Preferred solvents

The reaction is of the 8 2 type and works best with primary and secondary alkyl halides Elimination is the only reaction observed with tertiary alkyl halides Aryl and vinyl halides do not react Dimethyl sulfoxide is the preferred solvent for this reaction but alcohols and water-alcohol mixtures have also been used... 

Solubility and Solvent Resistance. The majority of polycarbonates are prepared in methylene chloride solution. Chloroform, i7j -l,2-dichloroethylene, yy -tetrachloroethane, and methylene chloride are the preferred solvents for polycarbonates. The polymer is soluble in chlorobenzene or o-dichlorobenzene when warm, but crystallization may occur at lower temperatures. Methylene chloride is most commonly used because of the high solubiUty of the polymer (350 g/L at 25°C), and because this solvent has low flammabiUty and toxicity. Nonhalogenated solvents include tetrahydrofuran, dioxane, pyridine, and cresols. Hydrocarbons (qv) and aUphatic alcohols, esters (see Esters, organic), or ketones (qv) do not dissolve polycarbonates. Acetone (qv) promotes rapid crystallization of the normally amorphous polymer, and causes catastrophic failure of stressed polycarbonate parts. 

Catalytic Reduction. Catalytic reduction usually takes place in solution, emulsion, or suspension in autoclaves or pressurized vessels after the catalyst is added, the vessel is pressurized with hydrogen (32,33). Water and methanol are the preferred solvents. In water the addition of alkaU hydroxide (34), alkah carbonate (35), or acid (36) has been recommended. 

Aromatic steroids are virtually insoluble in liquid ammonia and a cosolvent must be added to solubilize them or reduction will not occur. Ether, ethylene glycol dimethyl ether, dioxane and tetrahydrofuran have been used and, of these, tetrahydrofuran is the preferred solvent. Although dioxane is often a better solvent for steroids at room temperature, it freezes at 12° and its solvent effectiveness in ammonia is diminished. Tetrahydrofuran is infinitely miscible with liquid ammonia, but the addition of lithium to a 1 1 mixture causes the separation of two liquid phases, one blue and one colorless, together with the separation of a lithium-ammonia bronze phase. Thus tetrahydrofuran and lithium depress the solubilities of each other in ammonia. A tetrahydrofuran-ammonia mixture containing much over 50 % of tetrahydrofuran does not become blue when lithium is added. In general, a 1 1 ratio of ammonia to organic solvents represents a reasonable compromise between maximum solubility of steroid and dissolution of the metal with ionization. 

A direct method for introduction of a C-21 acetoxyl group into a 20-keto-pregnane is by reaction with lead tetraacetate at room temperature. Although originally the reaction carried out in hot acetic acid gave low yields, a careful study by Henbest has defined conditions so that yields as high as 86 % can be obtained at room temperature. The preferred solvent is 5 % methanol in benzene, with boron trifluoride etherate as catalyst. With either methanol or benzene, the yield is less than 4%. 

Dehydrobromination of 12 to -tetronic acid (13) in high yield could be achieved only under narrowly circumscribed conditions 62). The molar ratio of barium hydroxide to 12 needed to solubilize 12 and to neutralize the liberated hydrobromicacid must be at least two to one. Deionized water was the preferred solvent because of ease of workup. The yields were found to be concentration dependent. Concentrations of 12 in water of about 16% gave good yields of 13, whereas at 33% concentration the yield was much lower. One gram of 10% Pd-on-C per 33 g of 12 was required less catalyst gave incomplete reductions. 

The Ni(ii)/Cr(n)-mediated coupling reaction employs an excess of chromium(ii) chloride and a catalytic amount of nickel(n) chloride (0.1 %). The preferred solvent for this reaction is usually DMF, although THF, DMF/THF, or DMF/Me2S may also be used. The ability to activate the vinyl iodide (or related) substrate at ambient... 

Figure 1.14 Conceptual basis for environmentally preferable solvent selection.

Since water is the preferred solvent both in industrial technologies and biomedicine, the development of highly hydrophilic metal colloids has been a key step for a number of recently reported practical applications [182,203]. 

The remainder of this section deals primarily with selectivity optimization in isocratic liquid chromatography and with gradient elution Before entering these subjects proper, however, a discussion of the relevant chromatographic properties of solvents is in order as a framework for the intuitive selection of the preferred solvent or solvent mixtures for selectivity optimization. 

