Rate of hydrogen exchange

Truong T N 1997 Thermal rates of hydrogen exchange of methane with zeolite a direct ab initio dynamics study on the importance of quantum tunneling effects J. Rhys. Chem. B 101 2750... 

Lewis acids have been found to increase the rate of hydrogen exchange in halogen acids, and thus iodine increases the rate of hydrogen exchange in hy-... 

A different approach to the problem of hydrocarbon acidity, and hence carbanion stability, is that of Shatenshtein and Shapiro, who treated hydrocarbons with deuterated potassium amide and measured the rates of hydrogen exchange. The experiments did not measure thermodynamic acidity, since rates were measured, not positions of equilibria. They measured kinetic acidity, that is, which compounds... 

The reduction in the selectivity of forming C2HgN+ as the alkyl chain length is increased reflects the enhanced rate of hydrogen exchange processes which allow a deuterium atom to be transferred to a position in which it is incorporated in the immonium ion. 

Hung, H. C., Hoberman, H. D. Influence of steric specificity on the rates of hydrogen exchange between substrates of NAD-coupled dehydrogenases. Biochem. Biophys. Res. Commun. 46, 399—405 (1972). 

It should be noted that the rate of racemization (or the rate of hydrogen exchange in Section 10.1.1) is exactly the same as the rate of enolization, since the reprotonation reaction is fast. Hence, the rate is typical of a bimolecular process and depends upon two variables, the concentration of carbonyl compound and the concentration of acid (or base). 

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... 

IV. Comparison of the Rate of Hydrogen Exchange in Ammonia and in Amphoteric Solvents... 

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... 

These amines are stronger bases than ammonia, and the rate of hydrogen exchange with them is higher. Methylamine stands out among the amines, possibly because its dielectric constant is higher than that of the remaining amines (for the effect of the dielectric... 

We can also compare the rate of hydrogen exchange in liquid ammonia and in anhydrous hydrazine, ethylenediamine, and ethanolamine. Some physical properties of these solvents are given below (/jl = dipole moment). 

The rate of hydrogen exchange in the aromatic nucleus of t-butyl-benzene is lower by a factor of 100 than that in the CH3 group of toluene. For comparison, the second line shows the relative rate constants determined by Streitwieser and Van Sickle (1962a) for catalysis by a 0-06n solution of lithium cyclohexylamide in cyclohexylamine (49-9°). 

The rate of hydrogen exchange in the toluene nucleus is lower than in benzene by an average factor of 3 while in the durene ring it is lower by a factor of 250 (Shatenshtein and Izrailevich, 1962). 

Using the fact that the rate of anionic polymerization of a-methyl-styrene, initiated by potassium amide in liquid ammonia, is smaller than the rate of hydrogen exchange, it was possible to establish (Shatenshtem et al., 1962b) that hydrogen-deuterium exchange occurs... 

The rate of hydrogen exchange with a base depends, as we know, on the proton mobility of the hydrogen atoms in aromatic CH bonds, which is very dependent on the inductive shift of cr-electrons in the carbon skeleton of the aromatic ring. In hydrogen exchange with an acid (just as in other electrophilic substitution reactions in compounds... 

Other measures of protein flexibility have been found to correlate with thermal stability. One is resistance to proteolysis (Daniel et al., 1982 Fontana, 1988). Another is the quenching of buried tryptophan fluorescence by acrylamide, used in a study by Varley and Pain (1991). Both these processes are mediated by the same combination of local and global unfolding events that determine rates of hydrogen exchange. Their rates will depend on the ability of another molecule, acrylamide or a proteolytic enzyme, to penetrate into normally buried regions of the protein in order to either quench fluorescence or cleave peptide bonds. 

Br nsted coefficients, aA, obtained by a plot of kinetic acidity dependence against thermodynamic acidity dependence and 03, obtained by a plot of rate of hydrogen exchange of the aromatic substrates against substrate pAa. 

Fig. 2.3. Idealized plot of log rate of hydrogen exchange vs. -Ha expected if exchange occurred only on the base and conjugate acid.

Figure 1 Rates of hydrogen exchange standardized to 100C and pH 0 (log/<0).

In many examples of base-catalyzed exchange, the protonated azole is attacked by hydroxide ion to form an ylide in the rate-determining step, e.g., for imidazole (Scheme 62). Deuteration of imidazole is fast at the 2-position and much slower at the 4- and 5-positions. Rates fall off for N-unsubstituted imidazoles at high pH values because of the formation of unreactive anions. In the case of 1-methylbenzimidazole, the rate of hydrogen exchange in the 2-position is independent of the acidity over a wide range, in agreement with the mechanism shown in Scheme 62. 

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