Protonation equilibria for

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. 

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

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)... 

Takacs-Novak et al. (14) used a potentiometric assay in combination with UV spectroscopy and NMR to study the acid-base properties and protonation equilibria for lomefloxacin and other quinolone antibacterials. Okabayashi et al. used potentiometric measurements to determine the stability constants of metal ions with several quinolones including lomefloxacin (20). 

Potentiometric Methods. - Potassium trithiocarbonate has been used as a reductant (-S-S- reduction cleavage) for the potentiometric (and spectrophoto-metric) determination of the disulfides of dithio-phosphinic acids in DMF-H2O medium at millimolar levels.The protonation equilibria for N,N -diethylami-nomethylenephosphonic acid (88) were elucidated from both potentiometric titration and determination of the pH dependence of the NMR chemical shift (83 ip), and protonation constants for aminoalkanephosphonates RCH(NH2)P(0)(0Et)2 and the acidity constant of di(2-ethylhexyl)thiopho-sphoric acid (111) have also been obtained potentiometrically. 

The results of the analysis are summarised in Figure 24 and Figure 25. Again the protonation equilibria for the complex species are defined as formation constants, the logK values for the... 

Protonation equilibria for aliphatic sulphoxides in H2SO4, as determined by n.m.r., compare well with data obtained earlier by u.v. and c.d. methods. " " pKbh Values obtained similarly for sulphinates and sulphides have been reported values for sulphides are some 4 units more positive than for corresponding ethers, and a basicity order RjS >RSSR RSH emerges, in interesting contrast with the oxygen series (MeOH is more basic than MeOMe). C N.nLr. of DMSO and diethyl sulphoxide reveal O-protonation in strong acid since both CH, and CHz resonances are shifted to higher field.- "... 

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. 

Write the proton transfer equilibria for the following acids in aqueous solution and identify the conjugate acid-base pairs in each one (a) H2S04 (b) C6H5NH3+. anilinium ion ... 

Write the stepwise proton transfer equilibria for the deprotonation of (a) sulfuric acid, H2S04 (LA arsenic acid, H.As04 (c) phthalic acid. C6H4(COOH)2. 

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). 

FIG. 4 Thermodynamic equilibria for the interfacial distribution of a solute X which can be ionized n times, and X being its most acidic (or deprotonated) and its most basic (or protonated) forms, respectively. X and are the dissociation constants in the aqueous and organic phase, respectively, and P is the partition coefficient of the various species between the two phases. 

Proton Transfer and Electron Transfer Equilibria. The experimental determination used for the data discussed in the above subsections of Section IV.B were obtained from ion-molecule association (clustering) equilibria, for example equation 9. A vast amount of thermochemical data such as gas-phase acidities and basicities have been obtained by conventional gas-phase techniques from proton transfer equilibria,3,7-12-87d 87g while electron affinities88 and ionization energies89 have been obtained from electron transfer equilibria. 

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). 

The two protonation steps of the M02(CN)4 3 complexes (M=Tc(V), Re(V) equilibria (4) and (6) in Scheme 1) occur in acidic medium while the two protonation steps for the [M02(CN)4]4 (M = Mo(IV), W(IV)) occur in basic aqueous solutions. The pKa values which demonstrate the acid/base behavior of the complexes are given in Table II. Other aspects of these complexes are also presented therein but are discussed in later sections. 

There are many types of data in chemistry that are not specifically covered in this book. For example, we do not discuss NMR data. NMR spectra of solutions that do not include fast equilibria (fast on the NMR time scale) can be treated essentially in the same way as absorption spectra. If fast equilibria are involved, e.g. protonation equilibria, other methods need to be applied. We do not discuss the highly specialised data analysis problems arising from single crystal X-ray diffraction measurements. Further, we do not investigate any kind of molecular modelling or molecular dynamics methods. While these methods use a lot of computing time and power, they are more concerned with data generation than with data analysis. 

In aqueous solutions, whenever protonation equilibria are involved, the autoprotolysis of water needs to be incorporated into the model. Thus, for an acetic acid titration the model comprises two equilibria... 

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