, even under pure nitrogen. This effect is not due to water, since known amounts of water added to runs in freshly distilled dioxan do not produce the slow absorption period, Water does, however, increase the amount taken up in 13 minutes of rapid absorption. If oxygen is bubbled through an old sample of dioxan which shows the subsequent absorption period, just before its use in an autoxidation run, the subsequent absorption period is destroyed, and the amount of oxygen consumed in the initial period greatly reduced. The results of the autoxidation could not be reproduced very well from one preparation of dioxan to the next, so that the results concerning the effect of hydrochloric acid concentration have no quantitative value, but about twenty-five runs in freshly distilled dioxan, out of more than a hundred, show a qualitative trend similar to that found with benzyl alcohol, that is, some oxygen is taken up even with no hydrochloric acid present; the amount of oxygen used up increases with the acid concentration, and approaches a constant value a t high acid concentrations. Induced oxidation was also found, 1.01 millimoles of stannous chloride consuming 1.52 (average of ten runs) milliatoms of oxygen, an excess of 50.0 per cent. Water added to the stannous chloride-hydrochloric acid-dioxan system was found to increase the amount of oxygen absorbed during the first 16 minutes of the run. The amount of this increase was variable but ranged from 25 to 75 per cent for an addition of 0.10-0.30 g. of water. Some preliminary experiments were also made in the following solvents: diamyl ether, dibenzyl ether, p ,P'-dichlorodiethyl ether, nitrobenzene, diphenyl ether, and diethyl malonic ester. These all proved to be unsatisfactory from the standpoint of solubility, or reaction. THE ACTION OF POSITIVE AND NEGATIVE CATALYSTS I N NONAQUEOUS SOLVENTS
I n order t o correlate the reaction in organic solvents with the reaction in water, a few experiments were made with some positive and negative catalysts. Picric acid has been found to be a good inhibitor in aqueous solution, and it proved t o be one in non-aqueous solvents as well. It was used, qualitatively, to inhibit the reaction in the following solvents : benzyl alcohol, p , p'-dichlorodiethyl ether, and glacial acetic acid. Its inhibitory power can best be shown by the acetic acid experiments. I n this solvent one millimole of stannous chloride is oxidized in 2 minutes, whereas with 0.02 g. of picric acid added, less than 0.005 milliatom of oxygen is taken up in 20 minutes. m-Dinitrobenzene, p-aminophenol, and p-nitrotoluene were also found to be good inhibitors in acetic acid solution. All these
AUTOXIDATION OF STANNOUS CHLORIDE. I V
159
inhibitors seem to be more effective in acetic acid than they were in aqueous solution. Thiourea, which is a strong accelerator in aqueous solution, also catalyzes in benzyl alcohol, and in p , B'-dichlorodiethyl ether, but 0.01 M accelerates the reaction only about 150 per cent as compared with about 2000 per cent in water. However, this may in part be due to the low solubility of thiourea in these solvents. Tetraethyllead was found to accelerate the reaction in benzyl alcohol as it did in water, but willow charcoal had the anomalous effect of showing no catalytic effect in benzyl alcohol. DISCUSSION
In this paper further evidence has been presented to show that the chloro acid complexes are important in the mechanism of the autoxidation of stannous chloride. The approaching of a constant value for the rate of autoxidation with increasing concentration of either of the constituents of the complex indicates this, and it is further proven by the determination of complex formation in dioxan solution. The close analogy between the effect of hydrochloric acid on the reaction in water and organic solvents, and the identical action of inhibitors and accelerators show that the mechanism of the reaction is not affected appreciably by the solvents. The summation of the work presented in this series of papers leads the authors to believe that the autoxidation of stannous chloride may be represented by the series of reactions designating the typical autoxidation, viz., if A and A represent autoxidant molecules (in this case HSnC4 or H2SnC14)and * represents energy of vibrational activation (from thermal or photochemical sources), then
A02
+ A -+ 2 A 0
(5)
The symbol _A is used to show the transfer of two atoms of oxygen instead of an energy transfer. This mechanism was proposed by Bodenstein (3), and amplified by Backstrom and Beatty (2). I represents the inhibitor molecule, and the IO* molecule has a lower specific reaction rate with A than has the activated peroxide AOz". Thus the widely differing powers of various inhibitors in a reaction are due to their differing abilities to react with A and perpetuate the reaction chain. This also accounts for the ability of one substance to be an inhibitor for one reaction and an accelerator for another, since the effect the substance has will depend on the reac-
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ROBERT C. HARlNG AND JAMES H. WALTON
tive ability of its oxide (or peroxide) retutive to that of the autoxidant. In this general scheme, the reaction chain is reaction 1 followed by an alternation between reactions 2 and 3, while reaction 4 is the chain-breaking mechanism. Reaction 5 occurs subsequent to the chain, and may, under the proper conditions, be slowed sufficiently to permit the detection, and even the isolation, of the peroxide, as has been reported by many investigators in widely differing reactions. SUMMARY
1. Very pure dioxan (f.p. = 11.80 =tO.Ol°C.) has been prepared, and complex formation shown between stannous chloride and hydrochloric acid by means of freezing point lowering in this solvent. 2. A molecular compound of one mole of stannous chloride with one mole of dioxan has been identified by analysis. 3. The rate of autoxidation of stannous chloride in dioxan and in benzyl alcohol has been studied. I t was found to increase nearly linearly with the acid concentration up to a point where the acid concentration is close to that of the stannous chloride, and then to approach a constant value. 4. The postulation of the chloro acid complex as the form in which the stannous chloride is oxidized was confirmed by the above experiments, and by those in which the rate also approached a constant value with increasing stannous chloride concentration, a t a constant acid concent;ation. 5. Induced oxidation of dioxan and benzyl alcohol was demonstrated during the autoxidation of stannous chloride in those solvents. 6 . Several positive and negative catalysts were shown to have qualitatively the same effect in non-aqueous solvents as they have in aqueous solution. REFERENCES (1) ANSCHUTZ, L., AND BROEKER, W.: Ber. 69B,2844 (1926). (2) BACKSTROM, H. L. J., AND BEATTY, H. A. : J. Phys. Chem. 36,2530 (1931). (3) BODENSTEIN, M.: Z. physik. Chem. 12B,151 (1931). (4) HARINO, R. C., AND WALTON,J. H.: J. Phys. Chem. 37,133,375 (1933). ( 5 ) HERZ,W., AND LORENTZ, E.: Z. physik. Chem. 140A,406 (1929). (6) REID,E. W., AXD HOFFMAN, H. E.: Ind. Eng. Chem. 21, 695 (1929).
