Charge dispersive

Let us now return to the question of solvolysis and how it relates to the stracture under stable-ion conditions. To relate the structural data to solvolysis conditions, the primary issues that must be considered are the extent of solvent participation in the transition state and the nature of solvation of the cationic intermediate. The extent of solvent participation has been probed by comparison of solvolysis characteristics in trifluoroacetic acid with the solvolysis in acetic acid. The exo endo reactivity ratio in trifluoroacetic acid is 1120 1, compared to 280 1 in acetic acid. Whereas the endo isomer shows solvent sensitivity typical of normal secondary tosylates, the exx> isomer reveals a reduced sensitivity. This indicates that the transition state for solvolysis of the exo isomer possesses a greater degree of charge dispersal, which would be consistent with a bridged structure. This fact, along with the rate enhancement of the exo isomer, indicates that the c participation commences prior to the transition state being attained, so that it can be concluded that bridging is a characteristic of the solvolysis intermediate, as well as of the stable-ion structure. ... 

In general, the reaction between a phenol and an aldehyde is classified as an electrophilic aromatic substitution, though some researchers have classed it as a nucleophilic substitution (Sn2) on aldehyde [84]. These mechanisms are probably indistinguishable on the basis of kinetics, though the charge-dispersed sp carbon structure of phenate does not fit our normal concept of a good nucleophile. In phenol-formaldehyde resins, the observed hydroxymethylation kinetics are second-order, first-order in phenol and first-order in formaldehyde. 

Greater charge dispersal in the transition state may cause a greater rate of ethoxy-dechlorination for nitrohalobenzenes (38 and 39) than for chloropyridines (40 and 41) as discussed in Sections II,B, l,a and II, E, 2, c. The kinetic parameters are given in Table VIII, lines 2 and 5, and in Table II, lines 1 and 4. 

In general, resonance effects lead to the same result as field effects. That is, here too, electron-withdrawing groups increase acidity and decrease basicity, and electron-donating groups act in the opposite manner. As a result of both resonance and field effects, charge dispersal leads to greater stability. 

Effect of Solvent on Elimination versus Substitution. Increasing polarity of solvent favors Sn2 reactions at the expense of E2. In the classical example, alcoholic KOH is used to effect elimination, while the more polar aqueous KOH is used for substitution. Charge-dispersal discussions, similar to those on page 450, only partially explain this. In most solvents, SnI reactions are favored over El. The El reactions compete best in polar solvents that are poor nucleophiles, especially dipolar aprotic solvents" A study made in the gas phase, where there is no solvent, has shown that when 1-bromopropane reacts with MeO only elimination takes place no substitution even with this primary substrate." ... 

Alkene protonation at pore mouths can exclusively lead to secondary carbenium ions. In addition, the alkene standard protonation enthalpies increase with the number of carbon atoms inside the micropore because charge dispersive effects are supposed to be more effective on carbon atoms inside the micropores. 

As we have discussed, the LCFC approach predicts that all 1,1 -disubstituted molecules, where the two substituents are identical, are more stable than 1,2-isomers, regardless of the electronegativity of the substituent. On the other hand, Kollman proposed that the greater stability of 1,1-difluoroethylene relative to the 1,2-ds-isomer is a charge effect. An electrostatic depiction was provided in order to exemplify the argument, i. e. Kollman implied that better charge dispersal obtains in A than in B. 

Figure 5.16 Typical stress relaxation data for concentrated charge dispersions. Two models are shown, one based on a model for the relaxation spectra (Equation 5.59) and one based on an extended exponential (Equation 5.51)...

A note of caution should be sounded here. Whilst the curves shown in Figure 6.5 are characteristic of many charged dispersions it should be recalled that once we apply a sinusoid to a non-linear system the response need not be a sinusoid. As the strain is increased into the nonlinear region, the waveform passing through the sample becomes progressively distorted. The instrumental analysis in this case involves... 

Furthermore, in the addition to the 3,4-bond of 1,3-pentadienes, the anti stereoselectivity observed with both bromine and chlorine has been attributed to a tightly bridged bromonium ion intermediate involving less charge dispersal in the vinyl group. In support of this hypothesis, it has been noted that bromine addition to the terminal double bond of the 1,3-pentadienes occurs without isomerization of the internal cis or trans double bond15. 

