Polar transition state

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... 

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... 

The negative AS was explained in terms of the solvation of the transition state by the reaction medium. Dimerisation reduces the polar nature of acetic acid. Hence, in 1% acetic acid the polar transition state is thought to freeze the benzene molecules, thus producing a negative AS (—5.8 e.u./mol). When the acetic acid concentration is increased, the monomers of acetic acid solvate the transition state preferentially and the... 

It is easy to understand the lower reactivity of non-ionic nucleophiles in micelles as compared with water. Micelles have a lower polarity than water and reactions of non-ionic nucleophiles are typically inhibited by solvents of low polarity. Thus, micelles behave as a submicroscopic solvent which has less ability than water, or a polar organic solvent, to interact with a polar transition state. Micellar medium effects on reaction rate, like kinetic solvent effects, depend on differences in free energy between initial and transition states, and a favorable distribution of reactants from water into a micellar pseudophase means that reactants have a lower free energy in micelles than in water. This factor, of itself, will inhibit reaction, but it may be offset by favorable interactions with the transition state and, for bimolecular reactions, by the concentration of reactants into the small volume of the micellar pseudophase. 

It is well known that the surrounding solvent environment plays a crucial role in a chemical reaction. For example, the formation of tetraethylammonium iodide has been studied in many nonpolar and polar solvents. It is found that the rate of the reaction is quite sensitive to the solvent. From the least polar (hexane) to the most polar (nitrobenzene) solvent, the rate constant increases by 2700 times [1]. The polar transition state of this reaction is stabilized in a high dielectric constant medium. Since the... 

According to results from laser flash photolysis, the p-(methoxyphenyl) sulfanyl radical adds exclusively to the central atom in of 2,4-dimethylpenta-2,3-diene (If) with a rate constant of 1.1 x 10s M-1 s-1 (23 1 °C) (Scheme 11.6) [45], A correlation between the measured rate constants for addition of para-substituted arylsulfanyl radicals to allene If was feasible using Brown and Okomoto s o+ constant [46], The p+ value of 1.83, which was obtained from this analysis, was interpreted in terms of a polar transition state for C-S bond formation with the sulfanyl radical being the electrophilic part [45]. This observation is in agreement with an increase in relative rate constant for phenylsulfanyl radical addition to 1-substituted allene in the series of methoxyallene lg, via dimethylallene Id, to phenylsulfanylallene lh, to ester-substituted 1,2-diene li (Table 11.2). 

---