Isotopic labelled product

Proportions of Isotopically Labeled Products from Isomerization of Hexanes over 0.2% Platinum /Alumina Catalyst at 273°C ... 

Fig. 11. Pathways for isotopically labeled products from PN s and Hl3CN. Used by permission of the American Chemical Society (48).

IN SSITKA, Np and the mean surface residence time of these most active reaction intermediates (xp) are determined. After a step-change between two reactant streams containing different isotopes of a reactant without disturbing other reaction conditions or reaction (as long as an H2/D2 switch is not used), the distributions of isotopically labeled products are monitored using a mass spectrometer. Tp is first determined by integration of the normalized isotopic transient of a product relative to an inert tracer (usually Ar) that delineates gas phase hold-up (see Figure 1 for the case of methanation). Np is then calculated from... 

As stated earlier, SSITKA is carried out by tracking the change in the isotopic labelled product(s), following a step change in the isotopic label on a reactant. To maintain isothermal and isobaric reaction conditions, flow rate and pressure of the two isotopic labelled reactant flows are adjusted to be identical, prior to switching. A typical reaction system for isotopic transient studies is shown in Fig. 1. 

Figure 19.21. Transient rates of (a) old and (b) new isotopically labelled products. The filled area corresponds to the number of surface intermediates leading to these products.

The stereochemical relationship between the reactant and the product revealed by the isotopic labeling shows that oxygen becomes bonded to carbon on the same side from which H IS lost As you will see m this and the chapters to come determining the three dimensional aspects of a chemical or biochemical transformation can be a subtle yet powerful tool for increasing our understanding of how these reactions occur... 

A more recent experimental technique employs C as the isotopic label Instead of locating the position of a label by a laborious degradation procedure the NMR spectrum of the natural product is recorded The signals for the carbons that are enriched m are far more intense than those corresponding to carbons m which IS present only at the natural abundance level... 

Other mechanisms must also operate, however, to account tor the fact that 5-10% of the product is formed with retained configuration at the chiral center. Isotopic labeling studies have also demonstrated that the 3-bromo-2-butyl radical undergoes reversible loss of bromine atom to give 2-butene at a rate which is competitive with that of the bromination reaction ... 

When the l<, 2( -d2-labeled product (129) is subjected to alkaline equilibration to back exchange the 2i -label (for experimental conditions see section IT-B), the crystalline l< -di-4,4-dimethyl-5a-androstan-3-one (130) exhibits 6% do and 94% d isotopic composition. ... 

In a similar experiment, pentyl acetate was subjected to saponification with 0-labeled hydroxide in 0-labeled water. What product do you think became isotopically labeled here, acetate ion or 1-pentanol ... 

The mechanism outlined above is supported by experimental findings. An intermediate 5 has been isolated, " and has been identified by and N-nuclear magnetic resonance spectroscopy. Side-products have been isolated, which are likely to be formed from intermediate 4. N-isotope labeling experiments have shown that only the nitrogen remote from the phenyl group is eliminated as ammonia. 

The mechanism shown in Figure 21.7 is supported by isotope-labeling studies. When ethyl propanoate labeled with lsO in the ether-like oxygen is hydrolyzed in aqueous NaOH, the l80 label shows up exclusively in the ethanol product. None of the label remains with the propanoic acid, indicating that saponification occurs by cleavage of the C-OR bond rather than the CO—R bond. 

Complexed arenediazonium salts are stabilized against photochemical degradation (Bartsch et al., 1977). This effect was studied in the former German Democratic Republic in the context of research and development work on diazo copying processes (Israel, 1982 Becker et al., 1984) as well as in China (Liu et al., 1989). The comparison of diazonium ion complexation by 18-crown-6 and dibenzo-18-crown-6 is most interesting. Becker at al. (1984) found mainly the products of heterolytic dediazoniation when 18-crown-6 was present in photolyses with a medium pressure mercury lamp, but products of homolysis appeared in the presence of dibenzo-18-crown-6. The dibenzo host complex exhibited a charge-transfer absorption on the bathochromic slope of the diazonio band. Results on the photo-CIDNP effect in the 15N NMR spectra of isotopically labeled diazonium salts complexed by dibenzo-18-crown-6 indicate that the primary step is a single electron transfer. 

That this mechanism can take place under suitable conditions has been demonstrated by isotopic labeling and by other means. However, the formation of disproportionation and dimerization products does not always mean that the free-radical abstraction process takes place. In some cases these products arise in a different manner.We have seen that the product of the reaction between a carbene and a molecule may have excess energy (p. 247). Therefore it is possible for the substrate and the carbene to react by mechanism 1 (the direct-insertion process) and for the excess energy to cause the compound thus formed to cleave to free radicals. When this pathway is in operation, the free radicals are formed after the actual insertion reaction. 

The most straightforward experimental approach is isotopic labeling of certain atoms in the reactants. The detection method must distinguish between the possible isotopologs of the products. For example, in the reaction... 

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