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L. BARTON, S. K. WASON,AND R. F. PORTER
creases caused by rather small increases in sulfate ion content do suggest a significant change in the struc-
ture of the melt, why this should increase the silver ion activity is not evident.
Thermochemistry of Interconversion of H2B203 (g) and H,B,03(g) '
by Lawrence Barton, Satish K. Wason, and Richard F. Porter Departntent of Chemistry, Conell University, Ithaca, New York (Received April 86, 1966)
The thermochemical stability of gaseous H2B2Oa has been investigated. Alternate routes in the preparation of H2B203were employed to establish upper and lower limits in AHo for '/3B203(S). The methods the reaction HsB303(g) 1/z02(g) = H2B203(g) l/~B&(g) of chemical preparation involve the reaction of boroxine with oxygen and the reaction of diborane with oxygen under electrical discharge conditions. The molecules HzBz03and H3B303 have been shown to be precursors to each other. Mechanisms for the interconversion reactions have been suggested from results of isotopic substitution experiments in the H3B303-02and H2B203-B2Hsreactions using infrared and mass spectrometric techniques. From these observations, a mechanism for the formation of boroxine in the explosive reaction of pentaborane-9 with oxygen is proposed.
+
Introduction Reactions of gaseous boranes with oxygen are frequently explosive and lead to the formation of stable boron oxide. For this reason, it has been difficult to isolate partially oxidized compounds having both B-H and B-0 bonds. One interesting intermediate was observed in the reaction of BaHs with oxygen by Bauer and Wiberley.2 Later, Ditter and Shapiro3" identified the compound as H2B203 from its m a s spectrum. This compound is also observed as a product in the reaction of gaseous boroxine (HJ33Oa) and 0 2 . 3 b When mixtures of B2H6, H2B203, and 0 2 are heated, an explosive reaction occurs and boroxine is observed as a product.* The high temperature stability of boroxine was demonstrated from its preparation in a reaction of HzO(g) and elemental boron at temperature of about 1400°K.6 Taken collectively these observations suggest that Ha203 and HsBaOaare precursors to each other under different sets of experimental conditions, In the work described in this paper, certain aspects of tthethermochemistry and reaction mechanism The Journal of PhySical Chemistry
+
+
of the conversion of HzB20ato H&Oa and HJ3303 to HzB~O are ~ examined. Two procedures for the prepa ration of HzB20rB2Ha mixtures used in these studies are described below. Experimental It has been reported that H2B20ais unpredictably explosive in the condensed state. Thus, although very low pressures were used in this work, caution was observed at all times. The first method for the preparation of HzB203was (1) Work supported by the U. S. Army Research O5ce (Durham) and the Advanced Resesrch Projects Agency; preaented at the 149th National Meeting of the American Chemical Society, Detroit, Mich., April 1965. (2) W. H. Bauer and 8. E. Wiberley, Abstracts of Papers, 133rd National Meeting of the American Chemical Society, San Francisco, Calif., April 1958, p. 13L. (3) (a) J. F.Ditter and I. Shapiro, J. Am. Chem. SOC.,81,1022 (1959) ; (b) 9. I(.Wason and R. F. Porter, J. Phys. C h m . , 68, 1443 (1964). (4) G. H. Lee, W. H. Bauer, and 8. E. Wiberley, ibid., 67, 1742 (1963). (5) W. P. Sholette and R. F.Porter, ibid., 67, 177 (1963).
