Protium exchange

The majority of the more than 100 odorants (reviewed in [1]) synthesised for use as internal standards are labelled with deuterium. However, during the quantification procedure some deuterated odorants might undergo deuterium-protium exchange, which would falsify the results. Examples are 4-hydroxy-2,5-dimethyl-3(2H)-furanone (furaneol) [68, 69] and 3-hydroxy-4,5-dimethyl-2(5H)-furanone (sotolon) [70], which are consequently labelled with... 

Base-induced deuterium-protium exchange at the C-5 atom of N-substituted tetrazoles occurs readily via carbanionic type intermediates, e.g. (57). The reaction for 1-substituted tetrazoles had a Hammett p-value of -1-1.3 in piperidine-MeOD-DMF (69TL3377). In the acidic pH range 3-5, ylide t5q3e species (58 R = H) are also considered to be involved in the exchange arising from initial protonation at N-1 or N-2 followed by abstraction. 

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... 

Hauser et were able to isolate the intermediate dimeric halide when 9-chlorofluorene was allowed to react with only one equivalent of sodium amide in liquid ammonia. In t-butyl alcohol containing its potassium salt or a dilute aqueous solution of benzyltrimethylammonium hydroxide, the rate of formation of bifluorenylidene is second-order in 9-bromofluorene and first-order in the basicity of the reaction medium as measured by the ionisation of nitroaniline indicators . Under the same reaction conditions, protium exchange of 9-deutero-9-bromofluorene and elimination from 9-bromo-9,9 -bifluorenyl are much faster reactions than the conversion of 9-bromofluorene into bifluorenylidene. These facts are consistent with the displacement mechanism. 

The examples given in Fig. 5.14 show that for economic reasons, mostly internal standards labelled with deuterium are synthesized for IDA. The considerably more expensive carbon isotope 13 is introduced into the odorant (examples are the internal standards No. 11 and 12 in Fig. 5.14) only if a deuterium/protium exchange can occur in the course of analysis. This exchange would falsify the result. Another advantage of this isotope is the completely negligible isotope effect compared to deuterium. 

Dextran.—Tritium-protium exchange in dextran at 25 °C has been studied. ... 

Truitt MJ, Toporek SS, Rovira-Truitt R, White JL. Alkane C-H bond activation in zeohtes evidence for direct protium exchange. J Am Chem Soc 2006 128 1847-52. 

In another experiment, naphthalene-d8 was used to investigate the chemistry of hydrogen transfer between coal and nondonor solvent at 380°C. An analysis of the recovered naphthalene-d8 showed that approximately 4% of the hydrogen in the coal and in the naph-thalene-d8 exchanged. Most of the protium incorporated in the naphthalene-d8 was found in the a-position. The coal products contained approximately 2 wt % chemically-bound napththalene-d8. 

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 determine the principal pathway of exchange, isotopic exchange by functional position was examined. The lower halves of Tables II and III show the amounts of incorporation of protium into the spent Tetralin. In E10, 66% was in the position, 23% in the position, and 11% in the Har position. A similar distribution was found in El9. This preferential incorporation in the Hx position of Tetralin strongly suggests that the predominant path-... 

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. 

In order to explain the large specific incorporation of protium into the 1 position of the spent naphthalene-d8, the possible reaction mechanisms of exchange need to be examined. Naphthalene may accept a H atom from coal to initiate the reaction as shown... 

Deuterium-labelled Tetralin and deuterium were used to determine incorporation of hydrogen by structural position in the Tetralin and coal products. Approximately two-thirds of the protium incorporated into the Tetralin-di2 was found in the a-alkyl position, indicating that hydrogen exchange between the coal and tetralin involves the a-Tetralinyl radical. In the case of the coal products, the terms 2H >yFy and 1HX (1-Fy) were used to calculate the deuterium and protium contents or eacn structural position. Preferential incorporation was found in the a-aliphatic position of the soluble coal products, indicating that these positions participated preferentially in hydrogen exchange and addition. The... 

Perdeuteriated isopropyl, t-butyl, and t-pentyl fluorides were prepared from the corresponding deuteriated alkyl chlorides (bromides) by halogen exchange. The perdeuteriated alkyl fluorides were then used in the formation of the deuteriated carbonium ions under similar conditions as those for the protium complexes. The resonance spectra were obtained at 9 2 Mc/s and the data are summarized in Table 7... 

The relative merits of two possible reaction schemes have been discussed (20). The schemes considered involve either an associative n-complex (Fig. 1) or a dissociative 7r-complex (Fig. 2). These figures are drawn essentially as given by Garnett (20) they have been modified to include solvent molecules to fill and vacate coordination sites as required, and to make all the catalytic cycles in this review as comparable as possible.1 It should be noted that these catalytic cycles for H—D exchange are symmetrical, the second half of the cycle being the reverse of the first half with deuterium instead of protium. 

Although the pure titanium-hydrogen system exhibits the normal isotope effect, many titanium alloys show the inverse effect. The exchange of pro-tium-tritium mixture with the hydrided phase of these alloys has demonstrated an inverse protium-tritium isotope effect in Ti-V, Ti-Mo, Ti-Cr, Ti-Mn, and the ternary alloy TiCrMn (1). On the other hand, Ti-Co, Ti-Fe, and Ti-Ni systems exhibit the normal isotope effect. Clearly much can be learned from a study of these systems. 

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