Proton medium effects

L. M. Mukherjee, J. Phys. Chem., 76, 243 (1972). Standard potential of the ferrocene-ferricinium electrode in pyr dine. Evaluation of proton medium effect. 

O Connor and Cox and Yates have reviewed the many acidity function scales. A major use of acidity functions is for the measurement of the strengths of very weak bases. The procedure utilizes spectrophotometric measurements of the concentration ratio Cb/cbh+ in solutions of known acidity function and application of Eq. (8-89). One problem is the estimation of the spectra of the pure forms (protonated and unprotonated) of the base, for the spectra are subject to the medium effect, and corrections must be applied. Another problem is that the base... 

The rates increase up to a maximum at about 90 wt. % sulphuric acid (this point varies slightly according to the aromatic reactivity) and the increase with increasing acid concentration is consistent with the increase in the concentration of nitronium ions. The occurrence of a maximum indicates an opposing factor and is thought42 to be partly due to protonation of the aromatic (most of the measured compounds contain the group >X=0) but since it also occurs for PhNMe3, medium effects must be involved, i.e. the activities of the species present varies, whilst the concentrations remain the same. The kinetic equation for reaction of nitronium ion with an aromatic is... 

In Ref.125) the calculation of an activation barrier for reaction (21) in the gas phase is considered to be an error of the MINDO/3 method and the process is assumed to be activationless. But in respect to the medium effect a barrier of 54 k J mol-1 is obtain-ed which agrees again with the results from Huron-Claverie calculations. Bertran et al. calculated the influence of the solvation on the electrophilic attack of a proton 133) or a methyl cation 134,135) on ethene using a MINDO/3 supermolecule model. Smaller reaction enthalpies also result in solution than in the gas phase in addition to the appearance (H+ + ethene) or the increase (CH 4 + ethene) of an activation barrier1361. 

Reasonably reliable pATbh+ values for the protonation of weak bases or of weakly basic substrates can be obtained via equation (17), together with m slope parameters that can be used to classify basic molecules as to type, and for an estimate of the solvation requirements of the protonated base. Measurements at temperatures other than 25°C can be handled using equation (22), and enthalpies and entropies for the protonation can be obtained. Protonation-dehydration processes are covered by equation (26). Medium effects on the... 

Other reactions in which cations other than protons are catalyti-cally effective are esterification and acetal formation, catalyzed by calcium salts,277 and the bromination of ethyl cyclopentanone-2-carboxylate, catalyzed by magnesium, calcium, cupric, and nickel, but not by sodium or potassium ions.278 One interpretative difficulty, of course, is the separation of catalysis from the less specific salt effects. The boundary line between salt effects (medium effects) and salt effects (catalysis) is not sharp either in concept or experimentally. 

The kinetic solvent-isotope effects on these reactions are made up of primary and secondary kinetic isotope effects as well as a medium effect, and for either scheme it is difficult to estimate the size of these individual contributions. This means that the value of the isotope effect does not provide evidence for a choice between the two schemes (Kresge, 1973). The effect of gradual changes in solvent from an aqueous medium to 80% (v/v) Me2SO—H20 on the rate coefficient for hydroxide ion catalysed proton removal from the monoanions of several dicarboxylic acids was interpreted in terms of Scheme 6 (Jensen et al., 1966) but an equally reasonable explanation is provided by Scheme 5. 

Results for the composition of nuclear matter at temperature T = 10 MeV with proton fraction V/"1, = 0.2 are shown in Fig. 1, for symmetric matter Yp0t = 0.5 in Fig. 2. The model of an ideal mixture of free nucleons and clusters applies to the low density limit. At higher baryon density, medium effects are relevant to calculate the composition shown in Figs. 1, 2, which are described in the following sections. 

The effect of thermal pion fluctuations on the specific heat and the neutrino emissivity of neutron stars was discussed in [27, 28] together with other in-medium effects, see also reviews [29, 30], Neutron pair breaking and formation (PBF) neutrino process on the neutral current was studied in [31, 32] for the hadron matter. Also ref. [32] added the proton PBF process in the hadron matter and correlation processes, and ref. [33] included quark PBF processes in quark matter. PBF processes were studied by two different methods with the help of Bogolubov transformation for the fermion wave function [31, 33] and within Schwinger-Kadanoff-Baym-Keldysh formalism for nonequilibrium normal and anomalous fermion Green functions [32, 28, 29],... 

Some of the most important evidence for the two-step mechanism comes from studies of base catalysis, in this regard, reactions involving primary and secondary amines have played a central role1-5. The initially formed cx-adduct, 1, is zwitterionic and contains an acidic proton, which can be removed by a base which may be the nucleophile itself. Conversion of 1 to products can then occur via the uncatalysed k2 pathway or via the base-catalysed hl pathway. The influence of Brpnsted base catalysis, the experimental observation of 1,1- and 1,3-cr-adducts, the sensitivity of the system to medium effects, are some experimental evidence of the mechanism depicted in equation 1. 

Crepaux, D., Lehn, J. M., Dean, R. R. Nuclear Spin-Spin Interactions. X. Signs of Geminal and Vicinal Nitrogen-Proton Coupling Constants. Stereochemistry and Medium Effects on NH Couplings. Mol. Phys. 16, 225 (1969). 

