Exothermic reactions pathway

Stripping of chlorine from hydroxides such as Cl2Sn(OH)2 could eventually lead to gas-phase SnO or Sn02. However, at the relatively low temperatures typical of tin oxide CVD ( 873-973 K), we do not expect these oxides to form, based on the equilibrium calculations described above. Thus, the formation of tin hydroxides is not only thermodynamically favored (i.e., based on minimization of the Gibbs free energy), but there are also exothermic reaction pathways that we expect to be kinetically favorable. The primary tin carrier in the CVD process could therefore be a tin hydroxide. Complete conversion to Sn02 would most likely occur via reactions on the surface. 

The rate of reaction (4) has been estimated from computer models of complex systems. Thus, a rate constant of k(349 K) = 7xio cm mor - s was obtained in NH3 pulse radiolysis experiments [31]. A direct measurement in a flow reactor, in which NH and NH2 were detected by LMR and LIF, yielded the rate constant k(296 K) = (8 3) x 10 cm mol" s" [32]. At high temperatures (2200reflected shock waves. A rate constant of k(2500 K) = 3xio cm mor s" was obtained from the best fit of measured and calculated concentration profiles [12]. A temperature dependence of was concluded from the fact that reaction (4) is a radical-radical reaction [12]. The measured room-temperature value is consistent with this small negative temperature dependence. Besides the recombination NH + NH2 + M N2H3 + M, four exothermic reaction pathways are possible (N2H + H2, N2 + H + H2, N2H2 + H, and NH3 + N) as discussed in [32]. 

The reactions of primary amines and maleic anhydride yield amic acids that can be dehydrated to imides, polyimides (qv), or isoimides depending on the reaction conditions (35—37). However, these products require multistep processes. Pathways with favorable economics are difficult to achieve. Amines and pyridines decompose maleic anhydride, often ia a violent reaction. Carbon dioxide [124-38-9] is a typical end product for this exothermic reaction (38). 

In chlorinations either a substitution or an addition process can occur with the ultimate reaction pathway(s) determined by a combination of factors, which include the reaction conditions, the positions and natures of any substituents present, and the catalyst used. Uncatalyzed chlorination of benzothiadiazole is an exothermic reaction that gives rise to a mixture of isomeric tetrachloro addition products. These are converted in basic medium into 4,7-dichloro-2,1,3-benzothiadiazole (70RCR923). When an iron(III) catalyst is present 4- and 7-chloro substitution becomes the dominant process. Chlorination of a number of 4-substituted 2,1,3-benzothiadiazoles (43) using an oxidative process gave a combination of chlorinated and oxidized products. The 4-hydroxy, 4-amino-, 4-methyl-amino, and 4-acetoxy derivatives of 43 all formed the chloroquinones (44) (40-61% yields). With the 4-aIkoxy substrates both 44 and some 5,7-dichlorinated product were obtained (88CHE96). 

In highly exothermic reactions such as this, that proceed over deep wells on the potential energy surface, sorting pathways by product state distributions is unlikely to be successful because there are too many opportunities for intramolecular vibrational redistribution to reshuffle energy among the fragments. A similar conclusion is likely as the total number of atoms increases. Therefore, isotopic substitution is a well-suited method for exploration of different pathways in such systems. 

C02 elimination and the substitution with water were the most exothermic degradation reactions and this confirmed the observation of C02 evolved during degradation. Isomerisation reactions were also investigated and assessed for the possible reaction pathway exothermicity. The calculations did not try and assess the transition states or barriers along the reaction pathways. 

For a reaction to produce detectable CL emission, it must fulfill the following conditions (1) it should be exothermic so that sufficient energy for an electronically excited state to be formed (at least 180 kJ/mol for emission in the visible region) can be provided (2) there should be a suitable reaction pathway for the excited state to be formed and (3) a radiactive pathway (either direct or via energy transfer to a fluorophore) for the excited state to lose its excess energy should exist. 