THERMOCHEMISTRY OF INTERCONVERSION OF HzB203(g) AND HB303(g)
the reaction of boroxine with oxygen. Solid boroxine was prepared by passing H20(g) over a mixture of B(s) and &&(I) at a temperature of about 1400°K. and condensing the product in a bulb at liquid nitrogen temperatures. Hydrogen, which is one of the byproducts of the reaction, was removed by pumping. The apparatufr is illustrated in Figure 1. Solid boroxine was warmed to room temperature and diborane which is formed by partial decomposition was removed. Oxygen at 40 torr pressure was added to the bulb slowly through. a 1.25-mm. Teflon needle valve and the reaction vessel was then heated to 60". Product gases were withdrawn into a 10-cm. infrared cell and spectra were recorded on a Perkin-Elmer Model 337 grating infrared spectrophotometer. Bands due to BzHa and HzB203(g) were observed. No improvement in the yield of H2BzO3(g)was observed when the reaction products were re-analyzed after 24 hr. At room temperature the yield of HiB203was very low. When the reaction between boroxine and oxygen was carried out at -85" a slight explosion was observed. Infrared spectra of the products indicated bands due to BzHe(g) and H3B30a(g). This behavior is similar to that reported by Lee, Bauer, and W i b e r l e ~ . ~I n a separate experiment,6 lSO2was substituted for ordinary oxygen and the reaction with boroxine (containing ordinary oxygen) was carried out at 60" in the manner just described. A second method of preparation of HzB203 starting with diborane was developed. Oxygen was slowly added to diborane (20 torr pressure at room temperature) through a Teflon needle valve in a 300-ml. Pyrex bulb. An electric discharge was maintained near the neck of the flask with an ordinary Tesla discharge coil. The flow rate of oxygen, controlled by a pressure head of oxygen, had to be sufficiently slow to prevent "flashing" &e., rapid oxidation of diborane) and fast enough to cause the small intermediary concentrations of higher boranes to be oxidized rather than to polymerize further to give solid boranes. The correct oxygen flow rate was distinguished by the appearance of a white layer of boric oxide on the walls of the vessel, thickest near the point of discharge. A very slow flow rate of oxygen resulted in the formation of a yellow-white deposit which contained high molecular weight boranes. Under these conditions the yield of H&O3 was extremely low. The presence of the intermediary boranes B4H10, B&Hll,and BeHlo in concentrations of the order 4, 1.5, and 0.501,,, respectively, . . was shown by allowing diborane t o le&' slowly into a mass spectrometer in the presence of an electrical discharge. The preparadisOf these boranes from BzH6under charge conditions has been described by Kotlensky
3161
To Vocuum Line
It,
Infrared
Cell
W
Figure 1. Apparatus for the preparation of from the reaction of oxygen with boroxine.
H2B2O3
and Schaeffer.' Maximum yields of HzB203 were obtained when a ratio of B2Hs:02 of 1: 1.25 was used. The whole procedure required about 30 min.
Thermodynamic Stability of H&O,(g) Reaction of gaseous boroxine with 0 2 yields H2B203, B2Hs, and Bz03(s) as final products.ab The thermodynamic stability of HzB203(g)was determined from measurements of A$'" for the reaction H3B303(g)
+ '/zOz(g)
=
HzB&(g) '/gBzHdg)
+
+ l/sBzo&)
(1)
By utilizing the two procedures described above starting with either H3B303or diborane, it was possible to approach reaction 1 under different experimental conditions. In the first series of experiments, a reaction vessel (Figure 1) containing boroxine and oxygen was joined to the inlet of a mass spectrometer. The vessel was immersed in an oil bath that could be heated to about 150". A small quantity of gas was withdrawn and ion currents at m/e 27, 32, 71, and 83 were monitored. These correspond to the major ion peaks for B2He, 0 2 , HBz03, and HB303, respectively. Immediately following the mass spectral analysis, the total pressure in the reaction vessel was measured. Diborane and oxygen were the major constituents and their partial pressures were calculated from the total pressure and the relationship P B ~ H ~ /= P o1.27(1~)/ ~ (Ia)which was obtained by calibration with a low presSure sample of BzH6 and 0 2 of known composition. For reaction 1 we have (6) Sample contained 98% 1 8 0 e and was obtained from Yeda Research and Development Go. Ltd., Rehovoth, Israel. (7) W. V. Kotlensky and R. Schaeffer, J. Am. Chem. SOC.,80, 4167 (19%).
Voluma 69, Number 9 September 1966
L. BARTON, S. K. WASON,AND R. F. PORTER
3162
Table I: Thermodynamics of the Reaction BaOaHdg)
+ ‘/202(g) Temp. of K p determination,
Initial system
B2H6
+ 0 2 + discharge
B308Ha f
0 2
-+
B203H2(g)
+ ‘/&H6(g) + 1/3B20a(s) pol,
IHZBZOS
atm.
