Methanol, autoprotolysis

OXYGEN, OXIDES 0X0 ANIONS METHANE MONOOXYGENASE Methanol, autoprotolysis constant, AUTOPROTOLYSIS METHANOL DEHYDROGENASE... 

Autoprotolysis of the Solvent. While studying these proton transfers, there is another type that may be discussed at the same time, namely, the self-dissociation of the solvent itself. As is well known, highly purified solvents show at least a small electrical conductivity. In methanol, for example, it is generally recognized that this conductivity arises from the fact that, a certain number of protons havo been transferred according to the process... 

Proton Transfers in Various Solvents. The Autoprotolysis of Methanol. Formic Acid as Solvent. The Sulfate Ion. Autoprotolysis of Formic Add. The Urea Molecule. Sulfuric Add and Liquid Ammonia as Solvents. 

The AutoprotolySis of Methanol. The table gives the value log K = —16.6 for the autoprotolysis constant at 25°C. From this value we find... 

Given that the autoprotolysis constants of ethanol and methanol are 10 191 and 10-16-77, respectively, the background kQbs values for the lyoxide-catalyzed ethanolysis and methanolysis reactions of paraoxon at pH 7.3 (ethanol) and 8.3 (methanol) are 8.1 x 10-15 and 3.5 x 10 us 1, respectively... 

The recent introduction of non-aqueous media extends the applicability of CE. Different selectivity, enhanced efficiency, reduced analysis time, lower Joule heating, and better solubility or stability of some compounds in organic solvent than in water are the main reasons for the success of non-aqueous capillary electrophoresis (NACE). Several solvent properties must be considered in selecting the appropriate separation medium (see Chapter 2) dielectric constant, viscosity, dissociation constant, polarity, autoprotolysis constant, electrical conductivity, volatility, and solvation ability. Commonly used solvents in NACE separations include acetonitrile (ACN) short-chain alcohols such as methanol (MeOH), ethanol (EtOH), isopropanol (i-PrOH) amides [formamide (FA), N-methylformamide (NMF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA)] and dimethylsulfoxide (DMSO). Since NACE—UV may present a lack of sensitivity due to the strong UV absorbance of some solvents at low wavelengths (e.g., formamides), the on-line coupling of NACE... 

Although acid-base titrations in alcohol-water mixtures have been studied extensively, we do not consider them in detail since titration curves and indicator equilibria in ethanol-water and methanol-water mixtures can be calculated in the same way as in water. Values of the autoprotolysis constants of the mixtures, are close to for mixtures containing only a moderate amount of alcohol. On the other hand, even a trace of water in ethanol causes a large increase in J SH According to Gutbezahl and Grunwald, pXsH is 14.33, 14.88, 15.91, and 19.5 for ethanol-water mixtures containing 20, 50, 80, and 100 wt % ethanol. 

Thus, proton exchange is faster than in pure methanol because of the larger autoprotolysis constant. 

The most water-like of this class of solvents is methanol, for it maintains much the same nice balance of basic and acidic properties found in water. Its autoprotolysis constant is smaller than that of water (Table 3.3.4) because of its lower dielectric constant. Medium effects for transfer of ionisation equilibria from water to methanol are approximately constant for closely related acids. For six cation acids, the pyri-dinium ion and five methyl derivatives, the average medium ejffect is 0.06 0.02, small because the ionisation of these cations creates no new charge field. For phenol and thirteen of its derivatives the medium effect is 4.32 0.09 smaller values are obtained for nitrophenols, possibly because the anions are stabilised by dispersion interactions with methanol. For 23 carboxylic acids, aliphatic and aromatic, the average medium effect is 4.87 0.15. Values of the medium effect for individual acids are collected in Appendix 3.5.5. 

The acidity function H increases as the concentration of alkali metal methoxide in methanol is increased. Introducing the expressions for acidity and autoprotolysis constants into eqn. 3.5.11. we can write it in the form... 

The constants of several solvents are listed in Table III.3.7. Some examples of autoprotolysis are the reactions of water, methanol, acetic acid, and liquid ammonia ... 

The pH concept is most commonly used for dilute aqueous media however, a similar formalism can be extended to other systems. The extent of the pH scale, which in aqueous media can be described as 14 units, depends on the autoprotolysis constant of the amphiprotic solvent, so that the equivalent range, e.g., in methanol, equals 16.7 units, in sulfuric acid 2.9 units, and in acetic acid 14.5 units. In such solvents, as in water, the pH of neutrality corresponds to the middle of this range. Such reasoning cannot be extended to protophilic (e.g., pyridine, ethers), and aprotic (e.g., hydrocarbons) solvents, for which the logan+ scale is from one or both sides, respectively, unlimited. 

The pH scale of any amphiprotic solvent is limited by zero and pA ap values (A ap is the autoprotolysis constant of medium) it differs in a mixed solvent, for example, methanol-water, where different proton-transfer equilibria occur. Ionizable solutes dissolved in these mixtures are differently solvated, show different dissociation constants, and the pH scale of the medium changes with mobile-phase composition. 

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