Exchange reaction of methane-d4 with hydrogen chloride

Abstract

A shock tube coupled to a time-of-flight mass spectrometer was used to study the exchange reaction, CD4 + HC1 CDsH + DC1, over a density range 1.7-4.1 X 10"6 mol cm"3 in the temperature region 1600-2500°K. In the lower temperature range (1600-1900°K) DC1 exhibits a quadratic time-dependent growth rate while at higher temperatures (2200-2500°K) increase of DC1 is a linear function of time. The only chlorine-containing product is deuterium chloride. Rate constants extracted from the quadratic and linear regions yielded activation energies of 51 and 30 kcal mol"1, respectively. An atomic mechanism for the exchange predicts both time dependencies and the transition between them. Pyrolysis of methane occurred along with the exchange but the growth rate of the products ethane, acetylene, and diacetylene was shown not to be catalyzed by the presence of HC1. Acetylene was the major product of the pyrolysis. Exchange was observed in both major and. minor products. The ratio CHACEE was less than one when pyrolysis products were not detected and greater than one when they were present. This finding suggests the presence of methyl radical in the reaction mixture. The results are discussed in terms of an atomic mechanism.Professor Bauer and his coworkers studied the CH4 + D2 reaction in the temperature range 1440-1755°K (SPST) under conditions of negligible 02 impurities.6 Samples extracted from the reflected zone were analyzed with a mass spectrometer for CH4, D2, CHgD, and HD. Evidence for methane pyrolysis was not found. The reaction is reported to proceed through a four-center complex in accordance with the vibrational excitation mechanism proposed by Bauer.3a Burcat and Lifshitz (BL)6 studied the exchange system CH4 + CD4 in the temperature region 1340-1745°K (SPST). They report a zero-order dependence with respect to inert gas although Bauer found an order of 0.6 for the inert diluent. BL interpreted their results in terms of a methyl radical chain mechanism resulting from the pyrolytic initiation step CH4 CHg + H, followed by the exchange reaction CHa+ CD4-> CH3D + CD3.Several questions are apparent from a comparison of these two methane exchange reactions. Both studies are in the same temperature range, yet Bauer argues against the importance of pyrolysis in the exchange (1) (a) Support of this work by the National Science Foundation under Grant No. GP-23137 and also funds for equipment from NSF Departmental Science Development Program GU-2632 are gratefully acknowledged, (b) Paper presented in part at the Southwest

Self-exchange of carbon monoxide behind reflected shock waves

“…These reactions include the synthesis of HCN from CH4 and N2,1'2 the synthesis of acetylene from CH4,3 and the heterogeneous reduction of metal oxides (CuO, NiO, PbO, ZnO) in a hydrogen plasma. 4 The pressure range investigated was between 5 and 40…”

Exchange reaction of acetylene-d2 with hydrogen chloride

“…Hydrogen chloride was chosen as the exchange partner for this study since the positions of HC1 and DC1 occur in an easily focussed portion of the mass spectra and they do not undergo measurable dissociation at the temperature required for the exchange. 3 In addition to studying the pyrolysis of acetylene, the Kistiakowsky group also investigated the self-exchange, C2H2 4-C2D2, which they concluded proceeded via an atomic mechanism. 5 The Harvard group employed a timeof-flight (TOF) mass spectrometer to sample dynamically from the reflected shock zone and reported that the appearance of the exchange product C2HD was a quadratic function of reaction time.…”