Mixed solvent acidity

Note. The period of 5-8 hours recommended above for attaining an equilibrium between the vapour molecules of the mixed solvent and those absorbed by the paper strip is essential if accurate R values are required for identification of mixed amino-acids. To illustrate the separation, as in the above experiment, this period may be reduced to about 2 hours. 

If the substance is found to be far too soluble in one solvent and much too insoluble in another solvent to allow of satisfactory recrystallisation, mixed solvents or solvent pairs may frequently be used with excellent results. The two solvents must, of course, be completely miscible. Recrystallisation from mixed solvents is carried out near the boiling point of the solvent. The compound is dissolved in the solvent in which it is very soluble, and the hot solvent, in which the substance is only sparingly soluble, is added cautiously until a slight turbidity is produced. The turbidity is then just cleared by the addition of a small quantity of the first solvent and the mixture is allowed to cool to room temperature crystals will separate. Pairs of liquids which may be used include alcohol and water alcohol and benzene benzene and petroleum ether acetone and petroleum ether glacial acetic acid and water. 

Well, that s about as rounded an education on Leuckart reactions as Strike can give. Strike feels that after reading all of those similar, repetitious steps, one can start to get a good feel for where a product is at any given moment. Stuff like what happens to MDA when it s mixed with acid or base, or what happens to ketones (P2P) under the same circumstances. One can see now that it is possible to not only isolate safrole and P2Ps chemically but that the same can be true for the final MDA or meth freebase oil. Repeated washings with acid or base and solvent can effectively clean up a compound to an almost presentable state without the use of vacuum distillation, it can happen, one only needs have confidence in the chemistry. 

The chloranil dehydrogenation of A -3-ketones offers a convenient direct conversion to A -ketones. t-Butanol and xylene are the most suitable solvents. Slightly higher yields have been claimed with mixed organic acid-inert solvent systems, although somewhat lower yields (50-60%) are... 

There is increasing interest in copolymer systems, which, due to their chemical heterogeneity, may require very complex eluent systems in order to dissolve the sample and ensure that the separation ensues hy a pure size exclusion mechanism. In these examples, the PLgel is also compatible with eluent systems containing mixed solvents of different polarity (including water as a cosolvent up to 10% hy volume) and in organic solvents modified with acids or bases (e.g., acetic or formic acid, triethanolamine) as it is stable in the pH range of 1-14. 

Sn2 reactions with anionic nucleophiles fall into this class, and observations are generally in accord with the qualitative prediction. Unusual effects may be seen in solvents of low dielectric constant where ion pairing is extensive, and we have already commented on the enhanced nucleophilic reactivity of anionic nucleophiles in dipolar aprotic solvents owing to their relative desolvation in these solvents. Another important class of ion-molecule reaction is the hydroxide-catalyzed hydrolysis of neutral esters and amides. Because these reactions are carried out in hydroxy lie solvents, the general medium effect is confounded with the acid-base equilibria of the mixed solvent lyate species. (This same problem occurs with Sn2 reactions in hydroxylic solvents.) This equilibrium is established in alcohol-water mixtures ... 

Another issue important to the success of this chiral titanium reagent 31 was the discovery of a marked solvent effect. When the fumaric acid derivative is reacted with isoprene in the presence of 10 mol% of the titanium reagent 31 in toluene, poor optical purity results (36-68% ee). Interestingly the optical purity of the adduct greatly increased in the order benzene, toluene, xylenes, and mesitylene, with 92% ee obtained in the last. Mesitylene is difficult to remove, because of its high boiling point, and other solvents were screened in detail. As a result, the mixed solvent system toluene petroleum ether (1 1) was discovered to be very effective. 

The oxidation of n-butane represents a good example illustrating the effect of a catalyst on the selectivity for a certain product. The noncatalytic oxidation of n-butane is nonselective and produces a mixture of oxygenated compounds including formaldehyde, acetic acid, acetone, and alcohols. Typical weight % yields when n-butane is oxidized in the vapor phase at a temperature range of 360-450°C and approximately 7 atmospheres are formaldehyde 33%, acetaldehyde 31%, methanol 20%, acetone 4%, and mixed solvents 12%. 

As already indicated, ion exchange resins are osmotic systems which swell owing to solvent being drawn into the resin. Where mixed solvent systems are used the possibility of preferential osmosis occurs and it has been shown that strongly acid cation and strongly basic anion resin phases tend to be predominantly aqueous with the ambient solution predominantly organic. This effect (preferential water sorption by the resin) increases as the dielectric constant of the organic solvent decreases. 

