Rate of ionization

Application of Hammond s postulate indicates that the transition state should resemble the product of the first step, the carbocation intermediate. Ionization is facilitated by factors that either lower the energy of the carbocation or raise the energy of the reactant. The rate of ionization depends primarily on how reactant structure and solvent ionizing power affect these energies. 

If it is assumed that ionization would result in complete randomization of the 0 label in the caihoxylate ion, is a measure of the rate of ionization with ion-pair return, and is a measure of the extent of racemization associated with ionization. The fact that the rate of isotope exchange exceeds that of racemization indicates that ion-pair collapse occurs with predominant retention of configuration. When a nucleophile is added to the system (0.14 Af NaN3), k y, is found to be imchanged, but no racemization of reactant is observed. Instead, the intermediate that would return with racemization is captured by azide ion and converted to substitution product with inversion of configuration. This must mean that the intimate ion pair returns to reactant more rapidly than it is captured by azide ion, whereas the solvent-separated ion pair is captured by azide ion faster than it returns to racemic reactant. 

Both acetolyses were considered to proceed by way of a rate-determining formation of a carbocation. The rate of ionization of the ewdo-brosylate was considered normal, because its reactivity was comparable to that of cyclohexyl brosylate. Elaborating on a suggestion made earlier concerning rearrangement of camphene Itydrochloride, Winstein proposed that ionization of the ero-brosylate was assisted by the C(l)—C(6) bonding electrons and led directly to the formation of a nonclassical ion as an intermediate. 

Initiator reactivity orders can be explained on the baas of differences in the rate of displacement of MeX from Et2AlX MeX complexes by f-BuX and/or the rate of ionization of Et2 A1X f-BuX complexes. PIB yields decrease with increase of Mel or MeBr concentration. This poisoning effect has been attributed to the formation of propagation-inactive halonium ions. 

Among the experiments that have been cited for the viewpoint that borderline behavior results from simultaneous SnI and Sn2 mechanisms is the behavior of 4-methoxybenzyl chloride in 70% aqueous acetone. In this solvent, hydrolysis (i.e., conversion to 4-methoxybenzyl alcohol) occurs by an SnI mechanism. When azide ions are added, the alcohol is still a product, but now 4-methoxybenzyl azide is another product. Addition of azide ions increases the rate of ionization (by the salt effect) but decreases the rate of hydrolysis. If more carbocations are produced but fewer go to the alcohol, then some azide must he formed by reaction with carbocations—an SnI process. However, the rate of ionization is always less than the total rate of reaction, so some azide must also form by an Sn2 mechanism. Thus, the conclusion is that SnI and Sn2 mechanisms operate simultaneously. ... 

TABLE 10.12 Relative Rates of Ionization of p-Methoxyneophyl Toluenesulfonate in Various Solvents... 

Polar protic solvent will greatly increase the rate of ionization of an alkyl halide in any SN1 reaction. 

The preparation of di-w-butyl ether is illustrative (Scheme 2.6). No reaction occurred with n-butanol alone for 2 h at 200 °C. However, in the presence of 10 mol % n-butyl bromide, 26% conversion of the alcohol to the ether was obtained after 1 h, without apparent depletion of the catalyst. It is known that addition of alkaline metal salts can accelerate solvolytic processes, including the rate of ionization of RX [41]. This was confirmed when the introduction of LiBr (10 mol %) along with n-butyl bromide, afforded a conversion of 54% after 1 h at 200 °C. Ethers incorporating a secondary butyl moiety were not detected, precluding mechanisms involving elimination followed by Markovnikov addition. 

Nevertheless, chemical methods have not been used for determining ionization equilibrium constants. The analytical reaction would have to be almost instantaneous and the formation of the ions relatively slow. Also the analytical reagent must not react directly with the unionized molecule. In contrast to their disuse in studies of ionic equilibrium, fast chemical reactions of the ion have been used extensively in measuring the rate of ionization, especially in circumstances where unavoidable irreversible reactions make it impossible to study the equilibrium. The only requirement for the use of chemical methods in ionization kinetics is that the overall rate be independent of the concentration of the added reagent, i.e., that simple ionization be the slow and rate-determining step. 

