Reversible ionization

Trifluoromethanesulfonates of alkyl and allylic alcohols can be prepared by reaction with trifluoromethanesulfonic anhydride in halogenated solvents in the presence of pyridine.3 Since the preparation of sulfonate esters does not disturb the C—O bond, problems of rearrangement or racemization do not arise in the ester formation step. However, sensitive sulfonate esters, such as allylic systems, may be subject to reversible ionization reactions, so appropriate precautions must be taken to ensure structural and stereochemical integrity. Tertiary alkyl sulfonates are neither as easily prepared nor as stable as those from primary and secondary alcohols. Under the standard preparative conditions, tertiary alcohols are likely to be converted to the corresponding alkene. 

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. 

Fluorene and 9,9-dideuteriofluorene oxidized at the same rate in DMSO and in tert-butyl alcohol solution. This observation is consistent with a rapid, reversible ionization step. In tert-butyl alcohol the exchange of alpha deuterium atoms of dideuteriofluorene was measured (see experimental section) and a second-order rate constant for ionization calculated to be 0.12 = = 0.01 mole"1 per second. Under the conditions of this experiment the rate of oxygen absorption of undeuterated fluorene was approximately 1/50 the rate of deuterium exchange from the 9,9-dideuteriofluorene. 

There are at least three possible mechanisms for the spontaneous breakdown of hemiorthoesters, hemiacetals, and related species. Firstly, there may be a rapid and reversible ionization equilibrium followed by hydronium-ion catalysed breakdown of the anion (9) (Gravitz and Jencks, 1974). A necessary condition for this mechanism to be valid is that k2 calculated from kHi0 and Ka should fall below the diffusion controlled limit of c. 10loM 1s 1. The second mechanism (10) is similar to this but involves formation of the anion and hydronium ion in an encounter pair which react to give products faster than the diffuse apart (Capon and Ghosh, 1981). With this mechanism therefore the ionization equilibrium is not established and the rate constant for... 

Unlike with the breakdown of the more reactive hemiorthoesters a mechanism which involves a rapid and reversible ionization followed by a unimolecular breakdown of the monoanion appears to be a valid one for the hydroxide-ion catalysed breakdown of these nitrogen containing tetrahedral intermediates. Not only have the ionized forms of [124] and [126] been detected but the values of k l, the rate constant for reprotonation of the ionized form calculated as above, is always much greater than k2. 

Water molecules have a slight tendency to undergo reversible ionization to yield a hydrogen ion (a proton) and a hydroxide ion, giving the equilibrium... 

Because reversible ionization is crucial to the role of water in cellular function, we must have a means of... 

The degree of ionization of water at equilibrium (Eqn 2-1) is small at 25 °C only about two of every 109 molecules in pure water are ionized at any instant. The equilibrium constant for the reversible ionization of water (Eqn 2-1) is... 

The solvolysis mechanisms of 2,2-dimethyl-3-pentyl- and l-(l-adamantyl)-propyl sulfonates appeal- to involve partial reversible ionization to the ultimate ion pair followed by competing elimination and solvent separation, substitution products being formed from the separated ions.27 The lifetimes of simple tertiary carbocations may be some 100 times shorter than previously thought several 3-(4-methoxyphenyl)-l,l-dimethylpropyl species hydrolyse in 50% aqueous TFE with rate constants estimated at some 3.5 x 1012 s-1.28 Much elimination was also observed.28 Two studies concerning proposed carbocation intermediates in enzymatic processes are reported.29,30... 

As a result, the Stern-Volmer constant for reversible ionization Ko(AG,-) goes down long before the increasing AG, approaches the zero value. 

Figure 3.47. The free-energy dependence of the stationary rate constant for irreversible ionization k = ki defined in Eq. (3.22) (thick line) and the Markovian Stem-Volmer constant for reversible ionization Ko fromEq. (3.85) (dashed line). The open circles represent the non-Markovian Stern-Volmer constant of irreversible ionization Ko = K, from Eq. (3.27) or (3.372) for r = Tj. The energy of the excited singlet state is = 3.5 c and Xc = 35T. (From Ref. 107.)...

From Figure 3.50 we see how the reverse transfer to the excited state reduces cpp(a) in relationship to tp(cj), and how small (pa(cr) is compared to the value 1, which (p7( cr) takes in the case of irreversible transfer. However, the ratio of these quantities (3.393) remains unchanged at any rate of reverse transfer. This is a fraction of the free ions from the total amount of irrevocable products of reversible ionization. Each portion of photogenerated RIPs adds some free ions... 

Figure 3.51. The quenching constant for geminate reversible ionization Ks from Eq. (3.394) (solid line), compared to that for the irreversible one, k (dashed-dotted thick line). Dotted lines represent the FEG laws for the forward and backward kinetic constants, ko and kb.

