Carbon-14 tracer studies

The hetero ring in 4-(l -hydroxyalkylidene)-5-oxazolones is cleaved by alcoholic HCl to form alkyl a-acylaminoacylacetates, which cyclize to oxazole-4-carboxylates [Eq. (25)]. This rearrangement occurs directly in alkali, and a carbon-14 tracer study has substantiated a mechanism involving ring opening followed by the alternative ring closure. ... 

Novelli, A., Barrio, J. R. Carbon-14 tracer studies in the benzilic acid type rearrangement of 1-phenyl-and 1-(4-methoxyphenyl)-2-(3-pyridyl) glyoxal. Tetrahedron Lett. 1969, 3671-3672. 

Although many overall rearrangements can be formulated as a series of 1,2-shifts, both isotopic tracer studies and con utational work have demonstrated foe involvement of other species. These are bridged ions in which hydride or alkyl groups are partially bound to two other carbons. Such structures can be transition states for hydride and alkyl-group shifts, but some evidence indicates that these structures can also be intermediates. 

Even though acetate units, such as those obtained from fatty acid oxidation, cannot be used for net synthesis of carbohydrate in animals, labeled carbon from " C-labeled acetate can be found in newly synthesized glucose (for example, in liver glycogen) in animal tracer studies. Explain how this can be. Which carbons of glucose would you expect to be the first to be labeled by "Relabeled acetate ... 

Additional 14C-tracer studies were conducted to determine the specific isomer participation for TeNMe formation. In the dinitration mixt there are six isomers present 2,4-, 2,6-, 2,3-, 2,5-, 3,4- and 3,5-Dinitrotoluene (DNT). It was observed that 83% of the TeNMe formed from carbon-1 came from the DNT isomers derived from m-Nitrotoluene. The relative participation of DNT isomers for TeNMe formation is summarized in Table 1... 

Boutton, T.W. 1991 Tracer studies with C-enriched substrates humans and large animals. In Coleman, D.W. and Fry, B., eds.. Carbon Isotope Techniques. Academic Press, San Diego 219-242. 

Allen, M. B., and S. Ruben Tracer Studies with Radioactive Carbon and... 

Elemental carbon has many important applications. The diamond is a precious gem, known to mankind for ages graphite is used as an electrode and has numerous other applications carbon-14 isotope is used in carbon dating and the isotope carbon-13 in tracer studies and NMR. Carbon black is used in paints, pigments and inks. Activated carbon is used as an adsorbent for purification of water and separation of gases. Coke is used for electrothermal reduction of metal oxides to their metals. These applications are discussed below in more detail. 

Investigations of the mechanism of decarboxylation of hexuronic acids have largely involved kinetic and tracer studies. When either D-xylo-5-hexulosonic acid or D-glucuronic acid is converted into 27 in acidified, tritiated water, the resulting 27 contains 18% and 15%, respectively, of the activity of the solvent as carbon-bound tritium.21 Further degradation studies showed that the isotope is situated on the furan ring at either position 3 or 4, or both these atoms correspond to C-3 or C-4 of the starting uronic acid. 

Tracer studies with 13C showed that the carbon of methoxy species formed in CO hydrogenation appeared as the central C atom of isobutylene.531 Methoxy was shown to be reduced to surface methyl species that was transformed to surface acetyl group via insertion of CO into the methyl-metal bond. Aldol condensation of C2 oxygenates with formaldehyde affords iso-C4 products.532 Earlier results of a different approach for isosynthesis using metal-zeolite catalyst systems are summarized in a 1998 review.533... 

Experimental verification of Collie s hypothesis came many years later when isotopic hydrogen and carbon (2H, 3H, 13C, and 14C) became available. Tracer studies showed that long-chain fatty acids are made by plants and animals from CH3CO units by successively linking together the carbonyl group of one to the methyl group of another (K. Bloch and F. Lynen, Nobel Prize, 1964). 

This substance has been shown by tracer studies to be an efficient precursor of terpenes and steroids. Mevalonic acid has six carbon atoms, whereas the isoprene unit has only five. Therefore, if mevalonic acid is the precursor of isoprene units, it must lose one carbon atom at some stage. Synthesis of mevalonic acid labeled at the carboxyl group with 14C, and use of this material as a starting material for production of cholesterol, gives unlabeled cholesterol. Therefore, the carboxyl carbon is the one that is lost ... 

It is a basic assumption in tracer work that labelled and unlabelled molecules have identical reactivities. If the labelling involves one of the atoms attached directly to the valency bond concerned in the reaction, this assumption is not valid. The differences between the reactivities is referred to as an isotope effect. When the isotopes involved are those of hydrogen, the isotope effect is quite large for heavier elements, the effects are much smaller but in the case of carbon they can be detected in careful work. To some extent, isotope effects impose a limitation upon the accuracy of results obtained from tracer studies, but they can also lead to a fuller understanding of the mechanisms of reactions of certain types. In routine tracer work, it is advisable whenever possible to label molecules at sites remote from the points of reactivity. 

Within the carbonaceous deposit, there is mixing of both the carbon and the hydrogen as revealed by isotopic tracer studies. 

M.B. Allen and S. Ruben, Tracer Studies with Radioactive Carbon and Hydrogen. The Synthesis and Oxidation of Fumaric Acid, Journal of the American Chemical Society 64, 948-950, 1942. 

Further reports have appeared on the reaction of trivalent phosphorus compounds with acetylene dicarboxylates. In the first, alkyl diphenylphosphinites (e.g.57) are shown to react with dialkylacetylene dicarboxylates (e.g.58) in the presence of carbon dioxide to form 1,2-oxaphosphol-3-enes (e.g.59) which in the presence of excess phosphinite decompose via (60) to give di-ylids (e.g.61). On the other hand, the phosphoranes (62) from phosphonites and phosphites react with a further phosphorus component to give the ylids (63) which are readily converted by treatment with alcohol into phosphonates (65) apparently via ketene intermediates (64) as evidenced by and isotopic tracer studies. ... 

Similar tracer studies have been carried out using smaller compounds, such as acetate, although the interpretation of the results obtained from these biosynthetically more remote precursors is difficult, especially when they are used in the intact animal. Thus C -labeled carbonate, acetate-I-C, and acetate, butyrate, and caproate labeled at C-2, injected intravenously into lactating cows, produced lactose having similar specific activities in the D-glucose and D-galactose residues. This observation has been con-... 

Al-Ammar AS, Webb G (1978) Hydrogenation of acetylene over supported metal catalysts Part 1 - Adsorption of [ C] Acetylene and [ C] ethylene on silica supported rhodium, iridium and palladium and alumina supported palladium. J Chem Soc Earaday Trans 74 195 Al-Ammar AS, Webb G (1979) Hydrogenation of acetylene over supported metal catalysts Part 3 - [ C] tracer studies of the effect of added ethylene and carbon monoxide on the reaction catalyzed by silica-supported palladium, rhodium and iridium. J Chem Soc Faraday Trans 75 1900... 

Mortreux, A., Petit, F., Blanchard, M. Carbon-13 tracer studies of alkynes metathesis. Tetrahedron Lett. 1978,4967-4968. 

---