Acid-base equilibria anions

Anionic copolymerization of lactams presents an interesting example of copolymerization. Studies of the copolymerization of a-pyrrolidone and e-caprolactam showed that a-pyrrolidone was several times more reactive than e-caprolactam at 70 °C, but became less reactive at higher temperatures due to depropagation210 2U. By analyzing the elementary reactions Vofsi et al.I27 concluded that transacylation at the chain end occurred faster than propagation and that the copolymer composition was essentially determined by the transacylation equilibrium and the acid-base equilibrium of the monomer anion together with the usual four elementary reactions of the copolymerization. 

An aqueous solution of a soluble salt contains cations and anions. These ions often have acid-base properties. Anions that are conjugate bases of weak acids make a solution basic. For example, sodium fluoride dissolves in water to give Na, F, and H2 O as major species. The fluoride anion is the conjugate base of the weak acid HF. This anion establishes a proton transfer equilibrium with water ... 

FIG. 2 Model explaining the ability of tenidap to lower intracellular pH. Step 1 drug dissociation dictated by its pK value Step 2 entry of the conjugate acid of tenidap into the cell by diffusion, followed by reestablishment of the acid-base equilibrium Step 3 transport of tenidap anion out of the cell, activated by an anion transporter. (Reprinted from Ref. 107.)... 

Reactions in most functional micelles involve nucleophilic attack by an anionic moiety, e.g. oximate, hydroxamate, thiolate or alkoxide. Therefore it may be necessary to take into account the acid-base equilibrium which generates the anionic moiety. The simplest approach is to work at a pH such that deprotonation is essentially quantitative, but if this cannot be done the extent of deprotonation has to be measured directly or estimated. 

By comparing the approximate pA values of the conjugate acids of the bases with those of the carbon acid of interest, it is possible to estimate the position of the acid-base equilibrium for a given reactant-base combination. If we consider the case of a simple alkyl ketone in a protic solvent, for example, it can be seen that hydroxide ion and primary alkoxide ions will convert only a small fraction of such a ketone to its anion. 

HPTS is a highly water-soluble, pH-sensitive dye with a pK of 7.5 in aqueous solution [8], When in alkaline medium, pH > 7.5, acid-base equilibrium is totally displaced toward the anion form (3sPyO ) of the dye. The electronic character of 3sPyO remains unchanged after photo-excitation, and corresponds to a singlet-excited state [9], Fluorescence from this state undergoes a fast 0.4 ps Stokes shift and has a maximum at 515 nm and a lifetime of 5.3 0.1 ns [10],... 

In reactions with amine nucleophiles the reaction leads to a zwitterionic adduct that is in rapid acid-base equilibrium with its anionic form, Equation (45). In some cases, the zwitterion is strongly... 

The plot of the pH-dependence (Fig. 18) indicates qualitatively a participation of an intermediate acid-base equilibrium. Evaluation of rate constants kr and kg is made difficult by the inaccessibility of the dissociation constant of reaction (24 b) which corresponds to protonation of a radical anion. ESR would be a suitable method for the determination of the dissociation constants of at least the more stable radical anions. Another possibility for obtaining at least an approximate value of the equilibrium constant is the measurement of the shifts of the half-wave potentials of the more negative wave at potential 3 with pH. Because the half-wave potential of this wave is known to be sensitive to the... 

If the conjugate-base-dissociation mechanism is the correct one, the lower substitution rates observed with nitrite, acetate, and azide may be considered to arise because these weakly basic anions are less effective than methoxide in generating conjugate base CB in the preliminary acid base equilibrium. 

The alkoxide doesn t have to be made first, though, because alcohols dissolved in basic solution are at least partly deprotonated to give alkoxide anions. How much alkoxide is present depends on the pH of the solution and therefore the pKa of the base (Chapter 8), but even a tiny amount is acceptable because once this has added it will be replaced by more alkoxide in acid-base equilibrium with the alcohol. In this example, allyl alcohol adds to pent-2-enal, catalysed by sodium-hydroxide in the presence of a buffer. 

In the case of an anionic acid (z = —2 e.g. HSOj), the acid/base equilibrium (4-5) will be shifted to the right with an increase in relative permittivity of the solvent in which HA is dissolved. Here, the ionization will increase much more quickly with an increase in relative permittivity than in the preceding case. 

An especially challenging task is maintaining the selectivity of the method for separation of compounds whose elution time is very short, close to the dead time. In such cases, it is necessary to perform a preliminary review of the planned chromatographic conditions, including the composition of the analyzed material. For example, a typical eluent employed in anion-exchange chromatography (with pH of 8.5) is intended to facilitate the dissociation of separated compounds. Neglecting the time necessary to achieve acid/base equilibrium of substances loaded into the column in a neutral solution can result in their elution in the dead volume. The phenomenon is observed, for example, for MMA(V), whose consecutive dissociation constants are p/sTi 3.6 and p/sT2 8.22 [164]. 

