Encounter control

Alkyl radicals react in solution very rapidly. The rate of their disappearance is limited by the frequency of their encounters. This situation is known as microscopic diffusion control or encounter control, when the measured rate is almost exactly equal to the rate of diffusion [230]. The rate of diffusion-controlled reaction of free radical disappearance is the following (the stoichiometric coefficient of reaction is two [233]) ... 

The discrepancy between calculated and experimental AHf° value of 162 was resolved as it turned out that the latter has to be considered as the upper limit resulting from the assumption of the interception of 162 by 302 under encounter control. However, the numerical simulation of the experimental rate data was also perfectly successful by imposing an activation barrier of 7 kcal mol-1 on the trapping step [13], This brought the AH/ of 162 down to 78 kcal mol-1 above that of benzene [13], in good agreement with calculated values [13,116,117]. 

A reaction velocity equal to the rate of encounter of reacting molecular entities (also known as diffusion-con-trolled rate). For a bimolecular reaction in aqueous solutions at 25°C, the corresponding second-order rate constant for the encounter-controlled rate is typically about 10 ° M s See Diffusion Control for Bimolecular Collisions in Solution... 

ENCOUNTER-CONTROLLED RATE DIFFUSION-LIMITED REACTION CHEMICAL KINETICS DIFFUSION OF LIGAND TO RECEPTOR DIFFUSION OF MOLECULES INTO A PORE... 

STEREOCHEMICAL TERMINOLOGY, lUPAC RECOMMENDATIONS ENANTIOSELECTIVE REACTION ASYMMETRIC INDUCTION ENCOUNTER COMPLEX ENCOUNTER-CONTROLLED RATE DIFFUSION CONTROL FOR BIMOLECULAR COLLISIONS ENDERGONIC PROCESS ENDO-a (or j8)-N-ACETYLGALACTOSAMI-NIDASE... 

ENCOUNTER-CONTROLLED RATE SECOND-ORDER REACTiON CHEMICAL KINETICS ORDER OF REACTION NOYES EQUATION MOLECULARITY AUTOCATALYSIS FIRST-ORDER REACTION... 

For bimolecular second-order reactions and for trimolecular reactions, if the reaction rate is very high compared to the rate to bring particles together by diffusion (for gas-phase and liquid-phase reactions), or if diffusion is slow compared to the reaction rate (for homogenous reaction in a glass or mineral), or if the concentrations of the reactants are very low, then the reaction may be limited by diffusion, and is called an encounter-controlled reaction. 

For a fuUy diffusion-controlled (or encounter-controlled) reaction. 

An example of the application of this test to a compound that nitrates as its free base is provided by pyridine 1-oxide. Under an identical set of conditions, nitration of this N-oxide had a half life of 20 min, whilst 1-methoxypyridinium gave no nitro compound in 144 h. Two further criteria have been used to provide confirmatory evidence, namely comparison of the rate of nitration for the reactive species with the encounter controlled rate, and by determination of the Arrhenius parameters. 

Aryl radical additions to anions are generally very fast, with many reactions occurring at or near the diffusion limit. For example, competition studies involving mixtures of nucleophiles competing for the phenyl radical showed that the relative reactivities were within a factor of 10, suggesting encounter control,and absolute rate constants for additions of cyanophenyl and 1-naphthyl radicals to thiophenox-ide, diethyl phosphite anion, and the enolate of acetone are within an order of magnitude of the diffusional rate constant. ... 

On the assumptions that the triplet TMB biradical 37 is the reactive intermediate and that its reaction with O2 occurs at the encounter-controlled rate, the authors estimated that the triplet is more stable than the singlet by at least 4-5 kcal/mol, or more if the diffusion-limited trapping rate assumed is actually lower. 

The a-secondary IE of two deuteriums on the rate of base-catalyzed CD exchange of toluene, 3A ( PhC112D)/k(PhCD is 1.31, and the [3-secondary D IE on the rate of base-catalyzed a-C-D exchange of ethylbenzene, k(PhCHDCH3)//t(PhCHDCD3), is 1.11 0.03.58 Similarly, from the rates of base-catalyzed a-C-D exchange of tolucne-a,4-r/2, -a,2,4,6-c/4, and -a,2,3,4,5,6-d6 and with an assumption of linearity of IEs, the contributions of ortho, meta, and para deuteration lead to rate retardations of 2.4, 0.4, and 1.8%, respectively.59 These are all kinetic IEs, but to the extent that the transition state resembles closely the carbanion, or to the extent that the reverse reprotonation is encounter-controlled and independent of isotopic substitution, these kinetic IEs represent equilibrium IEs on acidity. The IEs were interpreted in terms of an electron-donating inductive effect of D relative to H. 

