Proton exchange reactions

Complexes 36 undergo a proton exchange reaction with protic reagents for example, a series of monomeric amido complexes are easily prepared using this route [45] (Equation 2.11 in Scheme 2.5). 

Exchange studies were carried out by solvolysing a series of butyl chlorides in 2m solutions of antimony pentafluoride in fluoro-sulfonic acid at —50° and —78°. The acid contained tracer levels of TjO and small amounts of water to provide sufficient nucleophiles to catalyse proton exchange reactions with some of the intermediates formed in the butyl system (Kramer, 1970, 1973). 

Pairs of conjugated acids and bases are always involved in proton exchange reactions (see p.30). The dissociation state of an acid-base pair depends on the concentration. Usually, it is not this concentration itself that is expressed, but its negative decadic logarithm, the pH value. The connection between the pH value and the dissociation state is described by the Henderson-Hasselbalch equation (below). As a measure of the proton transfer potential of an acid-base pair, its pKa value is used—the negative logarithm of the acid constant Kg (where a stands for acid). 

As the optimal H-Cl distance remains very close to its value in the isolated molecule also when approaching H2O, the proton exchange reaction H2O + HCl H30 + Ch results energetically unfavourable as further confirmed hy the data reported in the following paragraph. 

Oxides/Oxyhydroxides. For natural solids that are oxides or oxyhydroxides (e.g., quartz, Si02 goethite, a-FeOOH gibbsite, Al(OH)3), their water-wet surface is covered by hydroxyl groups (recall Fig. 11.2). These hydroxyl moieties can undergo proton-exchange reactions with the aqueous solution much like dissolved acids ... 

From Table 20.6 we conclude that independent of which model we use, typical transfer times are between a few tenths of a second and a minute. Proton exchange reactions of the form (see Section 8.2) ... 

Figure 20.11 Air-water exchange of an organic compound HA undergoing a proton exchange reaction. The conjugate base A cannot leave the water, but it contributes to the diffusive transport across the water-phase boundary layer. 1 = fast acid/base equilibrium (Eq. 8-6), 2 = diffusive transport of HA and A across water-phase boundary layer, 3 = Henry s law equilibrium of HA between water and air, 4 = diffusive transport of HA across air-phase boundary layer.

The proton exchange reaction can be assumed to be at equilibrium everywhere in the water. Thus the ratio of the total and neutral compound concentration is given by ... 

In Fig. 20.13 flux enhancement V / is shown as a function of the reaction/diffusion parameter q for different equilibrium constants Kr. Remember that q2 is basically the ratio of reaction time kr and diffusion time k (Eq. 20-52). Thus, q 1 corresponds to case (1) mentioned at the beginning of this section flux enhancement should not occur (V / = 1). The other extreme (vp 1, that is tT /w) was discussed with the example of proton exchange reactions (Eq. 8-6). We found from Eq. 20-49 that for this case the water-side exchange velocity v/w is enhanced by the factor (1 + Ka /[H+]). By comparing Eqs. 8-6 and 12-17 we see that for the case of proton exchange ATa/[H+] plays the role of the equilibrium constant KT between the two species. Thus, flux enhancement is ... 

Recent developments in enantioselective protonation of enolates and enols have been reviewed, illustrating the reactions utility in asymmetric synthesis of carbonyl compounds with pharmaceutical or other industrial applications.150 Enolate protonation may require use of an auxiliary in stoichiometric amount, but it is typically readily recoverable. In contrast, the chiral reagent is not consumed in protonation of enols, so a catalytic quantity may suffice. Another variant is the protonation of a complex of the enolate and the auxiliary by an achiral proton source. Differentiation of these three possibilities may be difficult, due to reversible proton exchange reactions. 

The systematic study of gas-phase proton exchange reactions between bases has led to the development of a new formalism describing the quantitative effects of substitution on the GBs of organic compounds200. Because of their formal simplicity and general importance, proton transfer reactions are excellent models for the study of other acid-base reactions, both in the gas phase and in solution201. 

A quantitative kinetic model of the polymerization of a-pyrrolidine and cyclo(ethyl urea) showed,43 that two effects occur the existence of two stages in the initiation reaction and the absence of an induction period and self-acceleration in a-pyrrolidine polymerization. It was also apparent that to construct a satisfactory kinetic model of polymerization, it was necessary to introduce a proton exchange reaction and to take into consideration the ratio of direct and reverse reactions. As a result of these complications, a complete mathematical model appears to be rather difficult and the final relationships can be obtained only by computer methods. Therefore, in contrast to the kinetic equations for polymerization of e-caprolactam and o-dodecalactam discussed above, an expression... 

There is a vast field in chemistry where the spin-boson model can serve practical purposes, namely, proton exchange reactions in condensed media [Borgis and Hynes, 1991 Borgis et al., 1989 Morillo et al., 1989 Morillo and Cukier, 1990 Suarez and Silbey, 1991], The early approaches to this model used a perturbative expansion for weak coupling [Silbey and Harris, 1983], Generally speaking, perturbation theory allows one to consider a TLS coupled to an arbitrary bath via the term ftrz, where / is an operator that acts on the bath variables. The equations of motion in the Heisenberg representation for the operators, daldt = i[H, ], have the form... 

As discussed previously, several types of reactive dications and superelectrophiles have been directly observed using NMR spectroscopy. These experiments have all used low temperatures (— 100°C to — 30°C) and superacidic conditions to generate the observable reactive dications and superelectrophiles. Some reactive dications and superelectrophiles are stable at low temperatures and can be directly observed by NMR, but at higher temperatures they readily cleave and decompose. The low temperatures also slow down proton exchange reactions and enable the ions to be observed as static species. 

The proposed mechanism for this catalytic asymmetric hydrophosphonylation is shown in Figure 35. The first step of this reaction is the deprotonation of dimethyl phosphite by LPB to generate potassium dimethyl phosphite. This potassium phosphite immediately coordinates to a lanthanoid to give I due to the strong oxophilicity of lanthanoid metals. The complex I then reacts (in the stereochemistry-determining step) with an imine to give the potassium salt of the a-aminophosphonate. A proton-exchange reaction affords the product... 

There is, however, one very important difference proton-exchange reactions are very fast, so if several different bases (proton sinks) are available in the same solution, they get filled from the bottom up. Redox reactions, in contrast, can be extremely slow. This means that an energetically-allowed electron transfer may not occur at an observable rate, and the outcome of an electron transfer when several oxidants are present in the same solution will be decided by kinetics rather than by thermodynamics. This explains why CI2, for example does not ordinarily reduce (i.e., decompose) H20, or why Pb is relatively inert to acids. 

In this example, one key objective would be to determine the concentrations of [AB], [A], and [B] that fit with the equilibrium constant. The values of ion activities, not shown (discussed later), will also be dependent on these concentrations at equilibrium. It is generally assumed that while most natural systems are far from equilibrium conditions, if the reactions between reactant and product states are rapid, equilibrium can be applied (Butcher and Anthony, 2000). For example, in aquatic systems, NH4+ and NH3(aq) are considered to be in equilibrium, as shown below, because the proton exchange reaction is so rapid (Quinn et al., 1988) ... 

Much more is becoming known about the rates of the physical processes in competition with proton exchange reactions in excited states. (For an excellent review see Henry and Siebrand, 1973.) The factors which determine the rate constants (k) for internal conversion and intersystem crossing are neatly summarized in the Golden Rule of time-dependent perturbation theory ... 

The Z E isomerization of enamines with primary or secondary amino groups may proceed either through a thermal mechanism or by a proton-exchange reaction involving the anion130. The calculations performed favour the thermal isomerization, therefore... 

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