Effect of negative charge

A ferroelectric crystal is one that has an electric dipole moment even in the absence of an external electric held. This arises because the centre of positive charge in the crystal does not coincide with the centre of negative charge. The phenomenon was discovered in 1920 by J. Valasek in Rochelle salt, which is the H-bonded hydrated d-tartrate NaKC4H406.4H 0. In such compounds the dielectric constant can rise to enormous values of lO or more due to presence of a stable permanent electric polarization. Before considering the effect further, it will be helpful to recall various dehnitions and SI units ... 

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 nature of the nucleophile plays a major role in the SN2 reaction but does not affect an S l reaction. Because the SN1 reaction occurs through a rale-limiting step in which the added nucleophile has no part, the nucleophile can t affect the reaction rate. The reaction of 2-methyl-2-propanoI with HX, for instance, occurs at the same rate regardless of whether X is Cl, Br, or 1. Furthermore, neutral nucleophiles are just as effective as negatively charged ones, so S 1 reactions frequently occur under neutral or acidic conditions. 

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