Diffusion-controlled trapping

Fig. 1 for stepwise solvolysis of R-X is due to the increase in ks (s ), with decreasing stability of the carbocation intermediate, relative to the constant value of az (M s ) for the diffusion-limited addition of azide anion. The lifetime for the carbocation intermediate eventually becomes so short that essentially no azide ion adduct forms by diffusion-controlled trapping, because addition of solvent to R occurs faster than escape of the carbocation from the solvent cage followed by addition of azide ion (k Now, the nucleophile adduct must form through a... 

A semiquantitative procedure used to estimate the lifetimes of carbocations and oxocarbenium ions by using diffusion-controlled trapping of the cations by nucleophiles . Ions of intermediate stability react with azide ions at a constant, diffusion-controlled rate and react with water by an activated process. The ratio of the products obtained from the azide path and the water path is dependent on the electronic characteristics of the cation. 

An important question is whether nucleophilic substitution at tertiary carbon proceeds though a carbocation intermediate that shows a significant chemical barrier to the addition of solvent and other nucleophiles. The yield of the azide ion substitution product from the reaction of 5-Cl is similar to that observed for the reactions of X-2-Y when this product forms exclusively by conversion of the preassociation complex to product. Therefore the carbocation 5 is too unstable to escape from an aqueous solvation shell and undergo diffusion-controlled trapping by azide ion. This result sets a lower limit of w fcj > -d 1.6 x 10 ° s (Scheme 2.4) " for addition of solvent to the ion pair intermediate 5" C1 . 

Diffusion-controlled trapping of carbocations benzylic cations... 

Values of for chloride ions have been determined by combining a rate constant for solvolysis ksoiv (for reactions for which the ionization step is ratedetermining) with a rate constant for the reverse reaction corresponding to recombination of cation and nucleophile. The latter constant may be found (a) by generating the cation by photolysis and measuring directly rate constants for reactions with nucleophiles or (b) from common ion rate depression of the solvolysis reaction coupled with diffusion-controlled trapping by a competing nucleophile used as a clock. 

The radical trapping technique relies on the almost diffusion-controlled trapping of carbon-centred radicals by stable aminoxyls (nitroxides) such as 1 (1, 1,3, 3-tetramethyl-2,3-dihydro-IH-isoindol-2-yloxyl) to forni stable alkoxyamine products 2 (Scheme 1). 

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

Stable radicals can show selectivity for particular radicals. For example, nitroxides do not trap oxygcn-ecntcrcd radicals yet react with carbon-ccntcrcd radicals by coupling at or near diffusion controlled rates.179,184 This capability was utilized by Rizzardo and Solomon181 to develop a technique for characterizing radical reactions and has been extensively used in the examination of initiation of radical polymerization (Section 3.5.2.4). In contrast DPPH, w hile an efficient... 

Prior to the development of NMP, nitroxides were well known as inhibitors of polymerization (Section 5.3.1). They and various derivatives were (and still are) widely used in polymer stabilization. Both applications are based on the property of nitroxides to efficiently scavenge carbon-centered radicals by combining with them at near diffusion-controlled rates to form alkoxyamines. This property also saw nitroxides exploited as trapping agents to define initiation mechanisms (Section 3.5.2.4). 

Reaction scheme, defined, 9 Reactions back, 26 branching, 189 chain, 181-182, 187-189 competition, 105. 106 concurrent, 58-64 consecutive, 70, 130 diffusion-controlled, 199-202 elementary, 2, 4, 5, 12, 55 exchange, kinetics of, 55-58, 176 induced, 102 opposing, 49-55 oscillating, 190-192 parallel, 58-64, 129 product-catalyzed, 36-37 reversible, 46-55 termination, 182 trapping, 2, 102, 126 Reactivity, 112 Reactivity pattern, 106 Reactivity-selectivity principle, 238 Relaxation kinetics, 52, 257 -260 Relaxation time, 257 Reorganization energy, 241 Reversible reactions, 46-55 concentration-jump technique for, 52-55... 

Among the evidence for this mechanism are the facts that other products are obtained when the reaction is run in the presence of competing nucleophiles, for example, p-ethoxyaniline when ethanol is present, and that when the para position is blocked, compounds similar to 23 are isolated. In the case of 2,6-dimethylphe-nylhydroxylamine, the intermediate nitrenium ion 22 was trapped, and its lifetime in solution was measured. The reaction of 22 with water was found to be diffusion controlled. ... 

Alkoxy (R0 ) radicals react at near diffusion controlled rates with trialkyl phosphites to give phosphoranyl radicals [ROP(OR )3] that typically undergo very fast -scission to generate alkyl radicals (R ) and phosphates [OP(OR )3]. In a mechanistic study, trimethyl phosphite, P(OMe)3, has been used as an efficient and selective trap in oxiranylcarbinyl radical systems formed from haloepoxides under thermal AIBN/n-Bu3SnH conditions at about 80 °C (Scheme 27) [64]. The formation of alkenes resulting from the capture of allyloxy radicals by P(OMe)3 fulfils a prior prediction that, under conditions close to kinetic control, products of C-0 cleavage (path a. Scheme 27), not just those of C-C cleavage (path b. Scheme 27) may result. 

Very recently, rate constants for scavenging of hydroxyl radicals by DMPO, and by the nitrone [18c], have been determined (Marriott et al., 1980) (see Table 5). As might be expected, the figures are close to the diffusion-controlled limit. The report of this work includes a concise and informative discussion of some of the difficulties with, and limitations of, the spin trapping method, especially where these relate to reactions involving hydroxyl radicals. 

Azide ion is a modest leaving group in An + Dn nucleophilic substitution reactions, and at the same time a potent nucleophile for addition to the carbocation reaction intermediate. Consequently, ring-substituted benzaldehyde g m-diazides (X-2-N3) undergo solvolysis in water in reactions that are subject to strong common-ion inhibition by added azide ion from reversible trapping of an o -azido carbocation intermediate (X-2 ) by diffusion controlled addition of azide anion (Scheme... 

MICROSCOPIC DIFFUSION CONTROL TRANSPORT NUMBER TRAPPING... 

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. 

Efforts have been made to eliminate diffusion-control of solid-solid reactions by using superlattices of nanometric dimensions as reactants. Formation of Cu MOgSeg from the superlattices of Cu, Mo and Se is one such example (Fister et al., 1994). The results reveal that superlattice reactant geometry could be used to kinetically trap the ternary phases which are thermodynamically unstable with respect to the binary phases. 

Figures 3.5 and 3.6 present schematic classification of regimes observable for the A + B —> 0 reaction. We will concentrate in further Chapters of the book mainly on diffusion-controlled kinetics and will discuss very shortly an idea of trap-controlled kinetics [47-49]. Any solids contain preradiation defects which are called electron traps and recombination centres -Fig. 3.7. Under irradiation these traps and centres are filled by electrons and holes respectively. The probability of the electron thermal ionization from a trap obeys the usual Arrhenius law 7 = sexp(-E/(kQT)), where s is the so-called frequency factor and E thermal ionization energy. When the temperature is increased, electrons become delocalized, flight over the conduction band and recombine with holes on the recombination centres. Such...

Tcrminadon is commonly diffusion-controlled, i.c., it is governed by the rate at which the reactive sites in growing radicals can come together rather than by chemical factors. In viscous media, termination may be so seriously impeded that both the overall rate of polymerization and the degree of polymerizadon increase markedly. In systems where the polymer is insoluble in the reacdon medium, polymer radicals may be trapped in the precipitated material and be able to grow but unable to participate in temunation processes. 

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