Hydrogen exchange constants

The solubility of hydrogen chloride in solutions of aromatic hydrocarbons in toluene and in w-heptane at —78-51 °C has been measured, and equilibrium constants for Tr-complex formation evaluated. Substituent effects follow the pattern outlined above (table 6.2). In contrast to (T-complexes, these 7r-complexes are colourless and non-conducting, and do not take part in hydrogen exchange. 

Hammett and Taft constants, S, 107 homolytic substitution, S, 5 hydrogen exchange, S, 57, 69-70 hydroxy... 

The rate coefficients obtained with hydrogen iodide show that the effectiveness of halogen acids in catalysing hydrogen exchange is HF > HBr > HI and, as noted above, this was attributed to the difference in dielectric constant of the media. 

A final, rather different example which fits in appropriately here, in that it involves hydrogen exchange, is the measurement of equilibrium and rate constants for the base-catalyzed isomerization of unsaturated sulfides, sulfoxides and sulfones193 ... 

Addition of 81-SH to 80-SS-81 led to formation of the homodisulfide compounds and an equilibrium, with an exchange constant of 1.8, was established. The presence of the templating (D)Pro(L)Val(D)Val tri-peptide in this mixture, shifted the equilibrium dramatically and the formation of the homodisulfide 80-SS-80 was amplified with a Keq=32. Since the templating tri-peptide was supported on polymer beads, the isolation of receptor 80-SS-80 (in 97% purity) was achieved easily by extraction of the beads. The formation of multiple hydrogen bonds between the template and the components of the DCL, led to the isolation of the best possible receptor available from the building blocks present in the equilibrated mixture. 

Fig. 2. Plot of normalized rate vs. the activity of silicic acid for the LAWABP1 (see Table 1) glass composition at two temperatures (26 and 40 °C). Rates are all computed at steady-state conditions. Boron and Na release rates are identical at low silica activities, then decrease, and become constant at or near saturation with respect to amorphous silica (vertical dot-dashed line). Note that the B rate decreases more than the Na rate. This behaviour can be rationalized as competition between two concurrent reactions alkali-hydrogen exchange and matrix dissolution (see text). Error bars represent 2-

The hydrogen ion probably falls, for a corresponding concentration, between Li+ and Ba++, and the ions of metals such as Ag, Cu, Zn, Pb, and Hi, or the rare earths, have a much stronger catalytic effect (6 ) which is further increased if the dielectric constant of the solvent is decreased by adding organic liquids such as acetone or dioxane 323). The degrees of dissociation (Table XII) and the exchange constants (Table XV) of the cations are also related to their catalytic activity (cf. Section IV,E,/,d). 

An estimate of a lower limit for the equilibrium constant for the formation of adducts from diazines and NH2 (K) has been obtained from the rate of hydrogen exchange of ammonia and from the NMR data. 9 The K value is found to be no smaller than 10s M. Pyrimidines bearing such groups as R2N or phenyl at position 2 display the same behavior as the parent compound and undergo attachment at position 4. The corresponding adducts have been characterized by C-NMR.90... 

Because of the apparent power of exchange at equilibrium to measure fluctuations in structure, there have been many attempts to use the procedure to determine pathways. We know from section Cl that this is a futile activity equilibrium measurements just give the relative thermodynamic properties of intermediates and not the pathway between them.53 The value of A Gex, the equilibrium constant between particular open and closed states of a protein derived from hydrogen exchange at equilibrium, is just such a thermodynamic measurement and does not give information about when that state is formed on a pathway. For example, con-... 

From (6.44), we see that in order to obtain a selective membrane, the value of the ion-exchange constant must be small and the sodium ion mobility in the hydrated layer relative to that of the hydrogen ion must also be small. The expansion of the selectivity coefficient to include selectivity to other ions involves inclusion of more complex ion-exchange equilibria, and the use of a more complex form of the Nernst-Planck equation. This rapidly leads to intractable algebra that requires numerical solution (Franceschetti et al 1991 Kucza et al 2006). Nevertheless, the concept of the physical origin of the selectivity coefficient remains the same. Electrochemical impedance spectroscopy has been successfully used in analysis of the ISE function (Gabrielli et al 2004). 

Hydrogen Exchange in Liquid Ammonia and Ethanol at 120° and Ionization Constants of Organic Compounds... 

The rate of hydrogen exchange depends on the protolytic properties of both the solvent and the substrate. In fact there is a correspondence between the magnitude of the rate constants for deuterium exchange with ND3 and the conventional ionization constants of hydrocarbons which were used by Conant and Wheland (1932) and by McEwen (1936) to obtain the first quantitative estimates of the acidity of hydrocarbons. To do this, they determined the equilibrium of metallation of hydrocarbons by organo-alkali metal compounds. This reaction was described by Shorygin (1910) and is represented by the equation... 

Rate Constants for Hydrogen Exchange with Liquid Ammonia and a Solution of Potassium Ethoxide in Ethanol... 

Alkoxides are very much stronger bases in anhydrous ethylenediamine than in alcohols (ethanol and ethanolamine). This can be seen from a comparison of the rate constants for hydrogen exchange in the CD group of triphenylmethane (Table 3) (Shatenshtein and Yakovleva, 1958). 

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