The previously described reactivity data for the five-membered heterocycles are gathered (in terms of o+ values) in Table 6.12 no data are given for nitration because the rates are encounter controlled and meaningless in terms of electronic effects. Among the other data, those for mercuriation, protiodemercuriation, and protiodeboronation are doubtful, and other qualifying aspects are noted in the table footnotes. The following main features are noteworthy. 

Encounter complex (or precursor complex) — is a complex of -> molecular entities produced at an - encounter-controlled rate, and which occurs as an intermediate in a reaction mechanism. If the complex is formed from two molecular entities, it is called an encounter pair . A distinction between encounter pairs and (larger) encounter complexes may be relevant in some cases, e.g., for mechanisms involving pre-association. 

Encounter-controlled rate — A -> reaction rate corresponding to the rate of encounter of the reacting -> molecular entities. This rate is also known as diffusion-controlled rate since rates of encounter are themselves controlled by -> diffusion rates (which in turn depend on the - viscosity of the medium and the dimensions of the reactant molecular entities). For a bi-molecular reaction between solutes in water at 25 °C an encounter-controlled rate is calculated to have a second-order rate constant of about 1010 dm3 mol-1 s 1. 

More recently, Stedman and coworkers have examined the kinetics of the nitrosation of hydrazine (Perrott et al., 1976), some methyl hydrazines (Perrott et al., 1977) and aryl hydrazines (Stedman and Uysal, 1977). Again (8) is obeyed with S = RNHNH3, and rate constants are believed to represent an encounter-controlled process. Since it is not possible to calculate the true rate constants for processes such as (9) because the equilibrium constant for HjNO formation is not known, arguments are based on the relative constancy of k-values (eqn 8) over a wide range of very reactive species. The products from the reaction of hydrazine itself are ammonia and hydrazoic acid in ratios which vary with the acidity. Both are thought to arise from a common intermediate as laid out in Scheme 4. With excess HNOj, hydrazoic... 

Thus reaction rates are again probably encounter-controlled, as is the case for some reactions in water where the N2O3 is generated in situ from nitrous acid (see Section 2). N2O3 is thought to be the effective reagent when nitric oxide... 

In the absence of added nucleophiles, nitrosation occurs virtually irreversibly by an acid-catalysed pathway, presumably by attack by HjNO or NO". The third order rate constant from the rate equation equivalent to (46) has a value of 840 dm moF s- at 31°C (c/. 456 and 6960 dm mol- s for cysteine and thiourea respectively at 25°C) which suggests that for this neutral substrate the reaction rate is somewhat less than that expected for an encounter-controlled process. There is a major difference between the nitrosation of alcohols and that of thiols in that, whilst the former reactions are reversible (with equilibrium constants around 1), the reactions of thiols are virtually irreversible. It is possible to effect denitrosation of thionitrites but only at high acidity and in the presence of a nitrous acid trap to ensure reversibility (Al-Kaabi et al., 1982). Direct comparisons are not possible, but it is likely that nitrosation at sulphur is much more favoured than reaction at oxygen (by comparison of the reactions of N-acetylpenicillamine and t-butyl alcohol). This is in line with the greater nucleophilicity expected of the sulphur atom in the thiol. For the reverse reaction of denitrosation [(52) and (53)], the acid catalysis observed suggests the intermediacy of the protonated forms... 

The results obtained for the weak acids phenylacetylene (Sect. 4.4) and chloroform (Sect. 4.6) have been interpreted on the basis that proton transfer to the carbanions is encounter controlled and the activation energy of the proton transfer step is small. For these carbanions the lone pair may be localized on carbon and little electronic rearrangement occurs on ionization, (XXXV) and (XXXVI). 

If an error is encountered control passes to statement 900. Check that the indices ir and kr correspond to the required block of integrals ... 

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