An example has, however, been reported (Alais et al., 1971) of a slow tautomerization of an N-protonated cation to a more stable C-protonated cation, which was observed by nmr and which is worth mentioning because it shows an interesting medium effect on the relative rates. The protonation of N,N-dimethylisobutylenamine [25] can lead to two types of cation the N-protonated... 

Large medium effects on the ultraviolet spectra of the protonated form of a number of other amides have been reaffirmed in some recent protonation studies (Farlow and Moodie, 1970 Congdon and Edward, 1972 Barnett and O Connor, 1973). These undoubtedly conceal evidence of tautomeric change. 

Ultraviolet spectra of benzoic acid in sulphuric acid solutions, published by Hosoya and Nagakura (1961), show a considerable medium effect on the spectrum of the unprotonated acid, but a much smaller one in concentrated acid. The former is probably connected with a hydrogen-bonding interaction of benzoic acid with sulphuric acid which is believed to be responsible for a peculiarity in the activity coefficient behaviour of unprotonated benzoic acid in these solutions (see Liler, 1971, pp. 62 and 129). The absence of a pronounced medium effect on the spectra in >85% acid is consistent with dominant carbonyl oxygen protonation. In accordance with this, Raman spectra show the disappearance in concentrated sulphuric acid of the carbonyl stretching vibration at 1650 cm (Hosoya and Nagakura, 1961). Molecular orbital calculations on the structure of the carbonyl protonated benzoic acid have also been carried out (Hosoya and Nagakura, 1964). 

In amine-imine systems (75 76) the mobile proton can in principle be located at either of the two basic nitrogen sites in the anion (77). Since the canonical form with aromatic (benzenoid) structure is polar in the imine (76b) and non-polar in the amine (75a), the amine structure should be favoured, particularly in non-polar solvents. This seems generally to be the case, although solvent and medium effects do not appear to have been investigated the results available at the present time refer mainly to aqueous and other polar solvents. 

Thus in the case that BH+ is H30+, Equation (14) becomes an identity reaction, for which there is no medium effect, and m =0. On the other hand, if BH+ is a protonated aniline, m = 1. These values provide fixed points on a scale of solvation energy changes associated with proton transfer between H30+ and the protonated base under study. 

Mechanisms, of proton transfer between oxygen and nitrogen acids and bases in aqueous solutions, 22, 113 Mechanisms, organic reaction, isotopes and, 2, 1 Mechanisms of reaction, in solution, entropies of activation and, 1, 1 Mechanisms of reaction, of /3-lactam antibiotics, 23, 165 Mechanisms of solvolytic reactions, medium effects on the rates and, 14, 10 Mechanistic analysis, perspectives in modern voltammeter basic concepts and, 32, 1 Mechanistic applications of the reactivity-selectivity principle, 14, 69 Mechanistic studies, heat capacities of activation and their use, 5, 121 Mechanistic studies on enzyme-catalyzed phosphoryl transfer, 40, 49 Medium effects on the rates and mechanisms of solvolytic reactions, 14, 1 Meisenheimer complexes, 7, 211... 

The immediate changes in UV spectra57 exhibited by the substrates on addition of mineral acids are consistent with a rapid protonation equilibrium, S+H+ < SH+, to form the conjugated acid. In order to interpret the rate data, one must first correct the observed values of k,j, for the amount of protonated substrate. Spectrophotometric methods are widely applicable for determination of the ionization ratio, I = CSH+ /Cs, of moderately basic substrates74. For A-f-butylbenzaldoxime and 2-/-butyl-3-phenyloxaziridinc, however, the rate of the hydrolysis reaction (t /2 = 1 min) at the maximum in the profile at 24.2 °C made it impossible to measure the zero-time absorption of the substrates. However, allowing for medium effects in the absorption spectra, the substrates appeared to be essentially fully protonated in solutions of CH+ > 2 M in all three acids. 

F. Hollfelder, A. J. Kirby, and D. S. Tawfik, On the magnitude and specificity of medium effects in enzyme-like catalysts for proton transfer,/. Org. Chem. 2001, 66, 5866-5874. 

The fact that valence band holes take over the role of the mobile reactive species from the intermediates X] in water+methanol mixtures with 48 and 80 mol % CH3OH and in water+acetonitrile mixtures with 13 mol % CH3CN can be attributed to a shift in equilibrium (1) due to a medium effect, leading to the immobilization of a major part of the intermediates XL Indeed, according to literature data (Feakins, 1963 Case, 1967 Das, 1981), the standard Gibb" —gy of the transfer of the proton from water to water +methanol and water+acetonitrile mixtures, as a function of the methanol and acetonitrile content,... 

The first section, under the heading solute-solvent interactions, considers the origin of the medium effect which is exhibited for reactions on changing from a hydroxylic solvent to a dipolar aprotic medium such as DMSO. This section is subdivided into two parts, the first concentrating on medium effects on rate processes, the second on equilibria of the acid-base variety. The section includes discussion of the methods used in obtaining and analysing kinetic and thermodynamic transfer functions. There follows a discussion of proton transfers. The methods and principles used in such studies have a rather unique character within the context of this work and have been deemed worthy of elaboration. The balance of the article is devoted to consideration of a variety of mechanistic studies featuring DMSO many of the principles developed in earlier sections will be utilized here. 

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