Mass Spectrometric Investigation of the Pyrolysis of Boranes. Tetraborane(10)¹

Abstract

The intermediates produced in the pyrolysis of "B4Hi0 were studied with an "integral furnace" mass spectrometer with reactor temperatures between 10 and 285°. The reactor pressure of B4H10 was ca. 10~5•2 atm. B4H8 was identified as an intermediate produced in the pyrolysis. Diborane as well as a combination of pentaboranes (B5H8 and B5Hn), hexaborane, a combination of heptaboranes, octaborane, decaborane, and possibly nonaborane also were observed. Monoborane and triborane were not observed, but quantities small compared to B4H8 could not be excluded.

Kinetic and mechanistic studies of the gas-phase thermolysis of hexaborane(12), and of its cothermolysis with other binary boranes and with CO, BH3·CO and H2

“…had suggested that B,H, was formed as a decomposition product. 22 The results of the present mass spectrometric analysis show that this is not the case and this conclusion was confirmed by "B NMR studies on pentaborane fractions separated (on the low-temperature column) from quenched reaction products.…”

Evaluating the Role of Triborane(7) As Catalyst in the Pyrolysis of Tetraborane(10)

“…13−16 Two reactions 1a and 1b, both first order in B 4 H 10 , have been claimed to be the rate-limiting step in the initial stages of pyrolysis. 17 20,21,23 Later publications by Greenwood and co-workers 4−6 were consistent with the proposal of reaction 1a as the rate-limiting step. However, Koski 24 excluded reaction 1a based on their observation that D 2 did not exchange with B 4 H 10 , leading Greatrex et al 4,5 to suggest that the observation was a ″worrying inconsistency in urgent need of reinvestigation.″ Nevertheless, Long 15 partially followed the mechanism of B 4 H 10 pyrolysis suggested by Koski. It is well established that the initial stage of B 4 H 10 pyrolysis is first order in the B 4 H 10 concentration in the temperature range 293 to 333 K, 4−6 with an observed activation energy of 23.7 kcal/mol.…”

A kinetic study of the gas-phase thermolysis of tetraborane(10)

“…Thus, the two-step mechanism which they proposed to explain the B4Hlo/B2H6 co-thermolysis, i.e. reaction (la) followed by reaction (12), does not by itself account for the observed stoicheiometry (13). This requires a third step involving the reactive intermediate (BH,}.…”