Solvent exchange hydrogen isotopes

The amounts of exchange and addition were calculated from a hydrogen isotope mass balance of the coal products, donor solvent and gas phase hydrogen. The starting and product weights of the coal and hydrogen compositions of the coal and coal products are shown in Table IV. From the values in Table IV, the net amount of hydrogen added to the coal, H, is... 

We chose the microwave-enhanced Raney Nickel catalyzed hydrogen isotope exchange of indole and N-methylindole as our substrates and D20, CD3COCD3, CD3OD and CDC13 as the solvents. The thermal reaction had already been the subject of a recent study [44], The microwave-enhanced method was some 500-fold faster than the corresponding thermal reaction (at 40 °C). Furthermore the pattern of labeling (Scheme 13.3) varied with the choice of solvent. Thus in the case of indole it-... 

Liquid hydrogen halides do not suffer from these disadvantages and have therefore been employed by us in preference to sulphuric acid in systematic studies of hydrogen isotope exchange reactions. Parallel investigations with solvents differing in their acid-base properties facilitate the elucidation of the effect of the medium on the reactivity... 

Comparison of results on hydrogen exchange in liquid ammonia and in other solvents can be used to obtain information on the nature and mechanism of hydrogen isotope exchange reactions. 

Isotope exchange and isotope effects. The vinylic hydrogen is lost and reintroduced from the solvent during the elimination-addition process. Incorporation of deuterium from deuteriated solvents is therefore expected. Depending on the elimination mechanism, hydrogen isotope effects would be also sometimes observed when RCX=CDY is compared with RCX=CHY. 

CH bonds in alpha position to carbonyl, sulfonyl, or nitro groups may undergo slow hydrogen isotope exchange with a hydroxylic solvent in the presence of a strong base. In another method of measurement of the deprotonation rate, a reagent (bromine or iodine) is added which reacts... 

The specific acid-catalyzed reaction behaves in many respects like the water reaction. Thus, unusually large negative entropies of activation are found for all but those substrates which react by the acylium ion path (Schaleger and Long, 1963). At least two water molecules are involved in the transition state, the presence of at least five exchangeable hydrogens in the transition state, is deduced from solvent isotope effects in mixed water-heavy water (Salomaa et al., 1964). From the dependence of both acid-catalyzed exchange and hydrolysis in concentrated sulfuric acid on the activity of water. Lane (1964) concluded that two water molecules must be involved in the transition state. Except for the extra proton, the transition states for the acid-catalyzed and water reactions are very probably similar. 

Solvent isotope effects. Much mechanistic information can be obtained about reactions involving proton transfer from solvent kinetic isotope effects, particularly in solvents of mixed isotopic composition. For practical reasons work is essentially confined to H/D effects, especially those in water. Unlike ordinary primary hydrogen isotope effects, solvent isotope effects have to take into account a host of exchangeable sites, subject to equilibrium as well as kinetic isotope effects. A key concept is that of the fractionation factor, (p, which is the deuterium occupancy of a site in a 1 1 H2O/D2O mixture more formally it is defined by equation l.l ... 

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