Pyrolyses of Aromatic Azines:  Pyrazine, Pyrimidine, and Pyridine

Abstract

The thermal decompositions of pyrazine, pyrimidine, and pyridine in shock waves have been investigated using the complementary techniques of laser-schlieren (LS) densitometry and time-of-flight (TOF) mass spectrometry (1600−2300 K, 150−350 Torr). A free radical chain reaction with initiation by ring C−H fission in the pyrolyses of all three azines is proposed. The measured C−H fission rates are compared and analyzed by RRKM theory. Barriers of 103 ± 2 kcal/mol for pyrazine, 98 ± 2 for pyrimidine, and 105 ± 2 for pyridine have been determined, supporting values lower than the barrier for C−H fission in benzene, 112 kcal/mol. The lower barriers for the azines are explained by the additional contributions of resonance structures of azyl radicals due to neighboring N−C interactions, which serve to further stabilize the azyl radicals. Detailed chain mechanisms are constructed to model the LS profiles and the TOF concentration profiles of the major products, hydrogen cyanide, acetylene, cyanoacetylene, and diacetylene. Of particular interest are the TOF observations and the mechanistic explanation of temperature dependent maxima seen in the formation of cyanoacetylene in the presence or absence of excess H2.

Structure Based Predictive Model for Coal Char Combustion

“…When considering the largest cumulative activation barrier, the 1,2-dioxetanyl radical pathway (via TS [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19] is even more favorable than "The B3LYP/6-31 G* geometry was used in each case.…”

“…Most studies with 8,9,10 pyridine or the diazines have focused on the pyrolysis of these compounds, and oxidative pyrolysis has been less well studied." C-H bond dissociation enthalpies @IDES) have been determined for some of these monocyclic azabenzenes, and a significant stabilizing effect on the radical has been noted when nitrogen is present in the ring.I2…”

“…For this work, the single particle combustion rate, R (gm carbon I sec), is modeled as: R = q 16s e-*RTp Pt mp (9) where k , is the preexponential factor for the surface rate constant (gm carbonls-cm2-atmn), S the total surface area I mass, (cm2 1 gm), E the intrinsic activation energy, P, the oxygen partial pressure at the outer particle surface (atm), n the empirical apparent reaction order for the range of P, examined, mp the carbon mass in the particle (gm), and q is the dimensionless effectiveness factor: where 4 is a generalized Thiele modulus for spheres [Froment and Bischoff, 19901: Equations 10 and 11 yield good approximate values of q for all exponents n in power law kinetics [Froment and Bischoff, 19901. The oxygen partial pressure at the particle surface, P,, is found by solution of mass transfer relations for boundary layer diffusion and ash film diffusion in series, as described elsewhere [Hurt et al, 19981.…”

Shock tube study of 1,3,5‐triazine dissociation and relaxation and relaxation of pyrazine

“…Considering that triple dissociations apparently occur in s-triazine and s-tetrazine, an immediate question is what are the barriers and rates of the analogous triple dissociations in say, pyridine and the two-nitrogen species pyridazine, pyrimidine and pyrazine? One of us was involved in complementary shock tube studies of the dissociation of these other than pyridazine [23,24], and from these experiments, as well as from other shock tube studies [25,26], they appear to dissociate by C H fission, and C H bond energies were derived from the kinetics of decomposition: pyrazine, 103 [23], and 96.5 kcal/mol [25,26], pyrimidine, 98 kcal/mol [23,24], and 95 [25,26] (these last refer to the weakest of the three C H bonds). These energies are uniformly lower than the values from current calculations [27,28], raising the question of whether the C H fission mechanism is completely dominant.…”

“…For this, we have chosen s-triazine, where it seems the triple-product channel is well established for the thermal reaction [10][11][12][13][14][15][16]. In any case, we note that any possible contribution from the chain reaction, as might be initiated by C H fission, can be clearly detected as acceleration with the laser-schlieren technique, as was seen in the studied one-and two-nitrogen heterocycles [23][24][25][26]. Although the s-triazine dissociation has already been investigated both experimentally and theoretically, a more complete study including high temperatures, and observing falloff, is needed to establish a good experimental high-pressure limit rate and barrier.…”