1414
H. S. UCHIDA,H. B. KREIDER, A. MURCHISON AND J. F. MASI
Vol. 63
KINETICS OF THE GAS PHASE DISPROPORTIONATION OF DIMETHOXYBORANE BY H. S. UCHIDA, H. B. KREIDER, A. MIJRCHISON AND J. F. MASI Callery Chemical Company, Callery, Pennsylvania Received January 69, 1959
A study of the rate of disproportionation of dimethoxyborane in the gas phase has been made by the manometric method. The reactipn 3(CH30)2BH e 2(CH30)3B 1/2B2He is found to be heterogeneous, and in a thoroughly cleaned and dried liter s herical Pyrex glass bulb, at low pressures and moderate temperatures, the rate is pro ortional to the square of the 1.69 X lo-" partiaypressure of dimethoxyborane. The rate constants at 40, GO and 80' are, respective&, 3.91 X and 4.97 X mrn.-l mh-1. From these values the activation energy is calculated to be 14.2 kcal. per mole of dimethoxyborane. The effect of a number of different adsorbing surfaces on the rate of disproportionation is considered. On the basis of the experimental results a most probable mechanism is discussed.
+
Introduction Previous work on the rate of disproportionation of dimethoxyborane in the gas phase was done by Burg and Schlesinger.' Although the reaction was not investigated extensively by them, they did present some helpful experimental results. 1. The stoichiometry for the disproportionation of dimethoxyborane to diborane and methyl borate is G(CHsO)*BH
B2He
+ 4(CHaO)gB
Analysis of Sample.-The disproportionated dimethoxyborane was passed through a -131' trap. The methyl borate and dimethoxyborane were fractionated out, and the diborane was expanded into a known volume and its molar quantity obtained by the perfect gas law. The methyl borate-dimethoxyborane mixture was hydrolyzed with an excess of methanol and the hydrogen evolved was collected with a Toepler pump in a known volume. The hydrogen formed is equivalent to the amount of dimethoxyborane on a one to one basis according to the stoichiometry of the hydrolysis. (CHa0)2BH
+ CHiOH --+ (CHa0)sB +
H2
2. The rate of disproportionation is pressure sensitive. 3. The white product, which is formed from the reaction of diborane and methyl alcohol and is analyzed to be a polymer of monomethoxyborane, increases the rate of disproportionation. The objects of this investigation were to determine the rate constants a t different temperatures, the mechanism of the reaction and means of accelerating the disproportionation.
Results and Discussion The disproportionation of dimethoxyborane was found to be heterogeneous. This was suspected a t an early stage, for reproducible results were obtained only when considerable care was taken in cleaning the reaction bulb. I n Fig. 1 the partial pressures of dimethoxyborane both in a clean glass bulb and in a bulb in which the surface area was increased five times by the addition of glass tubes are plotted against time. The curves clearly show Experimental the surface dependence of the reaction. SubseDimethoxyborane.-The dimethovyhorane was prepared quent measurements were confined to the use of a by the reaction of diborane and methyl alcohol and was purified in a low temperature Podbielniak column. Active one-liter spherical Pyrex glass bulb.
hydrogen analysis revealed the purity of the compound to be greater than 98%. The vapor pressure of pure dimethoxyborane at 0" is 275 mm. and was used as an additional check on purity. Dimethoxyborane does not disproportionate during this measurement if it is made within a half hour's time. Apparatus and Procedure.-For the static system, the apparatus consisted of a one-liter spherical Pyrex glass bulb with a mercury seal-off, which also served as a manometer. The sample was introduced as a vapor via a vacuum line. , was through the seal-off tube and into the liter I ~ i l b which immersed in a constant-temperature bath. When the desired initial pressure was obtained, the mercury was admitted into the seal-off and the pressure changes were followed by a cathetometer. Since the reaction was found to be heterogeneous the reaction bulb was thoroughly cleaned and degassed before starting each run. The bulb was washed before each run with cleaning solution, then distilled water and finally acetone. I t waa then pumped to dryness and heated with an air-gas flame under high vacuum. Whcn the effectof charcoal was studied, a weighed amount of charcoal was laced in the bulb and then degassed for an extended period ortime. The reactor for the flow system consisted of a 22-mm. Pyrex glass tube, 12 inches long, containing a thermometer and 12.7 g. of charcoal. The flow was regulated by a stopcock and a differential manometer containing mineral oil. Temperature was regulated by means of heating tape. The sample was introduced as a vapor and its pressure waa maintained at approximately 275 mm. by evaporating it from a bulb immersed in an ice-bath.
