Kinetics
and
Alkylation
of
Equilibria
in
the
Diborane
Preliminary Report
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LLOYD VAN ALTEN, G. R. SEELY, JOHN OLIVER, and D. M. RITTER Department of Chemistry, University of Washington, Seattle 5, Wash
The principal characteristics of the reactions of diborane with compounds of type B X have been known since the classical experiments of Schlesinger and his coworkers (1,6-8) were performed. Nevertheless numerous exact descriptions remain to be accomplished in terms of reaction velocities and equilibrium constants as well as the related mechanisms and thermodynamic quantities. Contributions have already been made through examinations of the decomposition of tetramethyldiborane (3) and dimethoxyborane (9). What is described here is a preliminary report of work on two phases concerning the reaction between diborane and trimethylborane and the decomposition of monomethyldiborane. 3
Experimental Gas partition fractometry was used for analysis in preference to infrared spectrophotometry and mass spectrometry as the method least complicated for complex mixtures. The column was loaded with a 50:50 mixture of 80-mesh firebrick and mineral oil (4) and the auxiliary apparatus was essentially that described previously (5). A typical fractometer pattern obtained at 0° is shown in Figure 1. The method proved suitable for the least stable component, monomethyldiborane, as is shown in Figure 2, where the two traces are those obtained by successive passes of the same sample through the column, in which the diborane contents were 0.3 and 0.8 mole %, respectively. The reactor was a cylinder 4.0 cm. in diameter of 173.8-ml. volume, immersed in agitated ice water contained in a 3-liter wide-mouthed Dewar bottle. At intervals aliquots were removed for analysis by expansion into a small adjacent volume, from which they were swept directly into the fractometer. The quantities present were estimated by graphical integration of the fractometer patterns. Where curves overlapped, the area assigned to each was estimated by comparison with curves obtained for each substance alone. The areas were corrected by factors proportional to the relative thermal conductivities with B H :B H Me:BMe : B H Me as 1:1.23:1.42:1.42. The values were checked in two different ways: from comparisons between diborane and trimethylborane during induction periods when no reaction occurred and from comparison between diborane and symmetrical dimethyldiborane based upon the stoichiometry of the monomethyldiborane decomposition. The corrected areas were normalized relative to a constant total boron content, and the values were converted to units of concentration in the reactor. The factors necessary to ensure accuracy were found to be constancy of the recorder base line and correct adjustment of areas at points of overlap. Constant Present address, Department of Chemistry, San Jose State College, San Jose, Calif. Present address, Shell Development Co., Emeryville, Calif. 'Present address, Department of Chemistry, Wayne State University, Detroit, Mich. 107 BORAX TO BORANES 2
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Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
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ADVANCES IN CHEMISTRY SERIES
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108
O >
TIME, MIN.
Figure 1.
Fractometer pattern for mixture of diborane derivatives 1. 2. 3. 4.
Diborane Monomethyldiborane Trimethylborane 1,1-Dimethyldiborane
potential across the bridge was ensured b y careful attention to the storage batteries; the thermal conductivity cells were carefully insulated and immersed i n oil to avoid stray current leaks. T h e line voltage to the recorder drive mechanism was passed through a voltage regulator. W i t h these measures taken, a base line was obtained
o >
TIME, MIN.
Me,) d(B H ) rAr (fc + for) KQ\ \ w x L 2 M 1 + r ) " Or] ( > « X '> 4
2
2
6
2
20
6
m
6
=
T T
B
Thus initially
P T i f n
H
B M e
and initially the kinetics should be pseudo second order, with ample provision for an induction period through necessity of feeding Reactions 1 and 2. F o r scheme B : - *i(BÄ) +
fc (B H )(BMe,) 0
2
6
an expression which contains no terms related to ohain components, and which upon integration yields an expression containing the constants k and k i n the logarithmic term. Only if ^ k will a simple second-order plot describe the data. The value (5) of k is about 7.5 X 1 0 s e c . , while the value of k can be taken as approximately equal to that obtained from the second-order treatment of the data. The value is 6.7 X 10~ cc. s e c . m o l e . Thus, k s k , and the alternative hypothesis B a p pears less likely than A . Determination of wall effect and an examination of pressure dependence must be carried out. Temperature dependence of a chain reaction has less significance than for ordinary first- and second-order reactions; however, determination of the over-all net activation energy, combined with estimates for the individual steps, might permit an estimate of the number of elementary steps contributing to the over-all reaction. x
a
a
- 6
x
6
- 1
- 1
- 1
a
x
a
BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
VAN ALTEN ET AL.