Many of the reactions of the weak carbon acids are reactions of the carbanion, the rate being the rate of ionization and independent of the concentration or nature of the reagent that determines what the product will be. 

Any resonance structure for the transition state in which the negative charge is very far removed from the departing proton should contribute less stabilization in the transition state than in the ion. Figure 2 shows the abnormally slow rate of ionization for nitromethane and nitroethane... 

The size of a spherically symmetrical ionization-bounded nebula (known as a Stromgren sphere ) can be found by equating the total number of recombinations in Case B to the total emission rate of ionizing photons from the central star(s) ... 

Studies of Equilibria and Shift Reagents.—N.m.r. studies of the exchange of halogen in boron trihalide adducts of trimethylphosphine, its oxide, and sulphide,49 and exchange of chloro- and methoxy-groups between methylphosphino and methyl-silyl or -germyl moieties,80 have been reported. The rates of ionization of phosphoranes... 

Kinetic and equilibrium acidities of several families of nitroalkanes have been discussed extensively in the chapter by Lewis172, where the effects of changing substituents and the nature of the base, together with the role of the solvent on rates of ionization and equilibria, have been considered. 

Oxidation of diphenylmethane in basic solutions involves a process where rate is limited by and equal to the rate of ionization of diphenylmethane. The diphenylmethide ion is trapped by oxygen more readily than it is protonated in dimethyl sulfoxide-text-butyl alcohol (4 to 1) solutions. Fluorene oxidizes by a process involving rapid and reversible ionization in text-butyl alcohol solutions. However, in the presence of m-trifluoromethylnitrobenzene, which readily accepts one electron from the carbanion, the rate of oxygen absorption can approach the rate of ionization. 9-Fluorenol oxidizes in basic solution by a process that appears to involve dianion or carbanion formation. Benzhydrol under similar conditions oxidizes to benzophenone by a process not involving carbanion or dianion formation. 

TITTe have reported that the triphenylmethide ion in dimethyl sulfoxide (DMSO) solution reacts with oxygen at a rate approaching the diffusion-controlled limit (k > 109 liters/mole sec.) (16). The triphenylmethide ion is actually more reactive toward molecular oxygen than the triphenylmethyl radical. Because of the reactivity of the triphenylmethyl anion toward molecular oxygen, it is possible to measure the rate of ionization of triphenylmethane in basic solution by the rate of oxygenation. 

The present work demonstrates that the oxidation of diphenylmeth-ane in basic solution follows a pattern similar to triphenylmethane and not to fluorene. At high concentrations of good electron acceptors it is possible to realize a situation wherein the rate of oxidation of fluorene is limited by and equal to the rate of ionization. The oxidations of benzhydrol and 9-fluorenol in basic solution are considered the difference in acidity of the methine hydrogens has a pronounced effect on the course of these oxidations. 

The following data would appear to substantiate this premise. At high nitroaromatic concentrations Reaction 12 should be able to compete with the reprotonation of the carbanion and the rate of ionization should become equal to the rate of oxygen absorption. Since the stoichiometry of the oxidation did not change on adding the nitroaromatic catalysts, the assumption that the absorption of only one molecule of oxygen occurred for each electron transfer step is legitimate. 

H. R. Gersmann (Koninklyke Shell Laboratories, Amsterdam, Netherlands) The results obtained by Russell correlate with those obtained by Gersmann and Niewenhuis (Organic Reaction Symposium, Cork, 1964) in the study of autoxidation of esters and ketones. Here weakly acidic esters also showed rates of ionization equal to the rate of oxidation as shown by the equality of the rate of racemization of an optically active ester to the rate of oxidation. 

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