Stoichiometry pathways

A second pathway involves the formation of a mixed-valent [Fe2+, Fe3+]-superoxo species that could react with a second molecule of 36 to form a peroxo-bridged tetranuclear [Fe +, Fe2+] cluster. Homolytic cleavage of the peroxo 0-0 bond followed by electron transfer and rearrangement would yield 37. These two pathways differ in their oxygen stoichiometry pathway 1 has a ratio of 02/reduced iron dimer of 1 1, whereas pathway 2 has a ratio of 1 2. Manometric measurements of 02... 

The influence of boron-bonded ligands on the kinetics and mechanistic pathways of hydrolysis of amine boranes has been examined (37,38). The stoichiometry of trimetbyl amine azidoborane [61652-29-7] hydrolysis in acidic solution is given in equation 10. It is suggested that protonation occurs at the azide ligand enabling its departure as the relatively labile HN species. 

Mitchell s chemiosmotic hypothesis. The ratio of protons transported per pair of electrons passed through the chain—the so-called HV2 e ratio—has been an object of great interest for many years. Nevertheless, the ratio has remained extremely difficult to determine. The consensus estimate for the electron transport pathway from succinate to Og is 6 H /2 e. The ratio for Complex I by itself remains uncertain, but recent best estimates place it as high as 4 H /2 e. On the basis of this value, the stoichiometry of transport for the pathway from NADH to O2 is 10 H /2 e. Although this is the value assumed in Figure 21.21, it is important to realize that this represents a consensus drawn from many experiments. 

At higher acidities (lower pH) decomposition is slower (ti/2 days or weeks) and the pathways are more complex. The stoichiometry, kinetics and mechanisms of several other reactions of H2N2O2 with, for example, NO and with HNO2 have also been studied. 

To understand the Na,K-pump mechanism it is obviously important to identify the cation pathway and the sites for binding and occlusion of Na and K relative to the intramembrane portion of the protein. The groups coordinating the cations should be identified and it should be known if the pump has independent sites for Na" and K" " or if the cations bind alternately to the same set of sites. With a stoichiometry of 3Na /2K per ATP split this would mean that two sites bind Na and alternately, while one site only binds Na". ... 

It is intriguing to note that this reaction scheme for the reduction of a sulphone to a sulphide leads to the same reaction stoichiometry as proposed originally by Bordwell in 1951. Which of the three reaction pathways predominates will depend on the relative activation barriers for each process in any given molecule. All are known. Process (1) is preferred in somewhat strained cyclic sulphones (equations 22 and 24), process (2) occurs in the strained naphtho[l, 8-hc]thiete 1,1-dioxide, 2, cleavage of which leads to a reasonably stabilized aryl carbanion (equation 29) and process (3) occurs in unstrained sulphones, as outlined in equations (26) to (28). Examples of other nucleophiles attacking strained sulphones are in fact known. For instance, the very strained sulphone, 2, is cleaved by hydride from LAH, by methyllithium in ether at 20°, by sodium hydroxide in refluxing aqueous dioxane, and by lithium anilide in ether/THF at room temperature. In each case, the product resulted from a nucleophilic attack at the sulphonyl sulphur atom. Other examples of this process include the attack of hydroxide ion on highly strained thiirene S, S-dioxides , and an attack on norbornadienyl sulphone by methyllithium in ice-cold THF . ... 

It was not possible from the images to decide on the eventual fate of the OH species. One possible pathway also not considered is the recombination of NH2 radicals and the desorption of hydrazine. If this did occur, then any arguments based on stoichiometries (e.g. OH to NH2) would not be valid. 

The hexose monophosphate pathway has several names just to confuse you. It s called the hexose monophosphate shunt or pathway (HMP shunt or pathway), or the pentose phosphate pathway, or the phospho-gluconate pathway (Fig. 15-1). The pathway in its full form is complicated and has complicated stoichiometry. Usually it s not necessary to remember all of it. The important points are that it makes NADPH for biosynthesis and riboses (C-5 sugars) for DNA and RNA synthesis. 

In the absence of nucleophile, the aryl halide undergoes a two-electron reductive cleavage according to an ECE-DISP mechanism (Scheme 2.21). The two-electron stoichiometry occurs because the aryl radical produced on the one-electron reductive cleavage is easier to reduce than the substrate. The competition between the ECE and DISP pathways is governed by the parameter... 

This section and the next are dedicated to the basics of the silicon-electrolyte contact with focus on the electrolyte side of the junction and the electrochemical reactions accompanying charge transfer. The current across a semiconductor-electrolyte junction may be limited by the mass transport in the electrolyte, by the kinetics of the chemical reaction at the interface, or by the charge supply from the electrode. The mass transport in the bulk of the electrolyte again depends on convection as well as diffusion. In a thin electrolyte layer of about a micrometer close to the electrode surface, diffusion becomes dominant The stoichiometry of the basic reactions at the silicon electrode will be presented first, followed by a detailed discussion of the reaction pathways as shown in Figs. 4.1-4.4. 

Cellular Cl- replenishment is maintained by a basolateral anion-exchanger (Cl /OH or Cr/HCOJ) or via the Na+/K+/2C1 co-transporter, whose activities are closely tied synergistically through action of Na+/K+-ATPase, K+ channels, and the IIC()7/Na+ co-transporter (with a 3 1 stoichiometry) that extrudes HCOJ [61]. Passive Cl- diffusion through the paracellular pathway can occur because of the greater mobility of Cl- than Na+ in the paracellular space ( <1.3) [13]. Electroneutrality is maintained by transcellular Na+ transport in the luminal to subluminal direction, accomplished by both the apical Na+/H+ exchanger (NHE-3) and a basolateral HCOJ/Na+ co-transporter (with a 3 1 stoichiometry). 

The presence of the OH group in alcohols makes alcohol combustion chemistry an interesting variation of the analogous paraffin hydrocarbon. Two fundamental pathways can exist in the initial attack on alcohols. In one, the OH group can be displaced while an alkyl radical also remains as a product. In the other, the alcohol is attacked at a different site and forms an intermediate oxygenated species, typically an aldehyde. The dominant pathway depends on the bond strengths in the particular alcohol molecule and on the overall stoichiometry that determines the relative abundance of the reactive radicals. 

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