Electrical effects

Although the interactions of charged, dipolar or polarizable groups have been investigated for various purposes, they have not often been utilized in the context of stereoselectivity. In fact, when coulombic effects were considered in the SN2 or E2 processes, their role was regarded as unimportant (Ingold, 1953 Cristol, et al., 1951). In view of the substantial electrostatic (field) effects estimated for polar substituents on the pA s of carboxylic acids (Tanford, 1958), metal-ion coordination (Basolo and Pearson, 1967), etc., it will be interesting to see what effects there may be on SS. 

In papers on the Walden inversion, Ingold s group (Harvey et al., 1960) established that electrostatic forces do not contravene the normal stereochemistry of SN2 reactions. Even in the favorable case of negative nucleophile (azide) and positive substrate (sulfonium salt), exclusive inversion occurs. Obviously, the quantal forces of orbital symmetry and energy obliterate the coulombic factor here. 

In any case, the problem of extricating a single classical factor at an Sn2 reaction site would be extremely difficult, as has been noted by Holtz and Stock (1965). These workers provide an estimate of remote dipolar acceleration in the displacement reactions of 4-substituted bicydo[2,2,2] octylmethyl derivatives. For the transition state 100 they 

In Fig. 25, we give scale diagrams for syw-periplanar, cmli-periplanar, and syn-cyclic dehydrochlorination of chloroethane with potassium alkoxide. The standard expression for an ion-dipole interaction is given in (194) (Amis, 1966), in which the negative and positive signs are 

The introduction of ion-ion and ion-dipole attractive terms in the syn-cyclic form of Fig. 26 adds a term —15-2/DB kcal mole-1. The comparison of syn-cyclic and anti forms yields AW — 5/D-e kcal mole-1. These electrostatic factors favoring the syn-cyclie form are exaggerated, since there is probably some compensating ion-pairing in the anti transition state. 

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Theory Cross-flow-elecfrofiltration can theoretically be treated as if it were cross-flow filtration with superimposed electrical effects. These electrical effects include elecfroosmosis in the filter medium and cake and elecfrophoresis of the particles in the shiny. The addition of the applied electric field can, nowever, result in some quahta-tive differences in permeate-flux-parameter dependences. 

With eveiy change in ion concentration, there is an electrical effect generated by an electrochemical cell. The anion membrane shown in the middle has three cells associated with it, two caused by the concentration differences in the boundaiy layers, and one resulting from the concentration difference across the membrane. In addition, there are ohmic resistances for each step, resulting from the E/I resistance through the solution, boundary layers, and the membrane. In solution, current is carried by ions, and their movement produces a fric tion effect manifested as a resistance. In practical applications, I R losses are more important than the power required to move ions to a compartment wim a higher concentration. 

Without coke backfill, the anode reactions proceed according to Eqs. (7-1) and (7-2) with the subsequent reactions (7-3) and (7-4) exclusively at the cable anode. As a result, the graphite is consumed in the course of time and the cable anode resistance becomes high at these points. The process is dependent on the local current density and therefore on the soil resistivity. The life of the cable anode is determined, not by its mechanical stability, but by its electrical effectiveness. 

The piezoelectric response investigation also provides direct evidence that significant inelastic deformation and defect generation can occur well within the elastic range as determined by the Hugoniot elastic limit. In quartz, the Hugoniot elastic limit is 6 GPa, but there is clear evidence for strong nonideal mechanical and electrical effects between 2.5 and 6 GPa. The unusual dielectric breakdown phenomenon that occurs at 800 MPa under certain... 

The piezoelectric polymer investigations give new physical insight into the nature of the physical process in this class of ferroelectric polymers. The strong nonlinearities in polarization with stress are apparently more a representation of nonlinear compressibility than nonlinear electrical effects. Piezoelectric polarization appears to be linear with stress to volume compressions of tens of percent. The combination of past work on PVDF and future work on copolymers, that have quite different physical features promises to provide an unusually detailed study of such polymers under very large compression. 

