Other Common Mechanisms

Many inorganic mechanisms involve pre-equilibria (Section 3.22). 

A mathematical analysis of such mechanisms shows that simple kinetics is only found when the equilibrium is set up rapidly and maintained throughout the reaction. In all other circumstances, complex kinetics would be expected, and computer analysis of the experimental data would be required. Fortunately, for most inorganic mechanisms the equilibrium is set up rapidly and maintained, and simple kinetics is observed. 

The situations discussed in this book are for reactions where the pre-equilibrium is set up rapidly and maintained. 

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Two other common mechanisms of resistance are known. Some cases of resistance of aerobic gramnegative bacilli to streptomycin are due to mutations in the proteins of the bacterial ribosomes. Streptococci, staphylococci, and Pseudomonadaceae resist aminoglycosides as a result of decreased transport of the aminoglycosides into the bacterial cytosol. 

The other common mechanism for substitution at saturated carbon, SnI (3.81a) also has its analogue in phosphorus chemistry. Moreover it is generally believed that, in the case of both elements, substitution reactions intermediate in mechanism between SnI and Sn2 may sometimes take place. In carbon chemistry, the SnI mechanism involves an intermediate planar carbonium ion. Since the nucleophilic entering group may attack either face of the planar carbonium ion with equal probability, a racemic mixture is expected to be obtained. In practice this is not always achieved completely, because the nucleophile may have attacked before the carbonium ion was produced. 

The major mechanism for moisture penetration into composite materials is diffusion, mainly direct diffusion of water molecules into the matrix and fibers. The other common mechanisms are capillary flow along the fiber/matrix interface, followed by diffusion from the interface into the bulk resin and fibers, and transport by microcracks. Capillary flow is mostly active only after debonding between fiber and matrix has occurred whereas transport of moisture by microcracks involves both flow and... 

Pentoxifylline is stmcturaHy related to other methylxanthine derivatives such as caffeine [58-02-2] (1,3,7-trimethylxanthine), theobromine [83-67-0] (3,7-dimethylxanthine), and theophylline [58-55-9] (3,7-dihydro-1,3-dimethyl-1 H-piirine-2,6-dione or 1,3-dimethylxanthine), which also show radioprotective activity in some instances, suggesting that methylxanthines as a dmg class may radioprotect through a common mechanism (see Alkaloids). In a retrospective analysis of cervical and endometrial cancer patients receiving primary or adjuvant XRT, no association between caffeine consumption and incidence of acute radiation effects has been found. However, there was a decreased incidence of severe late radiation injury in cervical cancer patients who consumed higher levels of caffeine at the time of thek XRT (121). The observed lack of correlation between caffeine consumption and acute radiation effects is consistent with laboratory investigations using pentoxifylline. 

The basic constituents of all commercial emulsion polymerization recipes are monomers, emulsifiers, and polymerization initiators. Other common components are modifiers, inorganic salts and free alkaH, and shortstops. The function of these different components and the mechanism of emulsion polymerization have been described (43,44). 

Perhaps more so than any other common metal, the mechanical properties of chromium (8,14—17) depend on purity, history, grain size, strain rate. ... 

There are two general theories of the stabUity of lyophobic coUoids, or, more precisely, two general mechanisms controlling the dispersion and flocculation of these coUoids. Both theories regard adsorption of dissolved species as a key process in stabilization. However, one theory is based on a consideration of ionic forces near the interface, whereas the other is based on steric forces. The two theories complement each other and are in no sense contradictory. In some systems, one mechanism may be predominant, and in others both mechanisms may operate simultaneously. The fundamental kinetic considerations common to both theories are based on Smoluchowski s classical theory of the coagulation of coUoids. 

However, it is known that the direct correlation functions have an exact long-range asymptotic form, arising due to intramolecular correlations in clusters formed via the association mechanism. This asymptotics is not included in the Percus-Yevick approximation. Other common liquid state approximations also do not provide correct asymptotic behavior of Ca ir). 

Not all the reactions in this chapter are actually nucleophilic substitutions. In some cases the mechanisms are not known with enough certainty even to decide whether a nucleophile, an electrophile, or a free radical is attacking. In other cases (such as 10-79), conversion of one compound to another can occur by two or even all three of these possibilities, depending on the reagent and the reaction conditions. However, one or more of the nucleophilic mechanisms previously discussed do hold for the overwhelming majority of the reactions in this chapter. For the alkylations, the Sn2 is by far the most common mechanism, as long as R is primary or secondary alkyl. For the acylations, the tetrahedral mechanism is the most common. 

Having in mind the various forms of the isokinetic relationship, we can also show its physical meaning in kinetics more clearly. Let us consider a reaction series with a variable substituent, solvent, or other factor. The term reaction series was discussed by Bunnett (14), with the conclusion that the common mechanism of all reactions is a necessary condition (12). However, this condition can seldom be ascertained, and best after finishing the whole analysis. At the beginning, it may be sufficient that the reaction products are invariable and the kinetic order equal. In addition, the structural changes should not be too large of course, this condition cannot be defined precisely. 

The isokinetic relationship can further yield a preliminary test of a common mechanism i.e., when one reaction deviates from the others, it follows to a high probability that its mechanism is different. The deviations are best seen in a plot like Figure 4, while in Figure 5, it is difficult to decide which of the straight lines does go through the common point of intersection. 

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