Equilibrium displacement mechanism

The Nature of the Equilibrium Displacement Mechanism for the Pfeiffer Effect in Inorganic Chemistry... 

Figure 1 shows the Pfeiffer Effect for the racemic complex [Cr(C20/j,) 3] " in the presence of d-cinchoninium chloride, and this figure also provides strong support for the equilibrium displacement mechanism described above. It should be noticed that the optical rotatory dispersion (ORD) of the complex in the Pfeiffer Effect (Figure 1) is essentially the same as that of the pure enantiomer resolved by conventional means, and it shows a marked Cotton Effect. Since the environment substance itself shows only a plain or normal optical rotatory dispersion in the visible region, the ORD in the Pfeiffer experiment must be due to an excess of one enantiomer of the complex over the other - which is what is postulated to occur in the equilibrium displacement mechanism. 

In support of the equilibrium displacement mechanism, a study of the Pfeiffer rotation of the system D, L-[ Ni ( o-phen) 3] 2+ with levo-malic acid as an environment substance in water produces some interesting results (Figure 2). The Pfeiffer Effect reaches its maximum in just over k days, and if, at that time, exactly as much dextro-malic acid is added to the system as the levo enantiomer already present, then this has the effect of removing (deactivating) the environment substance, so that the excess of the complex enatiomer has no alternative but to undergo racemization. It should be noted (Figure 3) that the rate of this racemization is identical to the racemization rate of the optically pure complex which has been resolved by conventional means. 

Further support for the equilibrium displacement mechanism arises from a study of the Pfeiffer Effect and the absolute configurations of the environment substances vs. the absolute configurations of those enantiomers of the complexes which are enriched during the appearance of the Pfeiffer Effect. Table 1 shows the sign of the optical rotation of the enriched enantiomer of the complex as a function of the absolute configuration of the environment substance. 

During the work on this equilibrium displacement mechanism, a question arose about whether the shift in the equilibrium (and in the equilibrium constant for this shift) can be altered by changing the concentrations and/or concentration ratios of the complex to the environment substance (7). The equation for this originally-proposed equilibrium shift is ... 

Consequences of the Snyder and Soczewinski model are manifold, and their praetieal importance is very signifieant. The most speetaeular conclusions of this model are (1) a possibility to quantify adsorbents ehromatographic activity and (2) a possibility to dehne and quantify chromatographic polarity of solvents (known as the solvents elution strength). These two conclusions could only be drawn on the assumption as to the displacement mechanism of solute retention. An obvious necessity was to quantify the effect of displacement, which resulted in the following relationship for the thermodynamic equilibrium constant of adsorption, K,, in the case of an active chromatographic adsorbent and of the monocomponent eluent ... 

Figure 7. Using a theoretically determined equilibrium fractionation to interpret measured isotopic fractionations in a hypothetical mineral-solution system. Three sets of data are shown. The theoretical equilibrium fractionation for this system is indicated by the gray arrow. The first set of data, indicated by circles, closely follow the calculated fractionation, suggesting a batch equilibrium fractionation mechanism. The second set of data (stars) is displaced from the theoretical curve. This may either indicate a temperature-independent kinetic fractionation superimposed on an equilibrium-like fractionation, or that the theoretical calculation is somewhat inaccurate. The third set of data (crosses) shows much greater temperature sensitivity than the equilibrium calculation this provides evidence for a dominantly non-equilibrium fractionation mechanism. For the first data set, the theoretical fractionation curve can be used to extrapolate beyond the measured temperature range. The second data set can also be extrapolated along a scaled theoretical curve (Clayton and Kieffer 1991).

