Competion ratios determination

The product ratio is therefore determined not by AG but by the relative energy of the two transition states A and B. Although the rate of the formation of the products is dependent upon the relative concentration of the two conformers, since AGJ is decreased relative to AG to the extent of the difference in the two conformational energies, the conformational preequilibrium is established rapidly, relative to the two competing... 

It is possible to measure equilibrium constants and heats of reaction in the gas phase by using mass spectrometers of special configuration. With proton-transfer reactions, for example, the equilibrium constant can be determined by measuring the ratio of two reactant species competing for protons. Table 4.13 compares of phenol ionizations. 

In hydroxyUc solvents, the reaction with aniline follows a bi-molecular course but is complicated by competing solvolysis. This is a striking result when compared with the behavior of picryl chloride, which is much more selective with regard to the same reagents (aniline and alcohol), and has been interpreted to mean that bond-breaking has made appreciable progress in the rate-determining step of the reaction of phosphonitrilic chloride. Furthermore, the same indication is obtained from the fact that in the reactions of the halides, the fluorine chlorine ratios are less than one. ... 

It should be noted that positional selectivity is never complete even when a clean reaction gives only one isolated product.Reaction occurs at all positions in proportion to the ratio of the rate constants. The difference between a clean reaction (e.g., rate 9 times that of a competing reaction) and one giving a troublesome mixture can be merely a moderate quantitative increase in one rate (e.g., to a 9 7 rate ratio) or a change in both rates (e.g., to a 3 4 ratio). Work such as that of Kauffmann and Boettcher on heteroarynes illustrates the potential of modern forms of chromatography for determining the true proportion of even very minor products. 

If, for the purpose of comparison of substrate reactivities, we use the method of competitive reactions we are faced with the problem of whether the reactivities in a certain series of reactants (i.e. selectivities) should be characterized by the ratio of their rates measured separately [relations (12) and (13)], or whether they should be expressed by the rates measured during simultaneous transformation of two compounds which thus compete in adsorption for the free surface of the catalyst [relations (14) and (15)]. How these two definitions of reactivity may differ from one another will be shown later by the example of competitive hydrogenation of alkylphenols (Section IV.E, p. 42). This may also be demonstrated by the classical example of hydrogenation of aromatic hydrocarbons on Raney nickel (48). In this case, the constants obtained by separate measurements of reaction rates for individual compounds lead to the reactivity order which is different from the order found on the basis of factor S, determined by the method of competitive reactions (Table II). Other examples of the change of reactivity, which may even result in the selective reaction of a strongly adsorbed reactant in competitive reactions (49, 50) have already been discussed (see p. 12). 

If there is a series of related reactants A each of which competes with A2 for B, the rate constant for each member can be evaluated analogously. If k2 is known, all of the kj s can be determined. If k2 is unknown, then the experiments yield only the ratios kjk2. Assigning (relative) = 1, the full pattern of reactivity of the A s toward B can be determined quantitatively. 

In this section indirect methods will be explored. One was given in Section 3.6. There, data were presented for two members of a large family of reactions of the phenyl radical with organic halides. Pairs of halides were used, and the ratio of two competing products was determined. Expanding the general scheme of Eq. (5-1) further, to use a pair of competing traps, we have... 

Since both reactions form phenol, Bunnett coined the phrase ad eundem competition, from the Latin, meaning (leading) to the same (product, singular). As such, a product ratio cannot allow the determination of h2oAoh However, if one adds thiophenoxide ion, PhS-, as a third competing reagent for benzyne, then diphenyl sulfide is formed,... 

The competing pathways to radical or carbenium ion derived products are determined, apart from experimental factors (see chap. 2), by the ionization potential of the radical. From product ratios and ionization potentials of the intermediate radicals, the conclusion could be drawn that such radicals with ionization potentials above 8 eV lead preferentially to coupling products, whilst those with ionization potentials below 8 eV are further oxidized to carbenium ions [8 c]. 

Thus, from simultaneous measurements of the unreacted fractions of the two competing species, one may readily determine the ratio of reaction rate constants. 

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

The values of ks/kp for partitioning of carbocations are most conveniently determined as the ratio of the yields of products from the competing nucleophile addition and proton transfer reactions (equation 1 derived for Scheme 2). The determination of these product yields has been simplified in recent years by the application of high-pressure liquid chromatography (HPLC). Typically, the product peaks from an HPLC analysis are detected and quantified by UV-vis spectroscopy. In cases where the absorbance of reactants and products is small, substrates may be prepared with a chromophore placed at a sufficient distance so that its effects on the intrinsic reactivity of the carbocationic center are negligible. For example, the aliphatic substrates [1]-Y have proved to be very useful in studies of the reactions of the model tertiary carbocation [1+].21,23... 

The simplest way to carry out a cyclisation reaction is one where the bifunctional reactant M is charged over a relatively short time into the reaction medium, where proper reaction conditions are set. Here short time means that the reaction proceeds to a negligible extent during the addition time. In the absence of competing reactions other than polymerisation, the outcome of a batchwise cyclisation experiment is almost entirely determined by the ratio between [M]0 and C, as shown by the plots reported in Fig. 1. 

Few experiments have attempted to follow variations in the rate constant, A nt. As discussed earlier, most experiments measure steady-state fluxes, and thus cannot distinguish a larger rate constant (kmt) from an increased number of carriers ( M — Rceii ). When metals compete for identical transport sites, it is possible to determine relative k-mi values from the ratio of the maximum uptake... 

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