Decomposition, quantitative reaction kinetics

The automation of the single dynamic DSC scan approach has provided an accurate, time efficient, routine method for obtaining quantitative reaction kinetics information for decomposition, polymerization and curing reactions. 

On the first part of this research, Advanced Chemical Oxidation, a quantitative estimation of direct ozonation and indirect free radical oxidation of dyes with assorted chromophores was studied through the examination of reaction kinetics in the ozonation process. The reaction kinetics of dye ozonation under different conditions was determined by adjusting the ozone doses, dye concentration, and reaction pH. The ozonation of dyes was found dominant by pseudo first-order reaction, and the rate constants decreased as the dye/ozone ratio increased. For all selected azo dyes, the dye decay rates increased as the initial pH of the solution increased, yet the decay rates of anthraquinone dyes would decrease in the same situation because of their insensible structure for ozone oxidation, formation of leuco-form, and higher solubility at a lower pH. The ozonation of dyes at a high pH contributed by hydroxyl free radicals was qualitatively verified by the use of a free radical scavenger. A proposed model, in another way, quantitatively determines the fraction of contribution for dye decomposition between free radical oxidation and direct ozonation. 

Both reaction kinetics and chemical equilibria of the formation/decomposition of 1,3,5-trioxane in aqueous formaldehyde solutions were studied by quantitative H NMR spectroscopy <2006MI910> a virtual reference signal of high stability was generated electronically and employed for the quantification of the small 1,3,5-trioxane proton signal. 

The problem was solved quantitatively by the decomposition of the IR optical stretching mode of the three-atom chain by model calculations taking the possible compositions and the frequency shift depending on the mass distribution in natural and isotope-enriched boron carbide into account (57). The determined concentrations of B12 and BnC icosahedra and C—B—C and C—B—B chains are shown in Fig. 26. Other chain compositions can be excluded. Toward the boron-rich limit of the homogeneity range, an increasing number of unit cells without chains arise. Two alternative models are compatible with the optical spectra completely chain-free unit cells and unit cells in which single B atoms saturate the outer bonds of the equatorial atoms of the adjacent icosahedra. Theoretical calculations of reaction kinetics based on the second version (132) satisfactorily confirm the results in Fig. 26. 

Some nitrate is also formed, thus the HOCl/NH stoichiometry is greater than theoretical, ie, - 1.7. This reaction, commonly called breakpoint chlorination, involves intermediate formation of unstable dichloramine and has been modeled kinetically (28). Hypobromous acid also oxidizes ammonia via the breakpoint reaction (29). The reaction is virtually quantitative in the presence of excess HOBr. In the case of chlorine, Htde or no decomposition of NH occurs until essentially complete conversion to monochloramine. In contrast, oxidation of NH commences immediately with HOBr because equihbrium concentrations of NH2Br and NHBr2 are formed initially. As a result, the typical hump in the breakpoint curve is much lower than in the case of chlorine. 

Measurements of overall reaction rates (of product formation or of reactant consumption) do not necessarily provide sufficient information to describe completely and unambiguously the kinetics of the constituent steps of a composite rate process. A nucleation and growth reaction, for example, is composed of the interlinked but distinct and different changes which lead to the initial generation and to the subsequent advance of the reaction interface. Quantitative kinetic analysis of yield—time data does not always lead to a unique reaction model but, in favourable systems, the rate parameters, considered with reference to quantitative microscopic measurements, can be identified with specific nucleation and growth steps. Microscopic examinations provide positive evidence for interpretation of shapes of fractional decomposition (a)—time curves. In reactions of solids, it is often convenient to consider separately the geometry of interface development and the chemical changes which occur within that zone of locally enhanced reactivity. 

The Avrami—Erofe ev equation, eqn. (6), has been successfully used in kinetic analyses of many solid phase decomposition reactions examples are given in Chaps. 4 and 5. For no substance, however, has this expression been more comprehensively applied than in the decomposition of ammonium perchlorate. The value of n for the low temperature reaction of large crystals [268] is reduced at a 0.2 from 4 to 3, corresponding to the completion of nucleation. More recently, the same rate process has been the subject of a particularly detailed and rigorous re-analysis by Jacobs and Ng [452] who used a computer to optimize curve fitting. The main reaction (0.01 < a < 1.0) was well described by the exact Avrami equation, eqn. (4), and kinetic interpretation also included an examination of the rates of development and of multiplication of nuclei during the induction period (a < 0.01). The complete kinetic expressions required to describe quantitatively the overall reaction required a total of ten parameters. 

