Oxidation puzzle

Experiment 66 is adapted from M. W. Pelter, R. M. Macudzinski, and M. E. Passarelli, A Microscale Oxidation Puzzle, Journal of Chemical Education, 77 (November 2000) 1481. 

The substituted 1,2,3,4-letrahydroisoquinolines studied by Knabe and Roloff (87) also produce some puzzling results difficult to rationalize when oxidized with mercuric EDTA. Compounds 88 and 89 are not attacked to... 

FIGURE 20.2 The organic acids observed by Krebs to be oxidized in suspensions of liver and kidney tissue. These substances were the pieces in the TCA puzzle that Krebs and others eventually solved. 

Another important piece of the puzzle came from the work of Carl Martius and Franz Knoop, who showed that citric acid could be converted to isocitrate and then to a-ketoglutarate. This finding was significant because it was already known that a-ketoglutarate could be enzymatically oxidized to succinate. At this juncture, the pathway from citrate to oxaloacetate seemed to be as shown in Figure 20.3. Whereas the pathway made sense, the catalytic effect of succinate and the other dicarboxylic acids from Szent-Gyorgyi s studies remained a puzzle. 

Burlet, S., Leger, L. Cespuglio, R. (1999). Nitric oxide and sleep in the rat a puzzling relationship. Neuroscience 92, 627-39. 

On oxidation of E8 the, exo-exo conformation of the neutrals changes and an exo-endo conformer is formed in several E82+ salts (refs. S,16,70 Se,53,71,72 Te72,73). Very puzzling features of these structures are the three transannular... 

The concept of color centers has been extended to surfaces to explain a number of puzzling aspects of surface reactivity. For example, in oxides such as MgO an anion vacancy carries two effective charges, V(2. These vacancies can trap two electrons to form an F center or one electron to form an F+ center. When the vacancy is located at a surface, the centers are given a subscript s, that is, Fs+ represents a single electron trapped at an anion vacancy on an MgO surface. As the trapping energy for the electrons in such centers is weak, they are available to enhance surface reactions. 

Both valence isomers easily give off an electron the first vertical ionization energies amount to only 7.5 eV and 6.35 eV (29), and therefore, both compounds can be oxidized in solution using the selective one-electron oxidizing system AICI3/H2CCI2 (3). The result, however, is puzzling in both cases, the tetrakis-(tert. butyl) cyclobutadiene radical cation is identified by its ESR spectrum (30). 

To find out what this air was, he performed further experiments. He did tests to see if it resembled nitrous oxide. It didn t. Still puzzled, Priestley turned to other work, including his experiments with sulfur dioxide, and then he had an idea. If nitrous oxide was mixed with air, the quantity of air was diminished (because another... 

Due to the accurate knowledge of the dose used, a control of the progressive extent of the radiolytic reduction was achieved. However, quite often, puzzling data were reported when the zero-valent metal was formed, such as an induction time for precipitation, a sensitivity of the radiolytic yields to the initial presence of added particles, and an unusually weak reproducibility [9,10]. Moreover, oxidation of silver atoms by molecular oxygen was observed [7], although the process was thermodynamically improbable for a noble metal such as silver. 

In subsequent research, it turned out that two-state reactivity can also provide a concept for the understanding of oxidation reactions way beyond the scope of gas-phase ion chemistry and can actually resolve a number of existing mechanistic puzzles. In enzymatic oxidations involving cytochrome P450, for example, changes in spin multiplicity appear to act as a kind of mechanistic distributor for product formation [27-29], and in the case of manganese-catalyzed epoxidation reactions, two-state scenarios have been put forward to account for the experimentally observed stereoselectivities [30-32], Two-state reactivity is not restricted to oxidation reactions, and similar scenarios have been proposed for a number of other experimentally studied reactions of 3d metal compounds [33-37]. Moreover, two-state scenarios have recently also been involved in the chemistry of main group elements [38]. The concept of two-state reactivity developed from the four-atomic system FeO /H2... 

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