Stability of negative charge

Zahler and elaborated in a series of papers by Miller and co-workers and in Bunnett s publications, many of which are cited in Section I, D. It should be pointed out that the effects of substituents on nucleophilic substitutions show important differences from their effects on other reactions or on equilibria which involve competition for a lone-pair of electrons on another group or stabilization of negative charge on some atom of the reacting moiety. The cr-constants for nucleophilic substitutions differ from those determined in the latter work in that they show the response of the substituent to a strong demand for stabilization of negative charge on the substituent itself, especially by resonance. 

Figure 6 Effect of low-molecular weight (3,400) po-ly(MA-CDA)s on the stability of negatively charged liposome. = unmodified MA-CDA-3.4K = MA-CDA-3.4K-A18 = MA-CDA-3.4K-H27.

The position of equilibrium can be easily predicted by comparing the relative stability of negative charges. 

Ketones, esters and nitriles but not aldehydes, which take a different route also can be reacted with LDA. See Table 17-1(6) for stabilization of negative charge on a C a to a C N group. 

SAMPLE SOLUTION (a) The best approach when comparing the acidities of different phenols is to assess opportunities for stabilization of negative charge in their anions. Electron delocalization in the anion of p-hydroxybenzaldehyde is very effective because of conjugation. 

A situation in which the pyridine nitrogen is not strictly the reaction site, but where the inductive stabilization of negative charge by the azonium site is the key role leading to reactivity is that of pyridine ylide formation, by deprotonation of the 2- and 6-positions of 3-substituted 1-methylpyridinium ions in D20 buffer solution catalyzed by OD. A reasonable correlation is produced using ox (p = 8.9) for the reactivity of... 

Figure 6.2 The mechanism of cytochrome-c-peroxidase complex formation, (a) Native enzyme, (b) Activated complex with the acid-base catalytic function of distal histidine (His) and stabilization of negative charge by arginine (Arg) residue of the active site, (c) Hypothetic intermediate oxene complex, (d) Complex I after intramolecular electron regrouping of oxene complex with Fe4+ and free radical X fragment formation.

Volodkin D, Mohwald H, Voegel J-C et al (2007) Stabilization of negatively charged liposomes by polylysine surface coating. Drug release study. J Control Release 117 111-120... 

Caution Electronegativity is the major factor when comparing the stability of negatively charged atoms in the same period of the Periodic Table. Other factors, which we will discuss in Section 2.5D and in future chapters, will become significant when comparing atoms in the same group (vertical column) of the Periodic Table. 

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