Huie et al. 106) have also determined rate constants by pulse radiolysis techniques for the oxidation of the above-mentioned cyano complexes by the carbonate radical at pH > 11.2, thus ensuring that only the anionic form in the acid/base equilibrium... 

In many papers the term "superoxide" (HOO ) is used simultaneously with "superoxide anion radicals" (Oa )- However, this is under the physiological conditions incorrect The pKa value of this acid-base equilibrium is 4.8 [227] and, therefore, there is only a very small contribution of HOO at physiological pH (7.4). Therefore, the term "superoxide anion radical" should be exclusively used. The superoxide anion radical is both, a one-electron oxidant and a one-electron reductant. The reactions of O2 with many different biological substrates were studied in detail by the radiation chemists and a summary of the obtained second order rate constants is provided in [228]. However, not a single carbohydrate is mentioned in this comprehensive survey since no reaction could have been observed [229]. 

The importance of the many add-base pairs in seawater in determining the acidity of the ocean depends on their concentrations and equilibrium constants. Evaluating the concentrations of an acid and its conjugate anion (base, Ba ) as a function of pH (pH = —log [H ]) requires knowledge of the equation describing the acid/base equilibrium (hydrogen ion exchange), the apparent equihbrium constant, K, and information about the total concentration, [Ba]x, of the acid in solution ... 

Alkoxides, Amides, and other Bases. The use of alkoxides as initiators has recently received some attention because of their ease of handling relative to metal alkyls. In principle such species might be expected to be active only with monomers capable of yielding a more stable anion. Potassium methoxide in dimethylsulphoxide/methanol solution will oligomerize 4-vinylpyridine and here initiation seems to proceed via an acid/base equilibrium rather than by addition to the vinyl double bond (Scheme 15). With methyl methacrylate lithium t-butoxide is an initiator although an inefficient one. Furthermore, unused... 

The choice of acid or base for solvent is simplified appreciably for melts containing complex ions (as a rule, they are anions), which are prone to the acid-base dissociation. Dissociation of this ion is assumed as the intrinsic acid-base equilibrium of a melt of such kind. In this case, the simpler eliminated anion will be considered as the base of the solvent and the coordinationally unsaturated residue will be the acidic particle of the solvent. Naturally, the division of particles formed by the auto-dissociation into acids and bases is made on the basis of the Lewis definition [13] an acid is the acceptor of an electron pair and a base is the donor of this electron pair. Ionic melts based on complex halides of gallium(III) [28], aluminium(III) [29] and boron(III) [30,31] may serve as examples of successful application of the above approach. The electron-deficient covalent halide (e.g. A1C13, BF3) in these melts is the solvent acid, and the corresponding halide ion is the base of the solvents ... 

Since the process of complexation between the melt anion and different cations of close radii proceeds to practically equal degrees, one may assume that for a set of oxides dissolved in the same ionic solvent the solubility value should be dependent on the degree of acidic cation-anion interaction which is the Lux-Flood acid-base equilibrium. In this case, the constant of the... 

Crucial questions concern the identity of the species that act as electron donors in forming the ArN02 and ArN022 intermediates. The simplest possibility is that ArN02, is formed via direct SET from the alkoxide ion (Scheme I, path a). The resulting alkoxy radical should rapidly give an alkyl radical via H abstraction from the solvent. The a-hydroxy radical so produced is in acid-base equilibrium with its conjugate base, the acetone radical anion. The equilibrium constant for the dissociation of the 2-propanol... 

Metrohm markets microprocessor-controlled Instruments for routine coulo-metric (KF 652 Processor) and amperometrlc-voltammetrlc (EP/KF 678 Processor) analyses. These Instruments are frequently used for research purposes. Thus, the Mettler Memotltrator and the Metrohm Tltroprocessor 636/Rod Stirrer 622 were used by Johansson et al. [62] to demonstrate that potentiometric two-phase titrations can be carried out in an automatic fashion. The typical background noise from the electrodes can be reduced by Introducing hydrophobic anions or cations in the aqueous phase. These ions also affect the acid-base equilibrium by extracting the sample ions as ion-pairs into the organic phase, which allows the conditional acidity constant (apparent K ) to be manipulated to make selective titrations possible. 

It would appear today, however, that the acid-base equilibrium of diazomethane with its anion is better known in the gas-phase than in solution The same group (DePuy et al., 1989) investigated reactions of the diazomethyl anion with CS2, COS, CO2, SO2, and with a series of a,)ff-unsaturated aldehydes and ketones — all in the gas phase using a flowing afterglow apparatus (DePuy and Bierbaum, 1981) and selected ion flow tube techniques (Van Doren et al., 1987). 

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