This mechanism clearly implicated alkane complexes as precursors to C-H activation but the IR absorptions of [Cp Rh(CO)Kr] and [Cp Rh(CO)(C6Hi2)] were not resolved and were presumed to be coincident. The temperature dependent data gave values of AH = 18 (or 22) kj mol for the unimolecular C-H (or C-D) activation step representing a normal kinetic isotope effect, kn/fco 10- However, an inverse equilibrium isotope effect (K /Kq 0.1) was found for the slightly exothermic pre-equilibrium displacement of Kr by CoHn/C Dn implying that C6Dj2 binds more strongly to the rhodium center than does C Hn-... 

Cellulase was found to be effective in the synthesis of artificial cellulose in a single-step reaction by polycondensation of /J-D-cellobiosyl fluoride (Scheme 13).123 The polymerization is a repetition of the transglycosylation reaction, which became predominant over the hydrolysis reaction when the enzymatic polycondensation was carried out in a mixed solvent of acetonitrile/acetate buffer (5 1, pH 5). This synthesis is therefore kinetically controlled as well as equilibrium controlled. The fi configuration of the Cl fluorine atom is necessary to form a reactive intermediate leading to a / (1—4) product via a double displacement mechanism .124 Thus, this method provided the first successful in vitro synthesis of cellulose, the most abundant biomacromolecules on the earth, the synthesis of which had been unsolved for one-half a century.123... 

Mechanistic interpretations The results of the dynamic and equilibrium displacement experiments are used to evaluate and further define mechanisms by which alkaline floods increase the displacement and recovery of acidic oil in secondary mode and the tertiary mode floods. The data sets used in the mechanistic interpretations of alkaline floods are (a) overall and incremental recovery efficiencies from dynamic and equilibrium displacement experiments, (b) production and effluent concentration profiles from dynamic displacement experiments, (c) capillary pressure as a function of saturation curves and conditions of wettability from equilibrium displacement experiments, (d) interfacial tension reduction and contact angle alteration after contact of aqueous alkali with acidic oil and, (e) emulsion type, stability, size and mode of formation. These data sets are used to interpret the results of the partially scaled dynamic experiments in terms of two-stage phase alteration mechanisms of emulsification followed by entrapment, entrainment, degrees and states of wettability alteration or coalescence. 

Acyl-pyrroles, -furans and -thiophenes in general have a similar pattern of reactivity to benzenoid ketones. Acyl groups in 2,5-disubstituted derivatives are sometimes displaced during the course of electrophilic substitution reactions. iV-Alkyl-2-acylpyrroles are converted by strong anhydrous acid to A-alkyl-3-acylpyrroles. Similar treatment of N-unsubstituted 2- or 3-acyIpyrroles yields an equilibrium mixture of 2- and 3-acylpyrroles pyrrolecarbaldehydes also afford isomeric mixtures 81JOC839). The probable mechanism of these rearrangements is shown in Scheme 65. A similar mechanism has been proposed for the isomerization of acetylindoles. 

Thus far we have discussed the direct mechanism of dissipation, when the reaction coordinate is coupled directly to the continuous spectrum of the bath degrees of freedom. For chemical reactions this situation is rather rare, since low-frequency acoustic phonon modes have much larger wavelengths than the size of the reaction complex, and so they cannot cause a considerable relative displacement of the reactants. The direct mechanism may play an essential role in long-distance electron transfer in dielectric media, when the reorganization energy is created by displacement of equilibrium positions of low-frequency polarization phonons. Another cause of friction may be anharmonicity of solids which leads to multiphonon processes. In particular, the Raman processes may provide small energy losses. 

If the antagonism is insurmountable, then there are a number of molecular mechanisms possible. The next question to ask is if the maximal response to the agonist can be completely depressed to basal levels. If this is not the case, then there could be partial allosteric alteration of the signaling properties of the receptor. Alternatively, this could be due to a hemi-equilibrium condition that produces a partial shortfall to true competitive equilibrium, leading to incomplete depression of the maximal response but also antagonist concentration-related dextral displacement of the concentration response curve to the agonist (see Figure 10.19a). The model (see Section 10.6.5) used to fit these data is discussed in Section 6.5 and shown in... 

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