Protonic equilibria

Streitwieser pointed out that the eorrelation whieh exists between relative rates of reaetion in deuterodeprotonation, nitration, and ehlorination, and equilibrium eonstants for protonation in hydrofluorie aeid amongst polynuelear hydroearbons (ef. 6.2.3) constitutes a relationship of the Hammett type. The standard reaetion is here the protonation equilibrium (for whieh p is unity by definition). For eon-venience he seleeted the i-position of naphthalene, rather than a position in benzene as the referenee position (for whieh o is zero by definition), and by this means was able to evaluate /) -values for the substitutions mentioned, and cr -values for positions in a number of hydroearbons. The p -values (for protonation equilibria, i for deuterodeprotonation, 0-47 for nitration, 0-26 and for ehlorination, 0-64) are taken to indieate how elosely the transition states of these reaetions resemble a cr-eomplex. 

THERMODYNAMIC STUDIES ON THE PROTONATION EQUILIBRIA OF SOME HYDROXAMIC ACIDS IN NaNOj SOLUTIONS IN WATER AND IN MIXTURES OF WATER AND DIOXANE... 

The protonation equilibria for nine hydroxamic acids in solutions have been studied pH-potentiometrically via a modified Irving and Rossotti technique. The dissociation constants (p/fa values) of hydroxamic acids and the thermodynamic functions (AG°, AH°, AS°, and 5) for the successive and overall protonation processes of hydroxamic acids have been derived at different temperatures in water and in three different mixtures of water and dioxane (the mole fractions of dioxane were 0.083, 0.174, and 0.33). Titrations were also carried out in water ionic strengths of (0.15, 0.20, and 0.25) mol dm NaNOg, and the resulting dissociation constants are reported. A detailed thermodynamic analysis of the effects of organic solvent (dioxane), temperature, and ionic strength on the protonation processes of hydroxamic acids is presented and discussed to determine the factors which control these processes. 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

One of the attractions of aprotic solvents is that the electron transfer behaviour of many compounds is much simpler than in protonic media. However, this is not always so for example, the quinone/hydroquinone couple is very simple in aqueous solution but it is complicated in aprotic solvents by the number of protonation equilibria which no longer lie well to one side as they do in aqueous solution (Bessard et al., 1970). 

The Henderson-Hasselbalch equation has great predictive value in protonic equilibria. For example,... 

Ground State Protonation Equilibria of the AvGFP Chromophore... 

An unusual pH dependence has been reported for the Gd111 complex of a tetraamide-based ligand with extended noncoordinating phosphonate side chains (Scheme 12).169,170 The relaxivity increases from pH 4 to 6, followed by a decrease until pH 8.5, then from pH 10.5 it increases again. The system, as well as isostructural lanthanide complexes, was characterized by various techniques such as 31P and 170 NMR and fluorescence measurements. The pH dependence could be attributed to protonation equilibria of the noncoordinating phosphonate groups, which can... 

Ru(edta)(H20)] reacts very rapidly with nitric oxide (171). Reaction is much more rapid at pH 5 than at low and high pHs. The pH/rate profile for this reaction is very similar to those established earlier for reaction of this ruthenium(III) complex with azide and with dimethylthiourea. Such behavior may be interpreted in terms of the protonation equilibria between [Ru(edtaH)(H20)], [Ru(edta)(H20)], and [Ru(edta)(OH)]2- the [Ru(edta)(H20)] species is always the most reactive. The apparent relative slowness of the reaction of [Ru(edta)(H20)] with nitric oxide in acetate buffer is attributable to rapid formation of less reactive [Ru(edta)(OAc)] [Ru(edta)(H20)] also reacts relatively slowly with nitrite. Laser flash photolysis studies of [Ru(edta)(NO)]-show a complicated kinetic pattern, from which it is possible to extract activation parameters both for dissociation of this complex and for its formation from [Ru(edta)(H20)] . Values of AS = —76 J K-1 mol-1 and A V = —12.8 cm3 mol-1 for the latter are compatible with AS values between —76 and —107 J K-1mol-1 and AV values between —7 and —12 cm3 mol-1 for other complex-formation reactions of [Ru(edta) (H20)]- (168) and with an associative mechanism. In contrast, activation parameters for dissociation of [Ru(edta)(NO)] (AS = —4JK-1mol-1 A V = +10 cm3 mol-1) suggest a dissociative interchange mechanism (172). 

Next, the quantitative analysis of the protonation equilibria is discussed. 

The molybdate ion, [Mo04]2 , is a weaker base than [V04]3 and protonation starts at pH s7. Acidification of molybdate leads to protonation and condensation reactions. At very low molybdate concentration (<10 4M) mononuclear species predominate. The protonation equilibria of the mononuclear species can be represented by the equations... 

The ionization and protonation equilibria for the doubly charged dimer have been determined in 3.0 M NaC104 medium but the values for these equilibrium constants can be expected to be affected by the great changes in ionic medium required (71)... 

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