Solvent acidity, measurement

Most organic compounds are bases, that is, they are capable of accepting a proton. The best-studied organic bases are the moderately strong ones, which will receive a proton in dilute aqueous solutions amines are the most important examples. The pKa value of the protonated base, referred to the infinitely dilute aqueous solution, is the usual measure of base strength, and the pH of the solution is a quantitative measure of solvent acidity, or ability to transfer a proton. 

When a solute is added to an acidic solvent it may become protonated by the solvent. If the solvent is water and the concentration of solute is not very great, then the pH of the solution is a good measure of the proton-donating ability of the solvent. Unfortunately, this is no longer true in concentrated solutions because activity coefficients are no longer unity. A measurement of solvent acidity is needed that works in concentrated solutions and applies to mixed solvents as well. The Hammett acidity function is a measurement that is used for acidic solvents of high dielectric constant. For any solvent, including mixtures of solvents (but the proportions of the mixture must be specified), a value Hq is defined as... 

A new acidity scale has been developed based on calorimetric measurement of A-methylimidazole and A-methylpyrrole in bulk solvents. A revised version of this method was shown to give better results in some cases. Another scale of solvent acidities was developed based on the hydrogen-bond donor acidities in aqueous DMSO. ... 

The a scale of hydrogen bond donor acidities measures the ability of a solvent to donate a proton in a solvent-solute hydrogen bond,... 

Analyses of Water Content. The water content of the PIQ starting materials was analyzed. The water content of amines was measured using a DuPont 321A moisture meter and those of the solvents were measured by Karl Fischer s reagent method. The water content of acid dianhydrides was measured by titrating the free acid. 

The water content in the starting materials was measured as follows. The water content of amines and solvents were measured using the DuPont 321A moisture meter and Karl Fischer s reagent methods, respectively. Since the water contained in acid dianhydride is considered to convert it to free acid. 

Procedure 1 We select several HA-type acids, for which pKa values of <7.5 are expected, and determine their pKa and K HAJ) accurately by method(s) other than potentiometry. If the selected acid is a nitro-substituted phenol that has no tendency to homoconjugate (p. 71), we dissolve various amounts of it in the solvent and measure the UV/vis spectrophotometric absorption for the phenolate anion formed by dissociation. For the conductimetric determination of pKa and K HA])), see Section 7.3.2. 

The impurity interacts with a substance participating in the process to be measured. An example is met when we determine the dissociation constant (pfQ of a weak acid HA by conductimetry. In the determination, we dissolve HA in the pure solvent and measure the conductivity of the solution containing dilute H+ and A formed by dissociation. If a basic impurity B is contained in the solution, it disturbs the measurement, producing ionic species by the reaction B+HA — BH++A . Because trace amounts of basic impurity are contained even in a purified solvent, conductimetric pK determination is practically impossible for HA with pfCa>7.5. 

In principle, the A 0(H) function is of limited interest for kinetic applications because the indicators are chemically very different from the organic substrates generally used. On the other hand, as the measurements are based on pH determination, the length of the acidity scale is limited by the pA" value of the solvents. However, very interesting electrochemical acidity studies have been performed in HF by Tremillon and co-workers, such as the acidity measurement in anhydrous HF solvent and the determination of the relative strength of various Lewis acids in the same solvent. By studying the variation of the potential of alkane redox couples as a function of acidity, the authors provide a rational explanation of hydrocarbon behavior in the superacid media.48... 

Acetonitrile (MeCN) was used as a model solvent to measure the stability of CPO in a solvent environment. Enzyme assays were performed as just described with the addition of MeCN. The experimental problem of salt precipitation was minimized by using citric acid (7.5 M) for pH adjustment and reducing the phosphate buffer concentration to lOmM. The concentration of the remaining reagents in the CPO assay remained the same. 

From the dependence of die measured coupling rate (k,) of acetylactone on the solvent acidity (HjO, MeOH, MCjSO, DMF and HjCCN) it was also found that in all these solvents it is the carbanion which is the most reactive species... 

Addition of SbFj makes the reaction too slow to measure and band broadening of the solvent peak increases with temperature. Therefore they concluded, in contrast to MacLean and Mackor, that intermolecular proton exchange with solvent (acid) is the main process causing the degeneracy of [289]. The value of 3 obtained was 13.7 — 15.5 kcal moh, depending upon the hydrocarbon concentration. The lower basicity of the solvent medium was proposed to explain the much lower reaction rate in FSO3H as compared with HF-BF3. 

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