The border between the irreversible and reversible ionization lies at a AG, value where the rate of the backward transfer to the ground state equals the rate of reverse transfer to the excited state kb(AG°) = kc(AG°). As a rule AG° is negative and not small, so that the border (indicated by the dashed line in Fig. 3.47) is far below the resonance reached at AG, = 0. To shift it up another channel of charge, recombination should be opened and its rate must be faster than kc. This ionic reaction may be parallel to that included in scheme (3.90) or recombination through the triplet channel proposed in Refs. 107 and 150. The latter is discussed in Section XI among other reactions affected by the spin conversion. 

The substitution of MET for IET is inevitable for studying the long-time asymptote of highly exergonic ionization, which is practically irreversible. However, in the case of reversible ionization, the restoration of excitations through ion recombination in the bulk hinders their decay, making it a much... 

The reaction scheme for the reversible ionization (3.354) should be essentially extended when accounting for the spin states of all species and their possible conversion in the radical ion pairs ... 

The first two represent the reversible ionization of the triplet excitations and accumulation of triplet RIPs. In the absence of the spin conversion, since there is no geminate recombination of triplet RIPs to the ground state these kernels are equal. R describes the recombination of triplet RIPs to the triplet excited states. The last kernel represents the recombination of ions to either the triplet or ground state, in proportion to the equilibrium weights of competing channels. 

The reversible ionization of certain amino and imidazole residues... 

All of the complexes 30-38 remain neutral and hexacoordinate in apolar solvents toluene, carbon tetrachloride. However, most of these compounds undergo reversible ionization when dissolved in hydrogen-bond donor solvents CD2C12, CDC13, CHFC12 (Eq. 17) 65 67... 

Like the mononuclear chelates,66 binuclear 55a,c,j (but not 55g ) undergo reversible ionization which is enhanced at low temperature, as is evident from the temperature-dependent 29Si NMR spectra (Fig. 24, Table XVIII). From the lowfield chemical shift already at 300 K (Table XVIII), relative to the mononuclear analogs 31a,c,j (Table XIV), it was concluded that significant ionization takes place at ambient temperature, to a much greater extent than in 31.69... 

A special case of reversible ionization of a hexacoordinate silicon complex has been described as a novel tautomeric equilibrium.41 It differs from the formation of siliconium-ion salts in that the positive charge resides on nitrogen, in a dimethylammonium cation, and not on silicon. The transsilylation of lg with 12 in equimolar concentrations leads to the pentacoordinate zwitterionic complex 13 (Eq. (10), Section II.B.5). However, when the molar ratio was 2 1, respectively, an equilibrium mixture of tautomers (58, 59) was obtained, as shown in Eq. (21). The same mixture was also obtained when a second mole-equivalent of lg was added to 13. 

The partial and reversible ionization of binuclear hexacoordinate silicon complexes 55a,c,j is described in Section III.A.5.ii. Like the mononuclear siliconium chloride salts, these can also form stable binuclear disiliconium salts (57a,c,j) by replacement of the chloride by other counterions, which are better leaving groups (triflate, bromide, or iodide, Eq. 37).69... 

Use of relatively weak Lewis acids which only partially, but rapidly and reversibly ionize covalent species... 

The proton released from HA is accepted by water to form the hydronium ion HjO. The reversible ionization reaction can be described by an equilib-... 

Recent studies on hydrazide-based hypercoordinate silicon complexes demonstrated the unusual flexibility of these compounds their tendency to reversibly transform between penta- and hexacoordinate compounds [1, 2] on the one hand and to irreversibly rearrange to more stable complexes [3] on the other. Thus, neutral hexacoordinate bis-chelate complexes (1) undergo reversible ionization in solution (Eq. 1), which is strongly dependent on a variety of factors temperature (ionization is enhanced at low temperatures), solvent (ionization takes place in hydrogen-bond donor solvents such as CHCI3, CH2CI2, and CHFCI2), the nature of the anion, the... 

Hydrogen sulfide and carbon dioxide (as carbonic acid and free carbon dioxide) are commonly found in well water. Even a low concentration of hydrogen sulfide can cause odor and taste problems. Hydrogen sulfide is a colorless gas which has a foul odor similar to rotten eggs and is slightly heavier than air (SG = 1.192). In water, molecular hydrogen sulfide is formed from the reduction, dissolves and disassociates in accordance with the reversible ionization reactions ... 

Where high resistivity particles are met in practice, the electrical operating characteristics and the difficulties of the reverse ionization phenomenon can be mitigated by the injection of chemical reagents into the flue gases ahead of the precipitator to modify the electrical resistivity. (Other reagents can be used to increase the cohesive properties of easily re-entrained materials if necessary to minimize re-entrainment.)... 

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