It is apparent, from the above short survey, that kinetic studies have been restricted to the decomposition of a relatively few coordination compounds and some are largely qualitative or semi-quantitative in character. Estimations of thermal stabilities, or sometimes the relative stabilities within sequences of related salts, are often made for consideration within a wider context of the structures and/or properties of coordination compounds. However, it cannot be expected that the uncritical acceptance of such parameters as the decomposition temperature, the activation energy, and/or the reaction enthalpy will necessarily give information of fundamental significance. There is always uncertainty in the reliability of kinetic information obtained from non-isothermal measurements. Concepts derived from studies of homogeneous reactions of coordination compounds have often been transferred, sometimes without examination of possible implications, to the interpretation of heterogeneous behaviour. Important characteristic features of heterogeneous rate processes, such as the influence of defects and other types of imperfection, have not been accorded sufficient attention. 

The trialkyltrlazenes are essentially protected diazo-nlum ions. They decompose cleanly and quantitatively to the dlazonlum ions and the corresponding amines over a wide pH range (M). Good kinetic data were obtained over the range of pH 6.9 - 8.3. In more acid solutions, the reactions are too rapid to measure by conventional kinetics. The decomposition reaction is subject to general acid catalysis. Thus, the trialkyltrlazenes will be a useful tool for the study of the reactive intermediates produced by the metabolism of dialkyl-nitrosamines. 

Because of the operating principles of the equipment, especially in the isoperibolic mode, complex calculation and calibration procedures are required for the determination of quantitative kinetic parameters and the energy release during decomposition. Also, for a reaction with a heterogeneous mixture such as a two-phase system, there may be mass transfer limitations which could lead to an incorrect T0 determination. 

Schindler and coworkers verified the formation of hydroxyl radicals kinetically and further RRKM calculations by Cremer and coworkers placed the overall concept on a more quantitative basis by verifying the measured amount of OH radical. An extensive series of calculations on substituted alkenes placed this overall decomposition mechanism and the involvement of carbonyl oxides in the ozonolysis of alkenes on a firm theoretical basis. The prodnction of OH radicals in solution phase was also snggested on the basis of a series of DFT calculations . Interestingly, both experiment and theory support a concerted [4 4- 2] cycloaddition for the ozone-acetylene reaction rather than a nonconcerted reaction involving biradical intermediates . 

The aminolysis of dibenzo[l,2]oxathiin 6-oxide 10 with primary and secondary amines in water was quantitively followed by the absorption at 270 nm in UV spectroscopy, from which the reaction was found to obey pseudo-first-order kinetics < 1999TL8901 >. Because of the lack of a distinct difference in the magnitude of y obs between primary and secondary amines, and between acyclic and cyclic amines, the aminolysis reaction must proceed in two steps the first is a fast formation of intermediate 102 followed by a second slow decomposition step to the reaction product 103 (Scheme 24). 

In a nutshell, it may be concluded that DTA, DSC and TGA have been used mainly to determine the thermal properties of explosives like melting points, thermal stability, kinetics of thermal decomposition and temperatures of initiation and ignition etc. Further, the properties which can be calculated quantitatively from the experimentally obtained values are reaction rates, activation energies and heats of explosion. DTA data of some explosives are given [46] in Table 3.6. 

Two excellent reviews <71AHC(13)235, 72IJS(C)(7)6l) have dealt with quantitative aspects of electrophilic substitution on thiophenes. Electrophilic substitution in the thiophene ring appears to proceed in most cases by a mechanism similar to that for the homocyclic benzene substrates. The first step involves the formation of a cr-complex, which is rate determining in most reactions in a few cases the decomposition of this intermediate may be rate determining. Evidence for the similarity of mechanism in the thiophene and benzene series stems from detailed kinetic studies. Thus in protodetritiation of thiophene derivatives in aqueous sulfuric and perchloric acids, a linear correlation between log k and —Ho has been established the slopes are very close to those reported for hydrogen exchanges in benzene derivatives. Likewise, the kinetic profile of the reaction of thiophene derivatives with bromine in acetic acid in the dark is the same as for bromination of benzene derivatives. The activation enthalpies and entropies for bromination of thiophene and mesitylene are very similar. 

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