Purification of the activation products (PMs). The methylamine activation product dissolved in methanol is purified by chromatography, first on a column of silica gel using a mixed solvent of chloroform/ethanol, followed by reversed-phase HPLC on a column of divinylbenzene resin (such as Jordi Reversed-Phase and Hamilton PRP-1) using various solvent systems suitable for the target substance (for example, acetonitrile/water containing 0.15% acetic acid). 

Hojo and colleagues155,156 have carried out numerous studies of ortho-effects, in particular on the ionizations of benzoic acids in DMSO-water mixed solvents. The ortho-effects are assessed by measuring the pKa values of ortho- and para- substituted benzoic acids in solvents containing from 0 to 95% v/v DMSO and expressing the results as equation 15 ... 

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... 

Cohen et have also made some observations on the exchange in water-sucrose and water-ethylene glycol mixed solvents containing perchloric acid (0.106 M). Over a range of dielectric constant 68 to 88, no alteration in the exchange rate was observed. 

The reaction between Tl(III) and U(IV) is one of the few redox reactions which have been studied in a mixed solvent. Solutions were kept under nitrogen. There are striking differences between the rate in aqueous perchloric acid and methanol-aqueous perchloric acid solutions. In the latter media the order with respect to Tl(III), U(IV), and H alters as the solvent composition is changed (Table 29). For 25% methanol-75 % water solvent the kinetic orders of 1.0, 1.5 and —1.33 with respect to U(IV), Tl(III), and H, respectively, are consistent with the existence of two competing pathr whose net activation processes are... 

Recovery of IC-0. Combine 20 g of the air-dried soil with 100 niL of a mixed solvent of methanol and 0.1 M ammonium chloride (4 1, v/v) in a 250-mL stainless-steel centrifuge tube, shake the mixture with a mechanical shaker for 30 min and centrifuge at 8000 r.p.m. for 2 min. Filter the supernatant through a Celite layer (1-cm thick) under reduced pressure into a 500-mL flask. Add 100 mL of mixed solvent of methanol and 0.5 M sodium hydroxide solution (4 1, v/v) to the residue and then extract and filter in the same manner. Combine the flltrates and concentrate to approximately 40 mL on a water-bath at ca 40 °C by rotary evaporation. Add 10 mL of distilled water and adjust the pH of the aqueous layer to 7 with hydrochloric acid. Transfer the aqueous solution into a 200-mL separatory funnel and shake the solution with 50 mL of mixed solvent of dichloromethane and acetone (1 1, v/v) for 5 min. Discard the mixed solvent and adjust the pH of the aqueous layer to 1.5 with hydrochloric acid. Extract the solution with three portions of 50 mL of diethyl ether. Combine the diethyl ether extracts and dry over anhydrous sodium sulfate. Concentrate to dryness on a water-bath at ca 40 °C by rotary evaporation. 

An analytical solution for molecules with alkaline functionality is acid/base titration. In this technique, the polymer is dissolved, but not precipitated prior to analysis. In this way, the additive, even if polymer-bound, is still in solution and titratable. This principle has also been applied for the determination of 0.01 % stearic acid and sodium stearate in SBR solutions. The polymer was diluted with toluene/absolute ethanol mixed solvent and stearic acid was determined by titration with 0.1 M ethanolic NaOH solution to the m-cresol purple endpoint similarly, sodium stearate was titrated with 0.05 M ethanolic HC1 solution [83]. Also long-chain acid lubricants (e.g. stearic acid) in acrylic polyesters were quantitatively determined by titration of the extract. 

The cosolvent will lower the dielectric constant of the mixed solvent, independent of the properties of the solute molecule. The ionization constant of acids will increase and that of bases will decrease (see Sections 3.3.3 and 3.3.4), the result of which is to increase the fraction of uncharged substance in... 

Shedlovsky, T., The behaviour of carboxylic acids in mixed solvents, in Pesce, B. (ed.), Electrolytes, Pergamon Press, New York, 1962, pp. 146-151. 

The one-step reaction of H2prCl6] with MeC02Li under 02 in a mixed solvent of acetic acid and acetic anhydride yields the Ir11 binuclear complex [Ir2(/u-02CMe)2Cl2(C0)2].483 Crystal-structure determinations of [Ir2(/x-02CMe)2Cl2(C0)2L2], (295), where L = MeCN, DMSO, and py, are reported. The one-electron oxidation product for (295), L = py, is EPR active at 77 K the odd electron occupies the 6Ir Ir orbital. 

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