Protium-deuterium exchange

In Figure 7, the labelled internal standards used for the quantification of eight selected key sesame odorants identified on the basis of AEDA results [46, 52] are shown. Most of them were labelled with deuterium since their preparation was relatively inexpensive. In the case of 4-hydroxy-2,5-dimethyl-3(2H)-furanone, the labelling had to be performed via the more expensive carbon-13 labelled intermediates [68], since the deuterated standard underwent significant protium-deuterium exchanges (unpublished data). 

The kinetics and equilibrium of protium-deuterium exchange between pentafluoro-ethane and aqueous hydroxide ion have been studied.125... 

In KNH2/NH3, protium-deuterium exchange takes place at the following relative rates ... 

Transition Metal Catalyzed Protium-Deuterium Exchange Reactions of (R)-(+> 1 -NpPhMeSiH (ref. 199) (eq. [561)... 

Kinetic studies of base-induced deuterium-protium exchange at the 5-C atom of 1-substituted tetrazoles and tetrazolium cations have indicated the presence of carbanionic intermediates in the exchange process.389 The reaction for 1-substituted tetrazoles had a p value of +1.3 in piperidine-MeOD—DMF.389 The pH profile for protium-deuterium exchange for l-methyltetrazole-5-d in aqueous solution has also been reported.390 In the pH range 3-5 the reaction was considered to involve either the free carbanion (167) resulting from D+ abstraction, the ylid (168) resulting from initial protonation at N-l or... 

Superacid DF-SbFs induces protium-deuterium exchange in isobutane [54b]. Strong acids (BF3, BFj-HjO, HF-BFj, CF3SO3H)catalyze carbonylation of alkanes including methane by carbon monoxide [54c], whereas sulfuric acid can induce carbonylation of iso- and cycloalkanes [54e,d]. In both cases, carboxylic acids are obtained. The elimination of molecular hydrogen from alkyl can occur (see a recent theoretical study of the Hz elimination from CzHs [54f]). 

Solvent Effects. Kinetic parameters for the reaction of [Pt(dien)Br]+ with 3-cyanopyridine have been determined in deuterium oxide solution, in order to make a comparison of reactivities in deuterium oxide and in ordinary water. To avoid any complications from protium-deuterium exchange between solvent and co-ordinated ligand, the perdeuterio-diethylenetriamine complex was used as substrate in deuterium oxide solvent. The kinetic parameters are compared in Table 1. The results were discussed in terms of an associative mechanism and the smaller solvating effects of D2O compared with HgO. ... 

In relation to the economic importance of acid catalysed hydrocarbon transformations such as cracking and isomerisation, the largest number of isotopic studies published thus far concern the exchange or migration of protium, deuterium and in alkanes. 

Hydrogen has two stable isotopes, protium and deuterium. According to the lUPAC classification, deuterium is denoted as or D. The natural abundance of deuterium is very low, about 156 ppm, and restricts the large-scale application of deuterium-exchanged compounds. However, deuterium-exchanged compounds are very useful and often easily obtainable model molecules for various investigations. 

Fig. 7 shows the variants of the mechanisms shown in Fig. 6, together with the form of expected plots of isotope effects as a function of pH. In reaction (17.62), only the chemical step, depict by the rate constant k, is sensitive to deuterium for protium isotope exchange in the substrate. 

Ion-pair effects have also been implicated in hydrogen-deuterium exchange reactions proceeding through highly delocalized fluorenyl anions. The stereochemical course of exchange of deuterium for protium in the chiral fluorene derivative 2... 

In this investigation, a labelled donor solvent was used to determine which structural positions in the coal products incorporate deuterium and to investigate the exchange of protium in the coal with deuterium in the donor solvent. It is important to understand this fundamental chemistry because a number of pilot plants use donor solvents (15-17). The yields of liquefied coal products may be improved through a detailed understanding of the hydrogenation mechanisms. 

To summarize the exchange and donation mechanisms of Tetralin, pathway 1 in Figure 2 is the predominant pathway of exchange and donation as determined by preferential incorporation of protium into the a-position of Tetralin. Tetralin-di2 loses a deuterium atom from the a-position, and a protium atom is incorporated into the a-position. This equilibrium is the exchange pathway. Continued loss of deuterium from the a-Tetralinyl radical eventually leads to naphthalene, and this reaction is the donation pathway. 

Tetralin. Therefore, either trans-Decalin was formed on the surface of the coal with deuterium from the solvent or gas, or it was formed by isomerization from cis-Decalin with accompanying isotopic exchange with a deuterium source. Because the protium content of the cis-Decalins is much greater than the trans-Decalins, cis-trans isomerization is not very important. 

The purpose of this experiment was to investigate the extent and the structural specificity of hydrogen exchange during the extraction of bituminous coal with naphthalene. Table I includes the data of an extraction experiment (E20) conducted with naphtha-lene-d8 using nitrogen as the cover gas. In the experiment, the reactants were heated at 380°C for 1 hour at 2200 psi the same apparatus was applied as in E19. After the run, the spent solvent was separated from the coal by distillation, and the coal and solvent were examined for deuterium and protium incorporation. 

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