Molar radioactivity

The thiazolecarboxylic acid structure (40) was also guessed in a similar way, from tracer experiments. The unknown compound was converted into the thiamine thiazole by heating at 100°C and pH 2. On paper electrophoresis, it migrated as an anion at pH 4. Tracer experiments indicated that it incorporated C-l and C-2 of L-tyrosine, and the sulfur of sulfate. The synthetic acid was prepared by carboxylation of the lithium derivative of the thiamine thiazole, and the derivatives shown in Scheme 19 were obtained by conventional methods. Again, the radioactivity of the unknown, labeled with 35S could not be separated from structure 40, added as carrier, and the molar radioactivity remained constant through several recrystallizations and the derivatizations of Scheme 17. 

From this observation of the inhibition by adenosine, and other observations, Newell and Tucker suspected the existence of a common synthetic pathway for adenosine and thiamine, and proved (with the help of a collection of mutants) that the bifurcation occurred after the 5-amino- l-(P-D-ribofura-nosyl)imidazole 5 -phosphate (46) step (Scheme 23). Finally, they found that 5-amino-l-(0-D-ribofuranosyl)imidazole (47), labeled with l4C in the imidazole ring, was incorporated into pyramine without significant loss of molar radioactivity by a mutant that is able to use this nucleoside (presumably after phosphorylation).53,54... 

FIGURE 9.24 Molar radioactivity of the reaction products when co-feeding n-hexa-decene-(l)-(l-14C) together with the synthesis gas to FT synthesis on cobalt. Catalyst 100Co-18Th02-100Kieselguhr (by weight), precipitated, 190°C, 1 bar, H2/CO = 2, Xc0 = 70%, 0.1 vol% of hexadecene-14C in the synthesis gas. 

With respect to the point mentioned last, special interest has been focused by several authors on the possible insertion of olefins. Of special relevance are results reported by Schulz and Achtsnit (77). These authors studied the FT catalysis on a cobalt catalyst and added ethylene marked with to the feed. The product was analyzed the specific molar radioactivities of the various fractions are shown in Fig. 3. It is seen that for higher carbon numbers the specific molar radioactivity increases linearly with the carbon number. Upon considering that, if ethylene can be inserted, the chance of this to happen increases linearly with the number of insertion steps that a chain undergoes during its life, this linear dependence is interpreted by the authors as proving ethylene insertion. 

The statement made by Schulz et al. (17) that a metathesis reaction (50) takes place therefore implies that carbenes must be present on the surface and can react with olefins under the conditions of the FT reaction. Once this is accepted, it follows that Eq. (48) provides a potential reaction path to transfer the atom from a terminally labeled a-olefin to the surface carbene. This evidence, therefore, suggests that x = 2, the insertion of surface carbene resulting in a linear increase of the molar radioactivity with the number of insertions, in agreement with the evidence in Fig. 4. 

In experiments involving 14C-labeled ethylene the molar radioactivities of products relative to that of reactant ethylene were determined by proportional counting of COL> obtained by combustion in 02 over CuO at 450°C. of products which had been purified by gas-chromatography over silica gel, as has been described previously (8). Bemstein-Ballentein proportional counter tubes, filled with P-10 gas at 1 atm. were used with a Tracerlab P30 amplifier and SC72 scaler. 

The hydrocarbon synthesis products in the C2-Cjo, C3-C10 C4-C10 ranges when ethanol, 1-pro-panol, or 1-butanol, respectively, were added to the synthesis gas had an approximately constant molar radioactivity equal to one-third to one-half of the molar radioactivity of the original alcohol. Thus, these results indicate that the primary alcohol adsorbed on the iron catalyst to act as an initiator to build up higher hydrocarbons. 

A constant molar radioactivity with increase in carbon number of the products was obtained when C-labeled ethanol or acetaldehyde was added to the syngas fed to a cobalt catalyst. On the other hand, the radioactivity showed a linear increase with carbon number when labeled methyl formate or formic acid was added to the synthesis gas thus, these compounds, like methanol, undergo decomposition to produce labeled CO which participates in the synthesis. 

C-Labeled ethene incorporation during synthesis with a Co catalyst at 195 °C produces hydrocarbons that contain a constant molar radioactivity (Figure 37). The added ethene underwent hydrogenation to ethane to the extent of 50 percent and 50 percent was converted to higher hydrocarbons. Eidus reports that comparing the published data indicates that ethene takes part in the synthesis with a cobalt catalyst to a greater extent than ethanol does under the same reaction conditions he proposed that the ethanol is... 