IHaBsOs
Kp
-AH, kcal./mole
2.9 X 3.9 x 10-3
1 . 5 X 10-6 1.1 x 10-6
1.3 X loa 3.5 x 103
9.6 X lo4 4 . 2 x 105
10.2 11.1
298
3.8 X
1.5 X
1.6 X lo3
5.0 X
lo4
9.9
298 373 373
2.0 X 7.9 X 9.4 X
2.3 X 1.4 X 1.4 X
1.2 X lo8 2.3 X 10’ 2.5 X lo2
2.9 X lo4 2.7 X l o a 3 . 1 X 10*
9.5 10.3 10.4
331 298
1 . 0 X lo-’ 8.2 X
1.1 X 10+ 6.1 X
6.2 X 4.4 X
lo8 lo2
3 . 8 X lo2 3.7 X lo3
7.0 8.3
298
6.7 X 10-2
3.5 X 10-8
2.2 X lo2
2.7 X l o 3
8.1
Sample history
OK.
Immediate analysis Immediate analysis Sample maintained at 323°K. for 30 min. Sample maintained at 298°K. for 36 hr. Samplewarmed to373”K. Sample warmed to 373°K.
298 298
Sample warmed to 331°K. Sample warmed to 331°K. Sample maintained a t 298°K. for 6 hr.
P B ~ ~ , atm.
lo-*
*
The partial pressure of H3B303 was too low for measurement by conventional methods. However, a good approximati.on to eq. 2 is given by
(3)
was estimated to be 1.0 1.0 cal./deg. mole at 300°K. This estimation was based on the Dah (six-membered ring) and CZv (five-membered ring) structures for HaB303 and H2B203, respectively. Uncertainties in the absolute entropies of HaBaOa(g) and H2B203(g) are in the vibrational contributions since the lowest vibration frequencies for the molecules have not been observed. However, at ordinary temperatures, the vibration terms will nearly cancel in evaluating the entropy difference for the two molecules. A somewhat similar comparison can be made for cyclohexane and cyclopentane where the entropy difference is only 1.3 e.u. at 300°K. Results of AHocalculations are shown in Table I.
where the pressure ratio of H2B-203 to HaBaOa in eq. 2 has been replaced by a ratio of total ion intensities. This is equivalent to assuming that the ionization cross sections for the two molecules are nearly equal. Equation 3 may be further simplified by the relation I E ~ B ~Interconversion o~/ of H2B20a(g)and H&O3(g) IH~B = ~l.6(I,1)/(I83) o~ which was obtained by analysis H&30a-1802Reaction. In Figure 2, we compare mass of complete mass spectral patterns. The high concenspectral patterns for the highest mass groupings for tration of B2H6 in the reaction products results largely H2B2Oa prepared in the reaction of boroxine with ordifrom the partial decomposition of solid boroxine. nary oxygen and with I8O2. The spectral pattern for When the sample is heated, the decomposition reaction HzB203 having the normal boron isotope abundance is also yields an excess of B20a(s) in the solid coating in similar to that reported by Ditter and S h a p i r ~ . The ~~ the reaction vessel. In another series of experiments, mass spectrum of HzB203 obtained from the reaction samples of H2Bz03were prepared from B2H6 and 0 2 with 1 8 0 2 shows that the isotope distribution in the by the electrical discharge technique. Partial presproduct is not statistical in all three oxygen atoms but sures of gaseous components in the products were is weighted towards the species H2B21802180.A determined mass spectrophotometrically. After corsmall quantity of HzBP03is present but the HzB21802160 rection or Hz(g) which is produced in the discharge, constitutes about 80% of the total amount of the data were treated in the same manner as for the H2BzOa. On the basis of this result, it is inferred that HaB30r02 reaction. O2 reacts without rupture of the 0-0 bond. To Equilibrium constants for reaction 3 computed from account for a homogeneous gas phase reaction between the experimental results, are listed in Table I. The H3Ba03(g)and Oz(g),we might assume that a biradical heat of the reaction was calculated from values of Keq and the standard entropy change. Entropies of gase(8) “Janaf Interim Thermochemical Tables,” The Dow Chemical ous 02, and solid B203 were taken from the Co., Midland, Mich. Data for B a s were taken from the tabulation “Janaf Tables.”8 A value of SoIIsBaOs - S o ~ z ~ zof~Dec. s 31, 1964; for Oa,March 31, 1961; for BSOa, Deo. 31, 1964. The JOUTW~ of Physical Chemistry
3163
THERMOCHEMISTRY OF INTERCONVERSION OF HZB20a(g) AND HaBaOdg)
100
-
50
c 01 .I
r
% .