(1) A. Burg a n d H. Schlesinger, J . A m . Chem. Soc., 66, 4020 (1933).
DATA FOR A
TABLE I TYPICAL DISPROPORTIONATION
Temperature 40" ; initial pressure 390.55 mm. Partial
Time, min.
0 5 GO 120 220 460 585 705 820 950 1370
Total pressure, mm.
390. G 390. 6 389.4 389.3 388.5 386.2 384.9 304.1 383.2 381.8 378.9
pressure of (CHa0)s-
RH,
Time,
mm.
min.
Total preaaure, mrn.
390.6 390. 6 383.3 382.7 377.9 364.1 356.6 351.5 346.4 338.0 320. G
1465 1535 1595 1604 1Y20 2035 2110 2205 2260 2800 2905
378.5 377.7 377.8 377.5 376.2 375.1 374.8 274.7 374.2 371.3 370.5
Partial preasure of
(CHuO)zBH, mm.
318.2 313.1 313.7 311.9 304.4 297.5 296.0 295.1 292.1 274.7 270.1
Three runs were made a t 40°, two a t 60" and three a t 80". Each run was begun with a different pressure of pure dimethoxyborane. A run consisted of measurement of total pressure and time a t frequent intervals up to 25-35y0 disproportionation. The data for a typical run are given in Table I. To calculate the partial pressure of dimethoxy-
1415
KINETICSOF GASDISPROPORTIONATION OF UIMETHOXTBOHANB
Sept., 1959
borane from the total pressure, the expreesion was used P = 6P-p - 5P0
where PT = total ressure Po = initiafpressure of dimethoxyborane
P = partial pressure of dimethoxyborane
This relationship was derived from the stoichiometry of the reaction. The gases were assumed to be ideal and there were assumed to be no side reactions. These assumptions were found to be valid up to a certain reaction time for each temperature, since the per cent. disproportionation calculated from the above expression agreed very well with that determined by analysis of the sample. At longer reaction times, deviations occurred due to the decomposition of diborane. The relationship between the initial pressures and the initial rates of disappearance of dimethoxyborane for the three curves obtained at 40" yielded an average value of 2.19 for the order of the reaction. That the reaction is of order 2 was corroborated by plots of the reciprocal of dimethoxyborane partial pressure vs. time; straight lines were obtained in all cases except the experiments at 80". The deviation from linearity in the latter case is attributed to the thermal decomposition of diborane. Significant amounts of hydrogen were found a t the ends of the runs, and after longer periods of time yellow solids were deposited on the walls. This deviation increased with increasing diborane concentration. The rate constants a t 40, 60 and 80" were calculated to be 3.91 X (mm.)-l (min.)-l, 1.69 X 10" (mm.)-1 (min.)-l and 4.97 X 10" (mm.) -l (min.) -1, respectively. These values were obtained by averaging the slopes of the lines obtained from a "second-order plot" (l/concentration versus time) for each temperature. These data were assembled into an Arrhenius plot (Fig. 2) from which rate constants at other temperatures are obtainable by interpolation or extrapolation. From the equation for the line shown log IC = -3100/T 3.50, the rate con-
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CHARCOAL ON T H E DISPROPORTIONATION
Per cent. disproportionation using 1 gram of charcoal Time (min.)
20 20 20 20
20 20 20b
Amt.
&Ha
(moles)
0.000595
Amt. DMB left (moles)
Initiafi DMB (moles)
0.0138 0.01737 .0140 ,01687 .000159 .0111 .01462 .000159 ,0135 .01446 Using 5 g. of charcoal 0.001901 0.00582 0.01722 .00213 .00757 .02040 .00110 ,01828 .00120 .000487
1500 2100 2700 3300 1200 1800 2400 3000 Time (min.). Fig. 1.-Partial pressure of dimethoxyborane us. time at 40': H-15, glass tube packing; H-17, unpacked bulb. 7.0 6.8 6.6 42
3
Y
a
f
6.4 6.2
2 6.0
a 0
-g~
5.8
I
5.6
TABLE I1
EFFECT OF
0 300 600 900
Temp.,
Disproportionation
OC.