R. A. Millikan (California Institute of Technology, Pasadena) work on the elementary charge of electricity and on the photo-electric effect. 

Reaction constant for the transmission of electrical effects through atom X to Y. 

Electrical effects were known to the ancients through the attraction that amber, when rubbed, had for lightweight objects. They were also aware of the seemingly unrelated phenomenon of lightning. There is the further possibility that a form of electric battci y was used for electroplating in Mesopotamia, but the evidence is meager and, even if true, there were no longterm consequences. 

We assume that the double bonds in 1,3-butadiene would be the same as in ethylene if they did not interact with one another. Introduction of the known geometry of 1,3-butadiene in the s-trans conformation and the monopole charge of 0.49 e on each carbon yields an interaction energy <5 — 0.48 ev between the two double bonds. Simpson found the empirical value <5 = 1.91 ev from his assumption that only a London interaction was present. Hence it appears that only a small part of the interaction between double bonds in 1,3-butadiene is a London type of second-order electrical effect and the larger part is a conjugation or resonance associated with the structure with a double bond in the central position. 

An example will show the nature of electrical effects (resonance and field) on reactivity. In the alkaline hydrolysis of aromatic amides (10-11), the rate-determining step is the attack of hydroxide ion at the carbonyl carbon ... 

It occasionally happens that a reaction proceeds much faster or much slower than expected on the basis of electrical effects alone. In these cases, it can often be shown that steric effects are influencing the rate. For example, Table 9.2 lists relative rates for the Sn2 ethanolysis of certain alkyl halides (see p. 390). All these compounds are primary bromides the branching is on the second carbon, so that field-effect differences should be small. As Table 9.2 shows, the rate decreases with increasing P branching and reaches a very low value for neopentyl bromide. This reaction is known to involve an attack by the nucleophile from a position opposite to that of the bromine (see p. 390). The great decrease in rate can be attributed to steric hindrance, a sheer physical blockage to the attack of the nucleophile. Another example of steric hindrance is found in 2,6-disubstituted benzoic acids, which are difficult to esterify no matter what the resonance or field effects of the groups in the 2 or the 6 position. Similarly, once 2,6-disubstituted benzoic acids are esterified, the esters are difficult to hydrolyze. 

The CT values are numbers that sum up the total electrical effects (resonance plus field) of a group X when attached to a benzene ring. The treatment usually fails for the ortho position. The Hammett treatment has been applied to many reactions and to many functional groups and correlates an enormous amount of data quite well. Jaffe s review article lists p values for 204 reactions, many of which have different p values for different conditions. Among them are reactions as disparate as the following ... 

This example shows that the insertion of a CH2 or a CH=CH group diminishes electrical effects to about the same extent, while a CH2CH2 group diminishes them much more. A p >1 would mean that the reaction is more sensitive to electrical effects than is the ionization of XC6H4COOH (p= 1.00). 

Another example of the coupling between microscopic and macroscopic properties is the flexo-electric effect in liquid crystals [33] which was first predicted theoretically by Meyer [34] and later observed in MBBA [35], Here orientational deformations of the director give rise to spontaneous polarisation. In nematic materials, the induced polarisation is given by... 

Taft (21) has suggested that the electrical effect of a substituent is composed of localized (inductive and/or field) and delocalized (resonance) factors. Thus we may write the substituent constant of the group X as... 

We had previously described the composition of the electrical effect by means of E, where... 

We believe that a more useful measure of the composition of the electrical effect may be defined as... 

Values of Pr for the substituent constants commonly used with eq. (1) are given in Table I. When correlations are made with eq. (1), the substituent constant used determines the composition of the electrical effect. 

The composition of the electrical effect in these sets is reported in the form of the p values in Table IV. Most of the sets studied show a value of Pr of 42, which corresponds roughly to that of the Op substituent constants. 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

Values of are given in Table VII. The value obtained for set 5-1 seems to show that the substituent effect in the vinylidene position is predominantly a localized effect. Further data are necessary to firmly establish the composition of the electrical effect. 

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