Transition state solvent influence

A fourth factor is related to the differential solvation energy of similar transition states. This influence, solvent-induced variation in relative reactivity, is measurable for chlorination (Stock and Himoe, 1962) and... 

The dissection of solvent influences on rate constants into initial-state and transition-state contributions, especially when combined with a knowledge of solvent effects on the thermodynamic properties of suitable model solutes, is a powerful method for the examination of transition states. In favorable cases with electrically neutral transition states, it is possible to estimate the degree of charge separation in the transition state and its position along the reaction coordinate. Dissections in terms of enthalpy and entropy are useful also in probing specific transition-state/solvent interactions. 

This is a question of reaction prediction. In fact, this is a deterministic system. If we knew the rules of chemistry completely, and understood chemical reactivity fully, we should be able to answer this question and to predict the outcome of a reaction. Thus, we might use quantum mechanical calculations for exploring the structure and energetics of various transition states in order to find out which reaction pathway is followed. This requires calculations of quite a high degree of sophistication. In addition, modeling the influence of solvents on... 

Important differences are seen when the reactions of the other halogens are compared to bromination. In the case of chlorination, although the same chain mechanism is operative as for bromination, there is a key difference in the greatly diminished selectivity of the chlorination. For example, the pri sec selectivity in 2,3-dimethylbutane for chlorination is 1 3.6 in typical solvents. Because of the greater reactivity of the chlorine atom, abstractions of primary, secondary, and tertiary hydrogens are all exothermic. As a result of this exothermicity, the stability of the product radical has less influence on the activation energy. In terms of Hammond s postulate (Section 4.4.2), the transition state would be expected to be more reactant-like. As an example of the low selectivity, ethylbenzene is chlorinated at both the methyl and the methylene positions, despite the much greater stability of the benzyl radical ... 

These isomerization processes may be dependent on the nature of the solvent. For example, the rotational barrier of the tetrazathiapentalenes 15.15 (ca. 16 kcal moF ) is influenced by the donor or acceptor ability of the substituents X and Y through the S N short contacts.Solvents with acidic protons increase the magnitude of the barrier, whereas solvents that are good Lewis bases decrease the size of the barrier, owing to solvation of the transition state. 

In this solvent the reaction is catalyzed by small amounts of trimethyl-amine and especially pyridine (cf. 9). The same effect occurs in the reaction of iV -methylaniline with 2-iV -methylanilino-4,6-dichloro-s-triazine. In benzene solution, the amine hydrochloride is so insoluble that the reaction could be followed by recovery. of the salt. However, this precluded study mider Bitter and Zollinger s conditions of catalysis by strong mineral acids in the sense of Banks (acid-base pre-equilibrium in solution). Instead, a new catalytic effect was revealed when the influence of organic acids was tested. This was assumed to depend on the bifunctional character of these catalysts, which act as both a proton donor and an acceptor in the transition state. In striking agreement with this conclusion, a-pyridone is very reactive and o-nitrophenol is not. Furthermore, since neither y-pyridone nor -nitrophenol are active, the structure of the catalyst must meet the conformational requirements for a cyclic transition state. Probably a concerted process involving structure 10 in the rate-determining step... 

The influence of the solvent on the decomposition rate of phenylpentazole (see Table II) supports the one-step four-centered 1,3-flssion of phenylpentazole according to 22. The correlation of the polarity of the transition states of 1,3-additions, which resemble the assumed transition state of the phenylpentazole decomposition (22), andsolvent effects has recently been discussed by Huisgen. ... 

For amide enolates (X = NR2), with Z geometry, model transition state D is intrinsically favored, but, again, large X substituents favor the formation of nt/-adducts via C. Factors that influence the diastereoselectivity include the solvent, the enolate counterion and the substituent pattern of enolate and enonc. In some cases either syn- or unh-products are obtained preferentially by varying the nature of the solvent, donor atom (enolate versus thioeno-late), or counterion. Most Michael additions listed in this section have not been examined systematically in terms of diastereoselectivity and coherent transition stale models are currently not available. Similar models to those shown in A-D can be used, however all the previously mentioned factors (among others) may be critical to the stereochemical outcome of the reaction. 

Finally it should be said that o-/p-ratios can be considerably influenced by the solvent in which the reaction is carried out. This can arise from changes in the relative stabilisation by solvent molecules of the transition states for o- and p-attack, but it may also involve the actual attacking electrophile being different in two different solvents the species actually added complexing with solvent molecules to form the electrophile proper—a different one in each case. This almost certainly occurs in halogenation without Lewis acid catalysts, e.g. in the chlorination of toluene at 25°, where f0./fp. ratios between 0-75 and 0-34 have been observed depending on the solvent. 

