Merit site

The selectivity of an electrophile, measured by the extent to which it discriminated either between benzene and toluene, or between the meta- and ara-positions in toluene, was considered to be related to its reactivity. Thus, powerful electrophiles, of which the species operating in Friedel-Crafts alkylation reactions were considered to be examples, would be less able to distinguish between compounds and positions than a weakly electrophilic reagent. The ultimate electrophilic species would be entirely insensitive to the differences between compounds and positions, and would bring about reaction in the statistical ratio of the various sites for substitution available to it. The idea has gained wide acceptance that the electrophiles operative in reactions which have low selectivity factors Sf) or reaction constants (p+), are intrinsically more reactive than the effective electrophiles in reactions which have higher values of these parameters. However, there are several aspects of this supposed relationship which merit discussion. 

ControUed release pharmaceuticals provide local or systemic treatment and differ in design depending on thek use and route of administration. The potential sites for controUed dehvery of therapeutic agents and some of the systems designed for those sites merit discussion. 

At the beginning of the contract, before work starts on site, the main contractor will produce a program of how he intends to carry out the works. In some bids the contractor is invited to state what his program will be if he is successful. This method has merit where the program is very tight. The most common type of program is a... 

On-site treatment in batch reactors using refinery and wood-preserving waste added to soil resulted in highly variable loss of PAHs over a year (Aprill et al. 1990). An extensive study (Ellis et al. 1991) examined a number of design features, and the relative merits of on-site and off-site treatments of a... 

Furthermore, ir-arene complexes of transition metals are seldom formed by the direct reaction of benzene with metal complexes. More usually, the syntheses require the formation of (often unstable) metal aryl complexes and these are then converted to ir-arene complexes. The analogous formation of w-adsorbed benzene at a metal surface via the initial formation of ff-adsorbcd phenyl, merits more consideration than it has yet been given. It is to be hoped that the recognition and study of structure-sensitive reactions will allow more exact definition of the sites responsible for catalytic activity at metal surfaces. The reactions of benzene, using suitably labeled materials, may prove to be useful probes for such studies. 

We have intentionally selected example reactions (Figs. 29-33) that would not usually be immediately obvious to a chemist. The examples chosen have all been concerned with rearrangements of various types, since their courses are frequently difficult to predict. It remains to emphasize that the reactivity functions contained in EROS perform perfectly well with other types of reaction. This is true, for example, with reactions that a chemist could derive directly from an analysis of the functional groups in a molecule. Thus, EROS predicts addition reactions to carbonyl compounds, nucleophilic substitutions, and condensation reactions, to name just a few examples. In all these reaction types, the possibility of assigning a quantitative estimate to the reactivity at the various sites via the reactivity functions is of particular merit. It... 

The dissociation of water coordinated to exchangeable cations of clays results in Brtfnsted acidity. At low moisture content, the Brrfnsted sites may produce extreme acidities at the clay surface-As a result, acid-catalyzed reactions, such as hydrolysis, addition, elimination, and hydrogen exchange, are promoted. Base-catalyzed reactions are inhibited and neutral reactions are not influenced. Metal oxides and primary minerals can promote the oxidative polymerization of some substituted phenols to humic acid-like products, probably through OH radicals formed from the reaction between dissolved oxygen and Fe + sites in silicates. In general, clay minerals promote many of the reactions that also occur in homogenous acid or oxidant solutions. However, rates and selectivity may be different and difficult to predict under environmental conditions. This problem merits further study. 

The effect of [Pt(NH3)s ] on the [Fe(CN)g ] oxidation of parsley PCu(I) is more difficult to interpret, but merits further comment. At pH 7.5, with [Pt(NH3)6]" (and its conjugate base) present at concentrations up to 1.31 mM, rate constants increase by a factor of 2.5 [96]. In the case of the [Co(dipic)2] oxidation (dipic is pyridine-2,6-dicarboxylate) [95], a smaller 8% increase is observed, drawing attention to the importance of size of charge. There are a number of possible explanations. These include the effect of association of [Pt(NH3)e] on the net protein charge and hence its interaction with [Fe(CN)g ] . Association of [Pt(NH3)g ] at the remote acidic patch may lead to [Fe(CN)g ] " using this site to enhance the rate. A further contributing factor may be the quite separate association of [Pt(NH3)g] and [Fe(CN)6] to give an adduct which is more redox active. 

A third, clearer explanation of the electron transfer, proton translocation cycle is given by Saratse. Each ubiquinol (QH2) molecule can donate two electrons. A hrst QH2 electron is transferred along a high-potential chain to the [2Fe-2S] center of the ISP and then to cytochrome Ci. From the cytochrome Cl site, the electron is delivered to the attached, soluble cytochrome c in the intermembrane space. A second QH2 electron is transferred to the Qi site via the cytochrome b hemes, bL and bn. This is an electrogenic step driven by the potential difference between the two b hemes. This step creates part of the proton-motive force. After two QH2 molecules are oxidized at the Qo site, two electrons have been transferred to the Qi site (where one ubiquinone (Qio) can now be reduced, requiring two protons to be translocated from the matrix space). The net effect is a translocation of two protons for each electron transferred to cytochrome c. Each explanation of the cytochrome bci Q cycle has its merits and its proponents. The reader should consult the literature for updates in this ongoing research area. 

Fin I sited welds after H>siwcld licaE Erca [merit..X... 

---