Gas-Phase Pyrolyses of 2-Nitropropane and 2-Nitropropanol:  Shock-Tube Kinetics

• Published in: The journal of physical chemistry. A, Molecules, spectroscopy, kinetics, environment, & general theory Molecules, spectroscopy, kinetics, environment, & general theory, 2000-02, Vol.104 (6), p.1207-1216
• Main Authors: Zhang, Yi-Xue, Bauer, S. H
• Format: Article
• Language: English
• ISSN: 1089-5639; 1520-5215
• DOI: 10.1021/jp993204e

The kinetics of the thermal decomposition of 2-nitropropane was investigated in a shock tube over the temperature range 970−1150 K under high dilution in argon at total pressures 4.5−6 atm. The decay of 2-nitropropane and the production of NO2 were followed spectrophotometrically at 276 and 405 nm, respectively. The unimolecular rate constant deduced from the loss of 2-nitropropane at the high-pressure limit is:  k ∞ = 1014.55±0.13 exp{(−54.2 ± 3.8 kcal/mol)/RT} s-1. The products of pyrolysis were identified and quantified with FTIR and GC. The major products are NO, CH3CHCH2, CO, CH3CHO, C2H4, and (CH3)2CO. The decomposition proceeds primarily through C−N bond fission, while the contribution from the five center elimination of HONO accounts for less than 20% of the total loss of 2-nitropropane. A reaction mechanism that consists of 81 elementary steps is proposed; it accounts quantitatively for the overall pyrolysis. Numerical simulations with the mechanism reproduce reasonably well the NO2 vs time profiles at different temperatures and the distributions of major reaction products over the temperature range studied. When the mechanism was subjected to a sensitivity and principal component analysis, it was found that only 58 reactions in the mechanism are needed to faithfully reproduce the observed product distributions. The fragmentation and reaction sequence in the pyrolysis is graphically presented so that the fundamental chemistry of the overall reaction can be readily visualized. The initial stage of the decomposition of 2-nitropropanol was also investigated following the above experimental protocol. The rate of the pyrolysis appears to be similar to that of the 2-nitropropane over the temperature range studied, but it has a much lower activation energy. The unimolecular rate constant can be expressed as 1011.29±0.78 exp{(−37.5 ± 3.6 kcal/mol)/RT} s-1. No pressure dependence was observed over the pressure range investigated (4−5.3 atm), and therefore it is presumed that the rate constant represents the high-pressure limit. Both the low A factor and the low activation energy suggest the involvement of the OH group in the first step of the reaction. NO2 production and subsequent consumption during the decomposition was also observed. The reaction products from this species are similar to those of 2-nitropropane.