In our detailed theoretical study of reaction pathways of the model A-azido-A-methoxyformamide 82b we showed that decomposition by loss of nitrogen was the energetically most favourable process with an EA of only 5.3 kcal mol-1 at B3LYP/6-31G. 36 In addition this step is exothermic by 42-44kcalmol-1. Thermal decomposition of 68b to methyl formate 67b and nitrogen has an EA of only 2.9 kcal mol-1 and is exothermic by 95 kcal mol-1. Overall, the conversion of 82b to methyl formate 67b and two molecules of nitrogen is thus predicted to be exothermic by 137-139 kcalmol-1. 

The coupling of SR with POX is termed autothermal reforming (ATR). The exact definition varies. Some define ATR as an SR reaction and a POX reaction that take place over microscopic distances at the same catalytic site thus avoiding complex heat exchanging (16). Others have the less restrictive definition that ATR occurs when there is no wall between a combined SR reaction and catalytic POX reaction. ATR is carried out in the presence of a catalyst that controls the reaction pathways and thereby determines the relative extents of the POX and SR reactions. The SR reaction absorbs part of the heat generated by the POX process reaction, limiting the maximum temperature in the reactor. The net result is a slightly exothermic process. 

Why is no addition product observed in the gas phase, in contrast to solution This is not a case of no endothermic reactions both the proton transfer reaction (6b) and the alkoxide addition reaction (6a) are exothermic pathways. When an exothermic reaction occurs in solution, the excess energy is passed to the solvent. In the gas phase, with no solvent available, the excess energy remains in the intermediate. This can result in an effective internal temperature for that intermediate of hundreds to thousands of degrees. If there is some other bond that can be broken to yield a product ion plus a neutral in a pathway that is exothermic with respect to the reactants, the intermediate will fragment by that method, and the observed product will be that fragment ion. This internal temperature is the reason for the very short lifetime of the intermediates mentioned above. 

In the mechanism of Giunta et al. CH3SnCl20H, formed by exothermic reaction of CH3SnCl20 with CH4, H2CO, or HO2, is a key species whose decomposition leads to gas-phase SnO and Sn02 formation. A potentially important pathway to form CH3SnCl20H not considered by these authors is... 

Unimolecular decomposition of ClsSnOO is highly endothermic (reactions Eq. 61 and Eq. 62 for example), but exothermic decomposition pathways involving H-atoms exist (Eq. 64 and Eq. 65). Reaction with Cl atoms is en-... 

The details of protonation of several alkyl-substituted phenanthrenes by superacids have been reported.73 The observed mono- and di-cations are usually in agreement with those predicted by AMI MO calculations. Molecular modelling studies have suggested a multi-step pathway for the sulfonation of toluene widi sulfur trioxide.74 Intermediate 71-complcx. Wheland intermediate and pyrosulfonate species (34) are suggested, the product (p-toluenesulfonic acid) arising from an exothermic reaction between toluene and the acid (35) fonned by a facile prototropic rearrangement of (34). The sulfur trioxide monosulfonation of isopyrene and some derivatives leads usually to sulfonated... 

Researchers at BASF have shown that microreactors can be utilized that give access to operating conditions that cannot be realized by means of macroscopic equipment. They succeeded in improving yield and selectivity in a highly exothermal two-phase reaction in connection with the synthesis of a vitamin precursor. At Degussa company, a microreactor test facility for proprietary reactions is under construction. The major focus in this context is the implementation of microreaction devices as powerful tools for process development and, in particular, for the evaluation of new reaction pathways. 

The first array-based technique was designed specifically to study reactions on solid phase catalysts as IR thermography.9,19 This approach utilizes IR sensitive FPA detectors to measure the temperature of catalysts under reaction conditions. This approach has the advantages of a theoretical high thermal sensitivity, typically several tens of millikelvin, and the ability to study both endothermic and exothermic reactions. The main disadvantage of this approach, however, is the lack of chemical information. It must be assumed that the temperature change is associated entirely with the desired reaction pathway. The presence of unexpected side reactions will not be detected in this approach, as long as they have similar thermal behavior as the reaction under study. 

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