Polarization transition

Metwally et al. [28] also studied the resin-catalyzed hydrolysis of ethyl formate in acetone-water mixtures at different temperatures. The experimental results indicated a linear dependence of the logarithm of rate constant on the reciprocal of the dielectric constant (Fig. 2). The decrease of dielectric constant may lower the concentration of the highly polar transition state and thereby decrease the rate [28]. 

Much of the kinetics and products work already described has been due to Banthorpe et al. who have produced a mechanism for the benzidine rearrangement42 which adequately explains the known facts. This has been called the Polar-Transition-State Mechanism and is currently accepted as being the most satisfactory description of the rearrangement. Other mechanisms have been proposed over the years and their limitations discussed (for detailed account see ref. 48). 

The Polar-Transition-State theory based on earlier ideas by Hughes and Ingold49, has as its main feature the heterolysis of the N-N bond in the mono- or di-protonated hydrazo molecule as the transition state is approached, with a de-localisation of the positive charge in the mono-protonated case and of one of the positive charges in the di-protonated case, viz. (16) and (17), respectively... 

The rate constants and their activation parameters are given in Table 9-2. As one would expect, the reaction is faster the more polar the solvent. This reaction is likely to have a polar transition state, which would be stabilized in a medium of high dielectric constant. A quantitative correlation will be given in Section 9.4. 

It is possible to take advantage of the differing characteristics of the periphery and the interior to promote chemical reactions. For example, a dendrimer having a non-polar aliphatic periphery with highly polar inner branches can be used to catalyse unimolecular elimination reactions in tertiary alkyl halides in a non-polar aliphatic solvent. This works because the alkyl halide has some polarity, so become relatively concentrated within the polar branches of the dendrimer. This polar medium favours the formation of polar transition states and intermediates, and allows some free alkene to be formed. This, being nonpolar, is expelled from the polar region, and moves out of the dendrimer and into the non-polar solvent. This is a highly efficient process, and the elimination reaction can be driven to completion with only 0.01 % by mass of a dendrimer in the reaction mixture in the presence of an auxiliary base such as potassium carbonate. 

This step was also part of the polar transition state mechanism see Ref. 610. 

A detailed examination of the kinetics of dimethylaminolysis of N3P3C16 by Krishnamurthy and co-workers has revealed that there is a gradual and subtle mechanistic change that occurs as the degree of replacement of chlorines increases (92). While the first chlorine replacement follows an Sn2 pathway involving the formation of a neutral five-coordinate intermediate [Fig. 8(A)], at the second stage the mechanism can be induced to follow a concerted path [Fig. 8(B)] by using acetonitrile as the solvent. The polar transition state of the concerted path reaction pathway is stabilized in acetonitrile. This postulate has sup-... 

On the other hand, many pericyclic reactions are accelerated by Lewis-acid catalysts. The acceleration has been attributed to a complex formation between the Lewis acid and the polar groups of the reactants that brings about changes in the energies and orbital coefficients of the frontier orbitals.6 The complex formation also stabilizes the enhanced polarized transition state. 

Again it appears that electronic factors (e.g., a polar transition state) are important in determining the dimerization. 

Enhanced decarboxylation in polar solvents may be due to stabilization of polar transition states and/or solvent coordination to the metal (20). Coordination of solvent or ligands may aid decarboxylation by weakening metal-oxygen bonding (10). It also reduces the electrophilicity of the metal, the consequences of which are considered later. 

The effect of solvent on regioselectivity was attributed to nonthermal effects, which are favored in nonpolar solvents and under solvent-free conditions, where products formed via more polar transition states would be expected to predominate. 

MW-expedited dehydration reactions using montmorillonite K 10 clay [70] (Schs. 6.20 and 6.21) or Envirocat reagent, EPZG [71] (Schs. 6.20 and 6.21) have been demonstrated in a facile preparation of imines and enamines via the reactions of primary and secondary amines with aldehydes and ketones, respectively. The generation of polar transition state intermediates in such reactions and their enhanced... 

For systems where the polarization dependence of the edge structure is already known, polarized measurements could be used to determine the sample orientation. For example, the intense pre-edge transition described for (acac) V 0 is also found in the isotropic absorption spectra of M 0(porphyrin) (M Ti,V,Cr) systems (33). Strongly polarized transitions like these could be used, for example, to determine the orientation of the porphyrin moiety within an ordered system such as a biological membrane or fiber (24). 

In many other reactions where ionic or polar transition metal catalysts are used it has been demonstrated that the use of polar and weekly coordinating ionic liquids can result in a clear enhancement of catalytic activity [32a]. 

The polar carbonyl group interacts with the polar transition state of the reaction between the peroxyl radical and the C—H bond of the aldehyde. This interaction lowers the activation energy of this reaction (see Section 8.1.4). As a result, all the three factors, viz., the strong RC(0)00—H bond formed, the weak C—H bond of the oxidized aldehyde, and the polar interaction in the transition state, contribute to lowering the activation energy of the reaction RC(0)00 + RCH(O) and increasing the rate constant of the chain propagation reaction (see Section 8.1.4). 

We see that the ester peroxyl radical is nearly five times more active than the cumyl peroxyl radical. Two reasons are possible for such a difference difference in the BDE of the formed O—H bond and polar influence of the ester group on the polar transition state. One can suppose that the BDE of the formed O—H bond in ester hydroperoxide is more than that of... 

Since ditriptoyl peroxide is electrically symmetrical, and since benzene is not outstanding in its ability to solvate polar transition states, it seems probable that the inversion reaction in this case is due to the rearrangement of an acyloxy radical rather than cation. It may be that failure to isolate comparable products from other peroxides under free radical conditions is due to competition from very fast substitution... 

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