Oxygen-18 labeling

In aqueous pyridine solution, most diaryl sulphoxides may be oxidized to the corresponding sulphones with (dichloroiodo)benzene in reasonable yields103. The reaction involves nucleophilic attack by the sulphoxide on the electrophilic chlorine-containing species, yielding an intermediate chlorosulphonium ion which then reacts with water producing the sulphone. If the sulphoxide is optically active, then an optically active sulphone is produced in excellent optical yield when the reaction is carried out in oxygen-18 labelled water104, as indicated in equation (33). 

Heesing and coworkers have reported the rearrangement of 0-alkylsulfinyl-iV-benzoyl-iV-phenylhydroxylamine (143) at — 70°C to the corresponding sulfonamide together with the o- and p-alkylsulfonyl derivatives 144 and 145 (equation 89). The reaction has been suggested to proceed by an intramolecular radical pair mechanism, as evidenced by experiments with oxygen-18 labeling and C-CIDNP effects. 

Wolter O, Willsau J, Heitbaum J. 1985. Reaction pathways of the anodic oxidation of formic acid on platinum evidenced by oxygen-18 labeling—A DEMS study. J Electrochem Soc 132 1635-1638. 

The photodegradation of para-aramid in an 0 atmosphere allows the differentiation between the accelerated experimental photooxidative conditions from its usual daylight exposure effects. This study illustrated an estimation of the rates of photooxidation of a commercial para-aramid product (i.e., DuPont s Kevlar-29 woven fabric) based on the oxygen-18-labelled carbon dioxide ( CC and CC ) decarboxylated from the sample. The oxygen-18-labelled atoms, which are inserted in the macromolecules, were analyzed for the photodegradation processes. This technique also allows the radial l O-distribution measurement from the fiber surface toward the fiber center. 

Reagents. Oxygen-18-labelled gas (99%) was purchased from ICON, a gaseous mixture of 100 ppm ethane and helium from Alltech, helium from Matheson, concentrated sulfuric acid (Ultrex) from J. T. Baker, and chloroform and dimethylacetamide (DMAc) from Aldrich. The reagents were used as received unless otherwise specified. 

When a sample of the same Kevlar-29 fabric was identically treated in l 02-atmosphere except in the absence of photolysis (in the dark) for 24 hours, the results of C02 evolved at 25° and 196°C from the GC-analysis were the same as the samples under photolysis (e.g., Table I). However, the gross difference was in the analyses of the GC-mass spectroscopic data, where no oxygen-18-labelled C02 isotopes evolved (i.e., there was no C02 or 48co2 above the natural background levels). ... 

This new analytical method determines the rate constant and activation energy of Kevlar s photooxidative processes. The 0.2 atm of oxygen-18-labelled environment in a solar chamber simulates the air-exposure under sunlight conditions. The technique also allows the radial 0-distribution measurement from the fiber surface toward the fiber center. The data from the accelerated experimental conditions in the solar chamber in an 02-atmosphere are differentiated from the usual daylight exposure effects. 

Additional evidence in favor of this suggestion was obtained when Wiberg (45) found, by use of oxygen-18 labels, that the reaction takes place by transfer of oxygen from the permanganate to the alkene i.e., if labeled permanganate is used, the oxygen in the product is also labeled. 

Electron-donating substituents make the aromatic subsU ate more reactive than benzene and lead to o,/ -orientation, while electron-withdrawing substituents decrease the reactivity and give mostly m-orientation products. The detailed mechanism of the formation of the a complex has been studied by oxygen-18 labeling of the sulfonyl oxygen in p-nitrobenzenesulfonyl peroxide. The ionic mechanism for aromatic substitution by sulfonyl peroxides has been confirmed by carrying out the substitution reaction in the presence of redox catalysts such as copper and cobalt salts and aluminum chloride. Small differences in the rate of the products can be found in the presence or absence of these additives, and it has been concluded that the ionic mechanism accounts satisfactorily for these results. ... 

Posner, G.H. and Oh, C. A regiospecifically oxygen-18 labeled 1,2,4-trioxane a simple chemical model system to probe the mechanism(s) for the antimalarial activity of artemisinin (Qinghaosu), /. Am. Chem. Soc., 114, 8328,1992. 

C. Crean et al., Oxidation of guanine and 8-oxo-7, 8-dihydroguanine by carbonate radical anions Insight from oxygen-18 labeling experiments. Angew. Chem. Int. Ed. 44, 5057-5060 (2005)... 

Wells (1971) showed through the addition of oxygen-18-labeled water to the reaction mixture that the point of attack occurs at the O-acyl bond as shown by the arrows in Figure 4-5. This conclusion was supported by the finding that oxygen-18 was found only in the liberated fatty acid. Though no direct evidence for the occurrence of an acyl-enzyme intermediate could be obtained, formation of such a complex could not be completely excluded. 

Ester hydrolysis is such an important reaction that chemists spent a lot of time and effort finding out exactly how it worked. If you want to know all the details, read a specialist textbook on physical (mechanistic) organic chemistry. Many of the experiments that tell us about the mechanism involve oxygen-18 labelling. The starting... 

Fig. 24A-C. Distribution of the oxygen-18 label in the reaction of Rh(NO)(S02)(PPh3)2 with Infrared...

The substituent effect of a methyl group on the rate coefficient of the basic hydrolysis of ethylene oxide is small. The kOH values for ethylene oxide, propylene oxide, and isobutylene oxide are almost the same (Table 9). It has been demonstrated by Long and Pritchard [150] with the aid of experiments in oxygen-18 labeled water that hydroxide ion attacks propylene oxide and isobutylene oxide predominantly at the primary carbon. Consequently, the base catalyzed hydrolysis of epoxides is a simple bimolecular nucleophilic substitution. 

For a discussion of the mechanism of acetal hydrolysis, experimental information about the position of bond cleavage is needed. According to the results of oxygen-18 labeling experiments, bond cleavage in the formation and hydrolysis of n-butyl benzal takes place between oxygen and aldehyde-carbon [170, 171], viz. 

The doubly labeled water method is used to determine the moles of COi produced as follows. The human subject drinLs a known quantity of deuterated water ( HjOj and oxygen-18 labeled water Hydrogen-2 is deuterium. 

When C2H5 is replaced with the more electron withdrawing CF3, the rate is slower, indicating that reaction does not proceed via allylic carbanions, the path for thermal isomerization of allylic esters. Oxygen-18-labeling experiments and the electronic effects are consistent with an internal ox3q>alladation mechanism ... 

A study of the fragmentation pattern of deuterium and oxygen-18 labeled derivatives of cytosine nucleosides established that a major fragmentation process occurred that was absent in the spectra of other nucleosides. The various fragmentation pathways for prostaglandins A, B, E, and F were de-termined by the use of specific derivitization with d -tetramethylsilane. 

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