Reactions Stille sequence

Calculations have shown that water bound to the zinc ion can lose a proton readily, but imidazole bound to the zinc ion cannot. There is still an unsettled question about the first ionization from the zinc-bound water molecule. This reaction seems to be much too fast and is dependent on buffer concentration. The role of the buffer is still unknown, but in some fashion it assists in the reaction. The sequence of reactions usually used to describe the reaction is as follows ... 

If this parallel (concurrent) reaction is faster than Step 2 in the above-mentioned reaction sequence, then it will be rate determining. If, however. Step 2 is faster it will still be rate determining. In a parallel reaction sequence, the faster reaction determines the overall rate. Although slower concurrent reactions still occur, less reactants are consumed by these slower reactions than by the fastest parallel reaction. 

In the first step, persulfide is reduced to sulfide, and in the second step, iron (II) is reduced to metallic iron. Each of the steps represents a two-electron reduction. However, the second step is kinetically faster than the first and under most applications, the reduction appears to be a single four-electron reduction, as summarized below. The reaction still follows the sequence shown above for slow discharge it is just that the intermediate Li2FeS2 discharges as quickly as it is formed. 

A mixed-valent dinickel monohydride 49 was found to react with bases ( BuOK, LiN(SiMe3)j) to produce dinickel(I) compounds 50 and 51, both of which feature a Ni(I)-Ni(I) bond (Scheme 10.21) [22]. Complex 50 features a triangular Ni P core with Ni(I)-Ni(I) bond length of 2.515(1) A (Entry 21, Table 10.2) while 51 features a fused Ni PC and NiPC bicyclic core, with a significantly shorter Ni(I)-Ni(I) bond (2.408(2) A) (Entry 22, Table 10.2) in comparison with that in 50. These reactions involve sequences of deprotonation, C-H/C-P bond activation, and C-H bond formation, although the mechanism is still ambiguous. 

An important point about kinetics of cyclic reactions is tliat if an overall reaction proceeds via a sequence of elementary steps in a cycle (e.g., figure C2.7.2), some of tliese steps may be equilibrium limited so tliat tliey can proceed at most to only minute conversions. Nevertlieless, if a step subsequent to one tliat is so limited is characterized by a large enough rate constant, tlien tire equilibrium-limited step may still be fast enough for tire overall cycle to proceed rapidly. Thus, tire step following an equilibrium-limited step in tire cycle pulls tire cycle along—it drains tire intennediate tliat can fonn in only a low concentration because of an equilibrium limitation and allows tire overall reaction (tire cycle) to proceed rapidly. A good catalyst accelerates tire steps tliat most need a boost. 

Although these results establish with reasonable certainty the reaction sequences leading from the sugar to the dye, no stoichiometric relationships have been shown to exist between the amounts of sugar employed and the quantities of dyes formed. Both tests are, therefore, still empirical,... 

From a reaction engineering viewpoint, semiconductor device fabrication is a sequence of semibatch reactions interspersed with mass transfer steps such as polymer dissolution and physical vapor deposition (e.g., vacuum metallizing and sputtering). Similar sequences are used to manufacture still experimental devices known as NEMS (for nanoelectromechanical systems). 

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