Deuterium exchange reactions rate constants

Analogous parahydrogen conversion and deuterium exchange reactions, catalyzed by NH2, have been observed in liquid ammonia (Wilmarth and Dayton, 61). The kinetics are of the same form as those of the OH -cat-alyzed reaction in water and the mechanism is open to similar interpretations. The NH2 -catalyzed reaction is much faster, its rate constant at —50° being 10 times that of the OH -catalyzed reaction at 100°. The assumption of equal frequency factors for the two reactions leads to a calculated activation energy for the NH2 -catalyzed reaction of about 10 kcal. This low value has been attributed to the much greater base strength of NH2 relative to OH . The results provide some support for the hydride ion mechanism. 

The effects of small changes in the molecular structure can be observed in the case of the related diarylamidines which are the nitrogen analogs of formic acid and which represent models for nucleic acids. In tetrahydrofuran, for N,N -di-(p-F-phenyl)amidine (DFFA) three forms were observed by NMR, a solvated s-cis-form and a solvated s-trans-form which is in fast equilibrium with a cyclic dimer in which a HH-transfer takes place [24] as illustrated in Fig. 6.43. Fortunately, at low temperatures, the s-cis- and the s-trans-forms were in slow exchange. The rate constants of the HH, HD and DD reactions were determined by dynamic H and i F NMR as a function of concentration, deuterium fraction in the mobile proton sites and of temperature. The dependence of the observed rate constants of the s-trans-form on concentration is depicted in Fig. 6.44. The solid lines were calculated using Eq. (6.39) from which the rate constants in the dimer as well as the equilibrium constants of the dimer formation could be obtained. The Arrhenius... 

The kinetics of the protophilic dedeuteriation of thieno[2,3-6]-[2- H]- and -[3- H]-thiophen, thieno[3,2-6]-[2- H]- and -[3- H]-thiophen, and benzo[fc]-[2- H]- and -[3- H]-thiophen have been measured. The activation parameters for the deuterium-exchange reactions of the isomeric thienothiophens with t-butyl alcohol catalysed by potassium t-butoxide were determined. The relative deuterium-exchange rate constants for the 2-position in thiophen, the isomeric thienothiophens, and benzo[/>]thiophen were found to be 1 10 9 4, whereas the corresponding relative values for the 3-positions are 1 94 10 65. These partial rate factors were compared with those obtained upon acidic exchange and interpretation was attempted. ... 

As an example we discuss predictions for the rate constant of the proton-deuterium exchange reaction between deuterated methane and the zeolite cluster of Figure 6.17... 

The hydrogen-deuterium exchange H2 + D2 — 2HD is the simplest heterogeneous reaction taking place on the surface of semiconductors. This reaction has been thoroughly studied experimentally. It has been shown that under the influence of illumination (other conditions being constant) the rate of the reaction is considerably changed. 

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... 

Yakushin and Shatenshtein (1960) and Yakushin et al. (1959) have determined the ratio of the rate constants for tritium and deuterium replacement by protium in ammonia solutions of fluorene and methyl 2-naphthyl ketone at 25°. Yakushin has obtained data for these reactions in anhydrous methylamine. Streitwieser et al. (1960, 1962b) have measured the ratio fcD/ T for the hydrogen exchange of ethylbenzene and toluene catalysed by a solution of lithium cyclohexylamide in cyclohexylamine at 49-9°. 

Here, too, the kinetics show the rate of halogenation to be independent of halogen concentration, but dependent upon ketone concentration and, this time, acid concentration. Here, too, we find the remarkable identity of rate constants for apparently different reactions fpr bromination and iodination of acetone, and exchange of its hydrogens for deuterium for iodination and racemization of phenyl sec-butyl ketone. 

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