However, the ready distortion of the ar-electron system provides an additional mechanism whereby the charge dispersal can reach the substituents. The greater substituent effects in ketones compared to the alcohols are therefore equally consistent with the loss of an oxygen nonbonding electron. Unsaturated substituents which can conjugate with the carbonyl double bond do not have the expected large effect in reducing... 

This may reflect that there is more charge dispersion in the addition cation from attack at C-2 than there is from attack at C-3. This is, of course, exactly the same argument as used above for C-protonation protonation (pyrrole acting as a base) also occurs at... 

If we protonate on N-3, we can predict resonance stabilization involving either of the adjacent amino functions. In each case we see charge distribution over three atoms using an N-C=N system. However, protonation on N-1 allows similar resonance stabilization, but increased charge dispersal emanating from the... 

The protonated alcohol loses water readily to form the benzyl carbocation, which is stabilized by charge dispersion to the ring. 

Nucleophilicity. A distinction is usually made between nucleophilicity and Lowry-Bronsted basicity [213]. The latter involves specifically reaction at a proton which is complexed to a Lewis base (usually H2O), while the former refers to reactivity at centers other than H. Linear correlations have been shown for gas-phase basicity (proton affinity) and nucleophilicity of nitrogen bases toward CH3I in solution [214] where the solvent is not strongly involved in charge dispersal. In each case, reaction of the base/nucleophile... 

Spherical latex particles with a reasonably well-defined number of charges per particle can be synthesized and used to study the non-Newtonian behavior of charged dispersions and related electroviscous phenomena (described in Chapter 4). The surface... 

From a technical standpoint, it is also important to note that colloids display a wide range of rheological behavior. Charged dispersions (even at very low volume fractions) and sterically stabilized colloids show elastic behavior like solids. When the interparticle interactions are not important, they behave like ordinary liquids (i.e., they flow easily when subjected to even small shear forces) this is known as viscous behavior. Very often, the behavior falls somewhere between these two extremes the dispersion is then said to be viscoelastic. Therefore, it becomes important to understand how the interaction forces and fluid mechanics of the dispersions affect the flow behavior of dispersions. 

The secondary electroviscous effect refers to the change in the rheological behavior of a charged dispersion arising from interparticle interactions, i.e., the interactions between the electrical double layers around the particles. 

In Section 4.7c we outlined the types of effects one can expect in the response of charged dispersions to deformation. In this section, we present some results for the viscosity of charged colloids for which electroviscous effects could be important. As mentioned above, we shall not go into the theoretical details behind the equations since they require a fairly advanced knowledge of fluid dynamics and, in some cases, statistical mechanics. Moreover, some of... 

The intrinsic viscosity [17] in the above expression includes the primary electroviscous effect. The experimental data of Stone-Masui and Watillon (1968) for polymer latices seem to be consistent with the above equation (Hunter 1981). Corrections for a for large values of kRs are possible, and the above equation can be extended to larger Peclet numbers. However, because of the sensitivity of the coefficients to kRs and the complications introduced by multiparticle and cooperative effects, the theoretical formulations are difficult and the experimental measurements are uncertain. For our purpose here, the above outline is sufficient to illustrate how secondary electroviscous effects affect the viscosity of charged dispersions. 

Both k and k 1 are used extensively in this chapter and Chapters 12 and 13 (as well as in the sections on the rheology of charged dispersions in Chapter 4). Table 11.3 lists numerical values for these quantities and the pertinent equations for their calculation for aqueous solutions at 25 °C. This table may be consulted as a source for k and k 1 values when these are required for exercises in these chapters. 

FIG. 13.3 Micrographs of local structure in a charged dispersion (a) an ordered region coexisting with a liquidlike region (b) an ordered crystalline structure, with a two-dimensional slice of the crystal shown. (Photographs courtesy of Dr. Norio lse, Fukui Laboratory, Rengo Co., Fukui, Japan.)... 

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