Unimolecular reactions, homogeneous

Decomposition and Decarboxylation. Maleic anhydride undergoes anaerobic thermal decomposition in the gas phase in a homogeneous unimolecular reaction to give carbon monoxide, carbon dioxide, and acetylene [74-86-2] in equimolar amounts. The endothermic... 

The homogeneous rate coefficient khomo is found to vary linearly with total pressure indicating that it is proportional to the rate coefficient of a unimolecular reaction in its second-order range. It is generally accepted that the initiating decomposition step is... 

The slow, thermal decomposition of hydrazoic acid in a static system has been studied by Meyer and Schumacher58. It turned out to be completely governed by heterogeneous catalysis. There are no studies on the kinetics of the homogeneous decomposition of this substance save for the investigation of its decomposition flame59. From the variation of flame properties with pressure it can be deduced that second-order reactions control the over-all rate. The unimolecular reaction... 

The thermal decomposition of F202 was studied by Schumacher and Frisch401 who found it to be a homogeneous, unimolecular reaction with the first-order rate coefficient given by... 

For temperatures in excess of 320 °C the gas-phase decomposition of anhydrous H0C103 (or of its dihydrate) is a homogeneous, unimolecular reaction close to its first-order limit at pressures in excess of 20 torr440. Levy440 obtained a value... 

For some time the thermal decomposition of phosphine at high temperatures was believed to be a homogeneous unimolecular reaction. It was studied by Trautz and Bhandarkar, who concluded that under the conditions of their experiments, namely in a 3-litre porcelain bulb, the reaction on the walls of the vessel was negligible above 945° abs., in comparison with the homogeneous reaction. 

Since the reactions, which are bimolecular when homogeneous, appear to be resolved into unimolecular reactions followed by a rapid combination of the free atoms, a somewhat indirect answer has to be given to the principal question, namely, the effect which the catalyst has on the energy of activation. 

It has been argued that insight into catalytic mechanisms can most readily be obtained by choosing reactions whose rate-determining step is unimolecular. The reactions best known to be unimolecular in homogeneous media are undoubtedly the solvolyses of tertiary butyl halides [161-163]. Their SN1 mechanisms are typified by the solvolysis of f-butyl bromide in an ethanol + water medium... 

Define the following expressions empirical method, metastable equilibrium, kindling temperature, thermostat, interface, dynamic steady state, unimolecular reaction, bimolecular reaction, homogeneous reaction, heterogeneous reaction. 

The thermal isomerization of cyclopropane into propene has been very extensively investigated from the experimental point of view, and a considerable amount of work has been done to apply the various theories of unimolecular reactions. The reaction rate is not influenced by the surface or by inhibitors such as nitric oxide or propene the process therefore appears to be a homogeneous unimolecular reaction. The main experimental results are summarized in Table 2. In this and... 

We prefer to explain the results of Herr and Noyes as well as those of Anderson and Rollefson on the pressure effect by the pressure dependence of the decomposition of the CH3CO radical. According to the Benson equation , based on the detailed theory of unimolecular reactions, the acetyl radical is in the fall-off region below a few atmospheres. O Neal and Benson demonstrated that the results, obtained in the absence of added foreign gases at low light intensities - , are in accordance with the assumption of a pressure-dependent decomposition and a homogeneous recombination of the acetyl radicals. 

For homogeneous unimolecular reactions, the reaction rate is proportional to the first power of the concentration of the species reacting. For a homogeneous bimolecular process, the observed reaction rate is the rate constant multiplied by the concentration of the activated complex, which is, in turn, proportional to the product of the partial pressures of the two reacting species. 

At low pH values, when additional protons are present, the separation step becomes reversible and one observes homogeneous proton recombination. The reaction under these conditions undergoes a transition from unimolecular (correlated pairs) to a bimolecular (or pseudo-unimolecular) reaction. The rate of this recombination reaction is expected to diminish with increasing concentration of inert salt, which screens the Coulombic attraction between the proton and the anion. In fact, the classical Bronsted-Bjerrum theory of salt effects puts all of the effect in the recombination reaction while predicting zero salt effect on the dissociation direction [7]. 

The kinetic description of ATRP has received considerable attention. There is a similarity between the descriptions of NMP and of ATRP. Both rely on reversible activation of dormant chains, albeit the activation in the case of NMP is a unimolecular reaction whereas in the case of ATRP it is a bimolecular reaction. If the reaction medium is homogeneous, ie activator and deactivator are soluble in the reaction mixture, the monomer consumption as a function of time can be described as follows. 

Central to catalysis is the notion of the catalytic site. It is defined as the catalytic center involved in the reaction steps, and, in Figure 8.1, is the molybdenum atom where the reactions take place. Since all catalytic centers are the same for molecular catalysts, the elementary steps are bimolecular or unimolecular steps with the same rate laws which characterize the homogeneous reactions in Chapter 7. However, if the reaction takes place in solution, the individual rate constants may depend on the nonreactive ligands and the solution composition in addition to temperature. 

Most of our knowledge about the kinetics of the homogeneous decomposition has come from shock-tube experiments. These have been performed in several laboratories under a variety of experimental conditions. However, their results are contradictory in some respects especially with regard to activation energy and on the question of the importance of chain reactions. In some cases the experimental conditions are such that consecutive reactions have to be taken into account or at least cannot be safely excluded. Until recently, one reason for the difficulty of reconciling the results of different investigators was that, if they were interpreted in terms of the unimolecular reaction48... 

The cis-trans isomerization of cyclopropanes is not restricted to the deuterium-substituted molecules, cis- and traws-l,2-Dimethylcyclo-propane have been shown to imdergo reversible geometrical isomerization as well as slower structural isomerization. All the processes are homogeneous and kinetically first order, and almost certainly unimolecular. The reaction scheme is shown below. 

Examples of unimolecular and termolecular homogeneous reactions will appear in subsequent chapters but the interpretation of the results yielded by kinetic studies of these reactions is much facilitated if the application of the theory of activation to the simpler example of bimolecular reactions is first considered in detail. 

This reaction also involves the elimination of carbon monoxide and the formation of a mixture of hydrocarbons, principally ethane and methane. It is homogeneous and conveniently measurable between 450° and 600° C. The decomposition is kinetically unimolecular over a considerable range of pressure, but at pressures below about 80 mm. Hg the velocity constant falls appreciably, in the manner which would be expected if Lindemann s theory were correct. In the region of pressure where the reaction is unimolecular the velocity constants (sec-1) are given by... 

D. F. Smith has shown that the racemization of pinene in the gaseous state, which can be observed by a polari-metric method, is a homogeneous reaction, and that it obeys the unimolecular law. Experiments were made... 

The reaction is homogeneous and unimolecular, with a heat of activation of 51,200 calories. 

The relation of homogeneous unimolecular changes to the corresponding catalytic reactions on surfaces will be discussed in a later section. 

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