Negative charge developing

Because in this case the complex is formed between two neutral species, it too is neutral, but a formal positive charge develops on the donor atom and a formal negative charge develops on the aeceptor atom. The result is to increase the effective electronegativity of the donor atom and increase the electrophilicity of the complexed fimctional group. 

An active-site zinc ion stabilizes negative charge development on the oxygen atom of acetaldehyde, leading to an induced partial positive charge on the carbonyl C atom. Transfer of the negatively charged hydride ion to this carbon forms ethanol. 

The common characteristics of the above mentioned heterocycles are electron withdrawing and a site of unsaturation that can stabilize the negative charge developed by the displacement reaction through resonance. For example, the thiazole activated halo displacement is similar to that of a conventional activating group as shown in Scheme 1. The activation is derived from the electron affinity and the stabilization of the negative... 

The small Hammett p value of +0.16 observed for a series of related meta- and para-substituted mandelic acids indicates that there is a very small negative charge development on the benzyl carbon in the transition state of the rate-determining step of the pyridine catalysed oxidation of mandelic acid. The large positive AS value (+24 e.u./mol) found for the catalysed reaction led Banetjee and coworkers to conclude that the transition state (Figure 5) is product-like . This conclusion is consistent with the small f n/f D that is observed in this reaction164. The Pb—O bond is shown to rupture in a heterolytic fashion because Partch and Monthony185 have demonstrated that pyridine diverts the reaction from a homolytic to a heterolytic mechanism. 

A similar analysis suggests the absence of significant negative charge development as well. The fact that p-haloethyl derivatives have only a weak deactivating effect, rather than an activating effect on SN2 reactivity (Hine and Brader, 1953 Hine, 1962 Streitwieser, 1962) implies that there is no... 

The sodium-potassium pump displays a curious stoichiometry. Three sodium ions are pumped from the inside and two potassium ions from the outside of a cell for each molecule of ATP cleaved. Thus, an excess of positive ions is pumped out with the result that a negative charge develops inside the cell and a positive charge accumulates on the outside. This action of the Na+,... 

Similarly, the negative charge developing on the alcohol expelled from the acetal can be stabilized by proton transfer from an acid. This is known as general-acid catalysis (equation 2.11). 

Most zeolites have an intrinsic ability to exchange cations [1], This exchange ability is a result of isomorphous substitution of a cation of trivalent (mostly Al) or lower charges for Si as a tetravalent framework cation. As a consequence of this substitution, a net negative charge develops on the framework of the zeolite, which is to be neutralized by cations present within the channels or cages that constitute the microporous part of the crystalline zeolite. These cations may be any of the metals, metal complexes or alkylammonium cations. If these cations are transition metals with redox properties they can act as active sites for oxidation reactions. 

When a thiocarbonyl undergoes a nucleophilic attack at carbon, a negative charge develops at S, which can accommodates it easily (remember that a sulfur atom can stabilize an adjacent negative charge35). On the other hand, thiophilic attacks give... 

See how similar these two mechanisms are. In particular, they are the same at the carbonyl group itself. Electrons move into the C=0 n orbital the C=0 bond becomes a C-O single bond as a negative charge develops on the oxygen atom. It should come as no surprise that the order of reactivity for enolization is the same as the order of reactivity towards nucleophilic attack. 

In the cleavage of the benzyl-silicon bond of 6 by base, negative charge develops on the benzyl carbon in the transition state, and electron withdrawal by X facilitates the reaction. When X is /)-H(C=C)2, the rate of cleavage is 3300 times as great as when X = H This corresponds to a value of 0-72 for the o constant, and indicates that the group can withdraw electrons strongly. 

To translate this schematic reasoning into sound chemistry in such a complicated case one cannot naively place pluses and minuses here and there to indicate positive and negative charge development. System I demands careful analysis in terms of the Woodward-Hoffmann symmetry rules since this is a thermally induced reaction. 

If the development rate by negatively charged developing agent ions is monitored until all the silver halide is reduced, it follows a typical S-shaped curve that has been interpreted as a consequence of autocatalysis caused by the increasing surface area of the developed silver. An alternative interpretation by Gavrik [23] holds that this autocatalysis is a consequence of simpler kinetics and that development is better represented by simple relations where developed silver mass is proportional to development time with an initial induction period and a final exhaustion period when all the silver halide has been reduced. Levenson [24], however, has reaffirmed the autocatalytic view. 

The loss of nitrogen might occur with the intermediate formation of ions or radicals. In the heterolytic process, a negative charge develops on carbon because of resonance stabilization by the nitro group and the nitrogen in the oxazole ring. 

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