Diels-Alder reactions are found to be little influenced by the introduction of radicals (cf. p. 300), or by changes in the polarity of the solvent they are thus unlikely to involve either radical or ion pair intermediates. They are found to proceed stereoselectively SYN with respect both to the diene and to the dienophile, and are believed to take place via a concerted pathway in which bond-formation and bond-breaking occur more or less simultaneously, though not necessarily to the same extent, in the transition state. This cyclic transition state is a planar, aromatic type, with consequent stabilisation because of the cyclic overlap that can occur between the six p orbitals of the constituent diene and dienophile. Such pericyclic reactions are considered further below (p. 341). 

It can be concluded that mBr depends on the magnitude of the charge at the transition state and also on its delocalization either by the substituents or by the solvent. It therefore seems difficult to separate these effects, since R, the measure of solvent assistance, depends also on the same factors. The idea that transition-state shifts contribute to m-variations is supported by substituent effects. Consequently, it would be useful to obtain p-m correlations to compare the influence of the solvent and the substituents in determining the position of the bromination transition state. 

Having a weak O—O bond, peroxides split easily into free radicals. In addition to homolytic reactions, peroxides can participate in heterolytic reactions also, for example, they can undergo hydrolysis under the catalytic action of acids. Both homolytic and heterolytic reactions can occur simultaneously. For example, perbenzoates decompose into free radicals and simultaneously isomerize to ester [11]. The para-substituent slightly influences the rate constants of homolytic splitting of perester. The rate constant of heterolytic isomerization, by contrast, strongly depends on the nature of the para-substituent. Polar solvent accelerates the heterolytic isomerization. Isomerization reaction was proposed to proceed through the cyclic transition state [11]. 

Since the reactants (R02 ketone) and the transition state have a polar character, they are solvated in a polar solvent. Hence polar solvents influence the rate constants of the chain propagation and termination reactions. This problem was studied for reactions of oxidized butanone-2 by Zaikov [81-86]. It was observed that kp slightly varies from one solvent to another. On the contrary, kt changes more than ten times from one solvent to another. The solvent influences the activation energy and pre-exponential factor of these two reactions (see Table 8.16). 

This relationship proves that the influence of solvents (decane, benzene, acetic acid, p-dichlorobenzene) on the rate constant is due to the nonspecific solvation of the reactants and the transition state. The values of the dipole moments of the TS calculated from experimenntal data within the scope of Equation (18.2) have the following values [87] ... 

We have seen that 1,2-H migrations in singlet carbenes may be affected by (e.g.) the participation of carbene precursor excited states, QMT, stabilization of the hydride shift transition state by polar solvents, and temperature. Here, we consider our third principal theme, the effect of substituents on the kinetics of carbenic rearrangements. We first examine the influence of bystander and spectator substituents (as defined in Eq. 22) on 1,2-H rearrangements of alkyl, alkylchloro, and alkylacetoxycarbenes. 

Generally speaking, the influence of solvent on reaction rates (equilibria) is determined by the difference between the effects < n the stability of transition states (products) and reactants. According to what Leffler and Grunwald (1963) call the first approximation, the free energy of a solute molecule RX is given by the sum of internal and solvent contributions, as shown in (59). The... 

A well defined theory of chemical reactions is required before analyzing solvent effects on this special type of solute. The transition state theory has had an enormous influence in the development of modern chemistry [32-37]. Quantum mechanical theories that go beyond the classical statistical mechanics theory of absolute rate have been developed by several authors [36,38,39], However, there are still compelling motivations to formulate an alternate approach to the quantum theory that goes beyond a theory of reaction rates. In this paper, a particular theory of chemical reactions is elaborated. In this theoretical scheme, solvent effects at the thermodynamic and quantum mechanical level can be treated with a fair degree of generality. The theory can be related to modern versions of the Marcus theory of electron transfer [19,40,41] but there is no... 

Thus far, these chiral ionic liquids do not appear to exert an influence, possibly because the structuring of the solvents around the reactions transition state is not rigid enough. These investigations are still in progress. 

Furthermore, although the intercepts k kiK/k- ) and the slope (kikjK/k-i) are equally influenced by the dimerization constant K in equation 28, this does not imply that they should show the same effect on changing the solvent. According to the dimer mechanism , it could be expected that the base catalysed decomposition of the transition state SB2, measured by Ag, should be more depressed by small additions of protic solvents than the spontaneous decomposition measured by Ag. Indeed, the overwhelming evidence on the classical base catalysis by amines shows that usually Ag is more important in aprotic than in protic solvents1. 

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