Charge development

The relation between p values and Aq values is determined by the extent of substituent interaction with the charge developed in the transition state and thus is related to the pattern of charge distribution within the ring itself (386). 

This suggests that as water attacks the bromonium ion positive charge develops on the carbon from which the bromine departs The transition state has some of the character of a carbocation We know that more substituted carbocations are more stable than less substituted ones therefore when the bromonium ion ring opens it does so by breaking the bond between bromine and the more substituted carbon... 

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. 

The electrophilic reactions of Table 7-5 are clear-cut examples of positive charge development and negative p values. 

Next we turn to the magnitudes of the p constants. Evidently if p = 0, there is no substituent effect on reactivity. Moreover because p = -I-1.000 by definition for the aqueous ionization of benzoic acids, we have a scale calibration of sorts. Wiberg gives examples of p as a measure of the extent of charge development in the transition state. McLennan" has pointed out that p values must first be adjusted for the transmission factor before they can be taken as measures of charge devel-... 

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... 

For illite (Figure 3B) the total amount of Cs adsorbed is two orders of magnitude larger (ca. 0.26 atoms/A2) due primarily to the permanent charge developed by substitution of Al for Si in the tetrahedral sites and to less efficient filtering of the solution caused by lower sample permeability. The maximum amount of hydrated Cs that can be accommodated on the illite surface is ca. 0.023 atoms/A, close to the amount... 

Various ab initio and scmi-cmpirical molecular orbital calculations have been carried out on the reaction of radicals with simple alkenes with the aim of defining the nature of the transition state (Section 1.2.7).2I>,j , 6 These calculations all predict an unsymmetrical transition state for radical addition (i.e. Figure 1.1) though they differ in other aspects. Most calculations also indicate a degree of charge development in the transition state. 

Ionic polymerisation is subdivided into cationic and anionic mechanisms depending on the charge developed in the growing polymer molecule. Typical catalysts for the former, the cationic polymerisation process, are Lewis acids such as AICI3 or BFj, which often require a co-catalyst, usually a Lewis base, in order to bring about polymerisation. 

When there is no conjugated substituent able to stabilize the positive charge development, the bromine atom is involved in the charge stabilization and the... 

The presence of the zeolite cavity dramatically lowers the activation energy for the protonation of toluene. It is mainly due to screening of the charges in the transition state due to the polarizable lattice oxygen atoms. In the transition state, a positive charge develops on protonated toluene. 

A Surface Science Instruments SSX-100 spectrometer (model 206), equipped with an aluminum anode whose radiation was monochromatized (AlKa, 1486.6 eV) and focalized, was used. The positive charge developed at the surface of the samples was compensated with a charge neutralizer adjusted at an energy of 8 eV. 

Mechanistic studies have been designed to determine if the concerted cyclic TS provides a good representation of the reaction. A systematic study of all the E- and Z-decene isomers with maleic anhydride showed that the stereochemistry of the reaction could be accounted for by a concerted cyclic mechanism.19 The reaction is only moderately sensitive to electronic effects or solvent polarity. The p value for reaction of diethyl oxomalonate with a series of 1-arylcyclopentenes is —1.2, which would indicate that there is little charge development in the TS.20 The reaction shows a primary kinetic isotope effect indicative of C—H bond breaking in the rate-determining step.21 There is good agreement between measured isotope effects and those calculated on the basis of TS structure.22 These observations are consistent with a concerted process. 

Direct connection of pendant heteroatom to polystyrene aryl is a synthetically more difficult, but often still feasible (37), alternative. However, though bonds from phenyl to many common heteroatoms are relatively strong, resonance stabilization of partial positive charge developed on an arylated atom activates it to leave other substituents alkyl anilium salts (12) and anilines (38), as well as phenolic esters (39), are relatively easy to cleave. Aryl linkages,... 

Of special interest is the civilian application of shaped charges developed initially for military purposes (see Chapter 15, p. 158). One such application is the blast furnace tapper illustrated in Fig. 14.11. In this... 

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