0 0
B
e 100
-.u .0
a
50
Time in minutes O
80 81 82 83 84
80 81 82 83 84
Mass m/e
Figure 2. M a s spectral data illustrating the effect of isotopic substitution upon the reaction of HsB~OS with 0,and HzBzOswith BZHG ( l0B/l1B = 1 :1.2 in BzHe).
H-BO2 is formed as a transient second product. This species, rather than HO-B = O(g), could be a precursor to B2Hsand B203, which are the stable final products. However, since Bzoa appears as a solid, it is possible that the reaction proceeds by a heterogeneous path. BZHrH&O3 Discharge Reaction. At ordinary temperatures and low pressures, reaction between diborane and HzBzO~was not observed. Results of a mass spectrometric study of the discharge reaction are illustrated in Figure 3. A continuous discharge maintained at the reaction vessel resulted in a diminution in HzB203and an increase in HaBaOa. When the discharge was firsit applied, the ion intensity of HB203+ did not change noticeably. This effect was observed whenever a small amount of oxygen was present initially. After this residual 0 2 had been removed by reaction with the intermediate boranes, the intensity of HB20a+ dropped rapidly as indicated. The effect of the discharge on the disappearance of BzH6 and appearance of B4Hlois also noted in Figure 3. Some information related to the mechanism of conversion of HzB2Oa to H&O, was derived from observations on reaction mixtures prepared by adding l0B2Hs to B z H c H ~ B z ~samples ~ containing the natural l0B/l1B isotope abundance. Mass spectral patterns obtained for these mixtures are compared with that for normal HjS303 in Figure 2. Analysis of these data show that the main product, other than that with the normal isotopic spectrum is HallB~loBOa. From the observed l0B,P1Bcomposition of the diborane, it is evident that boron exchange is not a major factor and that a preferred mechanism is involved. This behavior can be interpreted on the basis of the postulated reaction BHa(g)
+ HzBzOs(g) = H3BaOa(g) + Hzk)
(4)
Figure 3. Curves showing the variation with time of the concentrations of the major species involved in the reaction of BzHe with HZB203in an electrical discharge.
I
l
l
I
I
0
c
.c c
E
c
c ae
I I
4000
2000
1
1500 Frequency
I
1200
cm-1
I
l
l
L
1000 900
Figure 4. Infrared spectra of a mixture of B2Ha and H&OS taken at intervals during the c o m e of reaction in an electrical discharge. Lower curve is for the &a1 mixture of BaHa and H8BaOa.
The formation of higher molecular weight boranes under discharge conditions supports the supposition that BHs is the actual intermediate. Recently, direct evidence for BHs and BHa intermediates has been rep0rted.W The main features of the discharge reaction between BzH6(g) and HzBzOa(g) were confirmed by observing changes in the infrared spectra of a mixture of the two compounds. The discharge was applied to a small Pyrex thimble joined to a 10-cm. infrared cell. A (9) T.P.Fehler and W. 8.Koski, J. Am. Chm. Soc., 86,2733 (1984). (10) T.P.Fehler and W. S. Eoski, i W . , 87,409 (1966).