(%)
25 25
20.5 17.0 24.0 6.6
80 25 40 40
40
66.2 62.8 39.6
Using 10 g. of charcoalc 30 0.001862 0.000444 0.01180 40 m.6 Measured by P V T relationship in known volume at less than 276 mm. pressure. Not all B*HBremoved during analysis. Some white solids were present on surface of bulb after run.
5.4 5.2 5.0 3.5
3.4
3.3
3.2 3.1 3.0 l / T OK. X 108. Fig. 2.
2.9
2.8
2.7
stants at 25 and 200" were calculated to be 1.26 X (mm.)-l (min.)-l and 1.10 X (mm.>-l (min.) -l, respectively. The activation energy was calculated to be 14,200 cal. per mole from the slope of the line in Fig. 2. This value is of the right order of magnitude for a bimolecular reaction of measurable rate a t room temperature. It must be emphasized here that these rate constants apply only to the reaction carried out in a
11. S. UCHIDA, H. B. KREILWR, A4.MURCHISON AND J. F. MASI
141G
one-liter spherical Pyrex glass bulb, since the reaction is heterogeneous. A partial investigation was made of the effect of several adsorbing surfaces such as charcoal, glass tubes, glass wool, silica gel, magnesium strips and hydrogen reduced copper. Of these, charcoal w;is much the best catalyst. I n Table 11, some of the results obtained from the disproportionation of dimethoxyborane in the presence of charcoal in a static system are shown. The general conclusions obtained from the charcoal experiments are that the rate appears to be proportional to the amount of charcoal, temperature differences apparently do not affect the rate appreciably, and methyl borate decreases the rate since it is strongly adsorbed. To minimize this undesirable effect a flow system was constructed and the data obtained from it are listed in Table 111. TABLEI11 DISPROPORTIONATION OF (CH30)2BHI N ExDeriment No. 4 Run
no. 1 2 3 4
5 6
Time from start of run (min.)
Length of collection of sample (min.)
Temp. of run
60 120 180 240 305 325
20 20 20 20 20 30
'
A
(OC.)
(min.)
Disproportionation,
109 109 109 110 106 106
1.06 11.0 1.06 0.87 1.105 0.895
63.9 64.2 70.3 62.7 60.5 59.5
H
I
I
Step IV
'
H
CH2\o/B\o/ S' sI
CH3
+
Step V
%
At 110" and a residence time of approximately one minute, the rate of disproportionation remained near (30% per minute throughout the run. This proved to be the maximum rate obtainable under the conditions studied. Any proposed mechanism for the disproportionation of dimethoxyborane must account for the observed heterogeneity and the apparent second order. The following is suggested St,ep I Adsorption
Stev 111
FLOWSYSTEM
Reeidence time
.
Vol. 63
A
' A
B
S
Step V I
K 2BHa
BzHe
If the bridge-forming step (step 11) determines the rate, the reaction will be second order. An equilibrium is denoted in each step to account for the reversibility of the reaction. The presence of the mono-methoxyborane during the reaction was not detected; however, since the polymer of monomethoxybornne was observed by Schlesinger and Burg during the preparation of dimethoxyborane from methyl alcohol and diborane, it is possible that the mono compound is part of the reaction. The assumption that forward reaction 4 is very rapid is supported by the observation made by Schlesinger and Burg and in this Laboratory that the presence of this polymer hastens the reaction rate. This mechanism postulates that the methyl borate is adsorbed by the surface and thereby decreases the rate of reaction. In the case of charcoal catalyst methyl borate adsorption was observed to reduce the catalyst's effect on the rate of disproportionation over 50%. The mechanism also indicates that the orientation of the dimethoxyborane molecule is important, and suggests that an adsorbent whose active centers are the same distance apart as the oxygen-oxygen distance of the dimethoxyborane molecule and not of methyl borate would increase the over-all rate of disproportionation. e