C-Labeled propene incorporated to produce hydrocarbons with a constant molar radioactivity just as was observed with labeled ethene. However, propene initiated the formation of products (one of 12 molecules) to a smaller extent than ethene (one of four to five molecules). 

Table 1. Relative molar radioactivities (r%/m%) in the products after reacting a mixture of [ C]-Cyclohexane and Cyclohexene. ...

Fig. 2. Specific molar radioactivities in the components of feed and product when reacting a mixture of [ C]-n-hexane (65%) plus inactive 1-hexene (35%) and [ C -n-hexane (62%) plus inactive cyclohexane (38%). Pulse system, catalyst Pt black, carrier gas helium.

If the alkylating Ci unit is not radioactive, the molar radioactivity of the C7 product from the labelled feed should be 6/7 = 0.83, which is in good agreement with the result of Table 5. Comparing the results from hexane and inactive benzene,as well as the reverse mixture,indicates that alkylation of the ready aromatic ringl l as well as degradation of larger products (e.g., dimers) were less likely pathways. 

This technique has been developed for studying sequential reactions by Gal and co-workers. Theory and earlier results were summarised in a book, describing it as a dynamic method that utihsed the temporal changes of specific (or molar) radioactivities of assumed intermediates. 

Fig. 5. Relative molar radioactivities of n-alkanes ( ) and monomethylalkanes (o) in the presence of [ C]-ethene tracer as well as n-alkanes (X) and monomethylalkanes (A) produced in the presence of [ C]-propene during Fischer-ltopsch synthesis on Co catalyst. Re-drawn after Refs. 91, 92.

Experimental data indicated that the isotope exchange process is described by first order equations, and in most cases, e.g., on supported C0M0/AI2O3 catalysts by a superposition of two curves representing two types of sulfur mobility, the more and the less mobile i.e., rapidly and slowly exchangeable sulfur. In general, the H2S molar radioactivity a (in percent of the initial molar radioactivity of catalyst sulfide sulfur), as a function of the produced H2S-[X(cm )], is given (Fig. 7) as a superposition of curves... 

Experiments indicatedthat the molar radioactivity function of H2S production on M0S2 can be described by one equation (A = 100, i= 1) biexponential equations were required for promoted catalysts with different Aj and Aj. This indicates that the catalyst sulfur was not bonded homogenously, like that of the observations with H2 S or H2 S exchange.An important difference from those observations is that, in this case, we deal with H2 S, replaced by sulfur, formed in HDS, so that the H2S molar radioactivity represents sulfur formed on the (or via) cat-al Tically active sites. Consequently, the fact that function (7) is described by a biexponential equation with two A-s and A-s, indicates that there exist two types of catalytic sites, active in HDS on promoted Mo-based catalysts. 

In the foregoing examples the yields of the product or products have been determined when (1) the molar radioactivity of the product or of the addend and (2) the weight of addend have been known. In many experiments involving radioactive products the exact molar radioactivities (Aq, equation 2) of the materials produced may not be known. In such oases the double-dilution method may be useful, and both yield (x) and radioactivity A o) may be determined by means of the following relation ... 

The application of the radioactivity dilution technique to the determination of yield or radioactivity of a product is not without its drawbacks. The most serious of these may be illustrated as follows. Suppose a given reaction product consists of two compounds, A and B, both of which possess equal molar radioactivities. It is desired to determine the yield of A through the dilution method by the addition of a weighed... 

Upon oxidation of 32 with chromic acid to benzoic acid (35), followed by a Schmidt degradation of the benzoic acid, it was demonstrated that the carboxyl group of 35 contained 1/7, whereas the phenyl group contained 6/7 of the total molar radioactivity, a result consistent with the intervention of the tropylium ion (C). 

From the foregoing equation, k jk can be calculated for any fraction R. In the special case that A represents a very low concentration of radioactive reactant in the mixture A -f- A, where A and A differ only by isotopic substitution, R is the molar radioactivity of the product at timpi t divided by the molar radioactivity of the reactant at time zero. Collins and Lietzke (1959), using equation (33), have made plots of R vsf= 1 — e" for assumed values of k lk from 0-05 — 0-99. Some of these plots are shown in Figs. 4, 5 and 6, in which it should be noted that at/ = 0, — k /k. Thus for different values of the isotope effect... 

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