Volume 60, Number 9 September 1966
L. BARTON, S. K. WASON,AND R. F. PORTER
3164
series of spectra for one set of experiments is shown in Figure 4.
to the right, thus generating a transient concentration of BH3. Under such conditions, reaction 4 is possible. The presence of intermediate boranes in these explosive Discussion oxidation reactions has been postulated previously.2 ~ 1 2 As we noted in Table I, variations in AH" values for A calculation based on the heat of formation of HzBzO3 reaction 1 are dependent on the method of sample obtained here and published values for BH3 (Le,, preparation. The experimental data from BzHa18 kcal./mole) and H&03 indicates that reaction 4 is HzBz03 samples obtained by the discharge technique exothermic to the extent of 110 kcal./mole. Since indicate a higher stability for HzBz03(g) than the gaseous boroxine is unstable with respect to BzH6 data obtained from the H3B3O3-OZreaction. In the and B203at ordinary temperatures, reaction 4 provides electrical discharge experiment, the chemical mechanism a mechanism for the ultimate decomposition of HzBzOa for formation of HzB203 is assumed to involve the to Bz03and Hz. reaction of oxygen with the borane intermediates as The chemical behavior of H2B203with respect to its in the B6H9-02 r e a c t i ~ n . In ~ ~this ~ ~ case, the main formation from Ha308 and its precursor relationship products (aside from Hz)are BzH6(g) and H2BZO3(g) to H&03 is consistent with the CZv~tructure'~ while Oz(g) and H3B303(g) are in relatively small yield. 0 In the H3B3O3-O2reaction, is present initially in the reaction vessel and a high concentration of Oz /\ H-B B-H is arbitrarily fixed. Thus, the two sets of data tend to establish upper and lower limits on Kea and on the value of AH". From the results given in Table I, 0-0 we obtain for the "best value" of A H 0 2 g 8 ' , 9.5 f 1.5 This structure enables us to visualize the reaction of kcal./mole. This value was combined with the heats H3B303 and O2 as an "addition" of O2 to boroxine and of formation for BzHs(g),BB203(~),8 and H3B303(g)11 reaction 4 &s an insertion of a BH group into the of 7.5, -305.3, and -291 f 2 kcal./mole, respectively, HzB2O3 ring at the 0-0 bond. The mass spectral to give a heat of formation of -200.4 f 3.5 kcal./mole patterns in Figure 2 show that H2B21803is a minor for HzBz03(g)at 298°K. The stability of HzBZO3(g) product in the H3B30d8 O2 reaction. The origin of this could have been determined by a thermochemical species is not entirely clear but it could be obtained analysis of the reaction from a reaction of l8OZwith traces of B4Hlo or BsH9, '/3H3B&(g) '/zOz(g) = Hz&.03(g) (5) i.e., pyrolysis products from BzHa. Similarly, the production of small amounts of 'oBzllBOJ& in the However, this would not afford an independent check reaction of traces of oxygen with the higher boranes on the heat of formation of HzBz03(g)since the AH" produced from 10B-enriched diborane upon electrical for reaction 5 is a combination of the AH"for reaction 1 discharge. Interconversion reactions between HzBzO and AH" for the reaction and H3B803also may lead to some redistribution of '/mlBzHe(g) Bz03(S) = HaBaOa(g) (6) isotopic species. Appearance potential measurements gave an ionizaon which the heat of formation of HaB,Os(g) is based.11 tion potential of 13.6 f 0.2 v. for HzBzOa(g). The The observation of Lee, Bauer, and Wiberleyd that major ion fragments on electron impact are HBz03+, H3B303 is a product in the explosive oxidation of B6H9 HBO2+, and HBOf. Appearance potentials for these may be accounted for in a mechanism similar to that 15.5 0.2, 14.5 f 0.4, and 14.5 f 0.4 v., ions are proposed in reaction 4. Under the conditions of their respectively. The ionization potential of HzBzOs experiment, when oxygen is added to B6Hg in a 3 :1 13.5 v. reported for HaBaOa(g).' is close to the value of molar ratio, the products should contain BZH6, HzBz03, and unreacted oxygen. The explosion that occurs when (11) R. F. Porter and S. IC. Gupta, J. Phya. Chem.,68, 280 (1964). this mixture is heated should result in a sudden tem(12) A. T. Whatley and R. N. Peace, J. Am. C h m . Soc., 76, 1996 (1954). perature rise to shift the equilibrium
\ I
+
+
*
BzHe(g) = 2BH3(g)
The J w d of Phyeicd Chemistry
(13) C. C. Costain, W. V. F. Brooks,and R. F. Porter, "The Microwave Spectrum and Structure of HBnOa," to be published.