113
Alky Iat ion of Diborane
Decomposition of Monomethyldiborane. T h e decomposition of monomethyldiborane is seen to be a first-order reaction (Figure 8 ) . T h e velocity constants are pressure-dependent, but experiments have not yet been made at sufficiently high pressures to permit extrapolation to / ( 1 / P ) = 0 .
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.5
L
I
I
I
I
i o o " .9
"—T—r~~~^~~ 3 4 5 6 7
il
i
TIME, MIN. X 1 0
Figure 8.
s
First-order plot for monomethyldiborane decomposition
The reaction steps i n the decomposition are represented b y Equation 6 and the following additional reactions: (BH Me) % 2BH Me 2
2
(11)
2
hi
B H + (BH Me) ^ BH Me + B H Me 8
2
2
2
2
(12)
6
kn
The rate equation i s : d(B H Me) 2
6
[Wfc(B,H,Me) -
1
=
fe*iiW(BHtMe),(B,H Me)»] l
at (KQI* + / co/ ien -r «n ) which, with neglect of the second term for the reverse reaction, gives an initially firstorder process. I f the step represented b y k (Equation 12) can be neglected, the initial process becomes dependent only upon the step represented b y Equation 6. There appears latent i n this development the possibility of obtaining from the reaction an experimentally determined estimate of a bridge-bond dissociation energy. Values for the equilibrium constant for the reaction: n
2B H Me ^ B H« + (BH Me) 2
6
2
2
(13)
2
were calculated from the data i n Table I . Table I. Composition of Equilibrium Mixtures from Monomethyldiborane Decomposition at 0 ° K
C o n e n . , M o l e s / L . X 10« BsHiMe
BsHe
(BHiMe)j
1.14 0.59 3.41 2.19 6.92 6.48 4.03 1.50
0.36 0.17 1.08 0.58 1.83 1.71 1.05 0.42
0.27 0.13 0.85 0.55 1.81 1.69 1.00 0.39
X 10«
7.48 6.43 7.89 6.65 6.92 6.88 6.47 7.35 A v . 7.01
BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
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114
ADVANCES IN CHEMISTRY SERIES
Literature Cited (1) Bauer, S. H., J. Am. Chem. Soc. 78, 5775 (1956). (2) Burg, A. B., Ibid., 56, 599 (1934). (3) Cowan, R. D., Los Alamos Scientific Laboratory, Los Alamos, N. M., private communication. (4) Kaufman, J. J., Todd, J. E., Koski, W. S., Anal. Chem. 29, 1032 (1957). (5) Parsons, T. D., Silverman, M. B., Ritter, D. M., J. Am. Chem. Soc. 79, 5091 (1957). (6) Schlesinger, H. I., Burg, A. B., Ibid., 53, 4321 (1931). (7) Schlesinger, H.I.,Flodin, N. W., Burg, A. B., Ibid., 61, 1078 (1939). (8) Schlesinger, H.I.,Walker, A. O., Ibid., 57, 621 (1935). (9) Uchida, H. S., Kreider, H. B., Murchisen, A., Masi, V. F., Abstracts of Papers, 131st Meeting, ACS, Miami, Fla., April 1957, p. 5R. A MAJOR portion of this research was supported by the United States Air Force through the Air Force Office of Scientific Research of the Air Research and Development Command under Contract No. AF 18(600)-1541. Reproduction in whole or in part is permitted for any purpose of the United States Government.
BORAX TO BORANES Advances in Chemistry; American Chemical Society: Washington, DC, 1961.
