Chain reactions surface effects

Certain constituents when added to the reaction mixture, slow down the rate of reaction. This phenomena is called inhibition and constituent called inhibitor. Such an effect is similar to the negative catalysis. But the constituent usually undergoes chemical change, inhibition is the preferred term. Inhibition may occur in chain reactions, enzyme catalysed reactions, surface reactions or many reversible or irreversible reactions. A trace amount of an inhibitor may cause a marked decrease in the rate of reaction. The inhibitor sometimes combines with a catalyst and prevents it from catalyzing the reaction. 

Taking into account that ROS produced by irradiated fullerenes C60 may act only in the radius of their short diffusion existence, one may suppose that cytotoxic effect is determined by the interaction of fullerene C60 with the surface of cells and initiation of chain reactions of free radical peroxidation in membranes. That is why the influence of photoexcited fullerene C60 on the course of LPO process was studied and evaluated by the content of generated primary (diene conjugates) and final (Schiff bases) products. The content of diene conjugates in thymocytes was 17.7 4.2, inEAC cells was 21.1 1.3, andinL1210 was 12.8 3.1 nM/mg protein, and Schiff bases -56 7.9,46.5 4.5, and 36.6 4.6 rel. units/mg protein, respectively, and did not alter during 1 h incubation of the cells. 

Wall termination reactions immediately introduce a complexity to all chain reactions, namely, that the overall reaction rate can be a strong function of the size of the reactor. In a small reactor where the surface-to-volume ratio is large, termination reactions on surfaces can keep the radical intermediate pool small and thus strongly inhibit chain reactions (nothing appears to happen), while in a large reactor the surface-to-volume ratio is smaller so that the termination rate is smaller and the effective rate increases by a large factor (and the process takes oft). 

Results of these investigations demonstrate that changes of the reactor surface can be an effective method for directing chemical reactions. Thus, developing a theory of how heterogeneous factors influence liquid-phase chain reactions is one of the important lines of advancement in this area. Only a few years ago it was thought, almost a priori, that there are practically no heterogeneous factors in liquid-phase oxidation and that liquid-phase processes differ from vapor-phase processes in this respect. 

Coke formation is considered, with just cause, to be a malignant side reaction of normal carbonium ions. However, while chain reactions dominate events occurring on the surface, and produce the majority of products, certain less desirable bimolecular events have a finite chance of involving the same carbonium ions in a bimolecular interaction with one another. Of these reactions, most will produce a paraffin and leave carbene/carboid-type species on the surface. This carbene/carboid-type species can produce other products but the most damaging product will be one which remains on the catalyst surface and cannot be desorbed and results in the formation of coke, or remains in a noncoke form but effectively blocks the active sites of the catalyst. 

J. H. S. Green et al., J. Chem. Phys., 21, 178 (1953). These authors claimed to suppress chain reactions in EtBr, w-PrBr, w-BuBr, and f-BuBr by use of cyclohexene. It is not clear whether or not such suppression is completely effective or whether heterogeneous reaction on the glass surface persists. See also J. Chem. Soc., 2455 (1955). 

Surface Initiation or Termination, The surface acts to initiate or terminate radicals or ions which diffuse out into the homogeneous phase, gas or liquid, where a chain reaction lakes place. The behavior is certainly characteristic of the activity of glass and quartz surfaces in a great many chain reactions, particularly gas-phase oxidations such as H2 + O2. When initiation takes place rapidly at the surface, it is likely that termination is also effective there. Such systems can be recognized by the fact that they react at specific rates which are nearly independent of the surface/volume ratio, i.e., zero order with respect to surface or catalyst. 

Using mass-spectrometric techniques Vreeland and Swinehart studied the decomposition down to pressures below 10 torr. They found the normal pressure fall-off down to about 10 torr, but at lower pressures the rate coefficients were abnormally high and were essentially independent of pressure. By studying this phenomenon in packed vessels they concluded that the reaction was not becoming a surface process at these pressures, and that it occurs by two competing mechanisms. It is possible that at low pressures a chain reaction becomes important, the surface bringing about initiation and termination so that there is no overall surface effect further experimental work is needed to test this possibility. 

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