Some fairly broad generalizations can be made about the selection of certain preferred solvents for liquid chromatography from the relatively large number of liquids that might be employed as solvents. A suitable solvent will preferably have a low vi jslty, be compatible with the detection system, be readily available in a pure form, and if possible, have a low flammability and toxicity. Since detection in HFLC occurs on-line,... 

Grignard reagents add to nitriles and, after hydrolysis of the reaction mixture, a ketone is obtained, with hydrocarbons being the preferred solvent for this reaction.90... 

Ketones can also be prepared from acyl chlorides by reaction at low temperature using an excess of acyl chloride. Tetrahydrofuran is the preferred solvent.91 The reaction conditions must be carefully controlled to prevent formation of tertiary alcohol by addition of a Grignard reagent to the ketone as it is formed. 

The polymer/additive system in combination with the proposed extraction technique determines the preferred solvent. In ASE the solvent must swell but not dissolve the polymer, whereas MAE requires a high dielectric solvent or solvent component. This makes solvent selection for MAE more problematical than for ASE . Therefore, MAE may be the preferred method for a plant laboratory analysing large numbers of similar samples (e.g. nonpolar or polar additives in polyolefins [210]). At variance to ASE , in MAE dissolution of the polymer will not block any transfer lines. Complete dissolution of the sample leads to rapid extractions, the polymer precipitating when the solvent cools. However, partial dissolution and softening of the polymer will result in agglomeration of particles and a reduction in extraction rate. 

The SEC mechanism demands only an isocratic (constant composition) solvent system with normally a single solvent. The most frequently used organic solvents are THF, chloroform, toluene, esters, ketones, DMF, etc. The key solvent parameters of interest in SEC are (i) solubility parameter (ii) refractive index (iii) UV/IR absorbance (iv) viscosity and (v) boiling point. Sample solutions are typically prepared at concentrations in the region of 0.5-5 mg mL-1. In general an injection volume of 25-100p,L per 300 x 7.5 mm column should be employed. For SEC operation with polyolefins chlorinated solvents (for detector sensitivity and increased boiling point) and elevated temperatures (110 to 150 °C) are required to dissolve olefin polymer. HFIP is the preferred solvent for SEC analysis of polyesters and polyamides. 

A moderate diastereoselectivity was observed in these reactions where a mixture of diastereomers could be generated.58 The reactivity of the halides followed the order of tertiary > secondary primary and iodide> bromide (chlorides did not react). The preferred solvent system was aqueous ethanol. The process was suggested to proceed by a free radical mechanism occurring on the metal surface under sonochemical conditions. Efforts to trap the intermediate [A] intramolecularly gave only a very low yield of the cyclization product (Scheme 10.4).59... 

Quaternary ammonium azides will displace halogens in a synthesis of alkyl azides. Dichloromethane has been used as a solvent, although this can slowly form diazido-methane which may be concentrated by distillation dining work-up, thereafter easily exploding [1]. An accident attributed to this cause is described, and acetonitrile recommended as a preferable solvent, supported polymeric azides, excess of which can be removed by filtration are also preferred in place of the tetrabutylam-monium salt [2]. A similar explosion was previously recorded when the quaternary azide was generated in situ from sodium azide and a phase transfer catalyst in a part aqueous system [3,4],... 

Shinohara et al. [299] have described a procedure based on gas chromatography for the determination of traces of two, three, and five-ring azarenes in seawater. The procedure is based on the concentration of the compounds on Amberlite XAD-2 resin, separation by solvent partition [300], and determination by gas chromatography-mass spectrometry with a selective ion monitor. Detection limits by the flame thermionic detector were 0.5-3.0 ng and those by gas chromatography-mass spectrometry were in the range 0.02-0.5 ng. The preferred solvent for elution from the resin was dichloromethane and the recoveries were mainly in the range 89-94%. 

Different procedures of this dehydrogenation are thoroughly discussed in the monograph (4). It is only necessary to note here that the process is carried out mainly as halogenation-dehydrohalogenation. The intermediate hydroximoyl halide is frequently not isolated (Scheme 1.3). The reaction is convenient for both the generation of unstable nitrile oxides (in the presence of a dipolarophile) and the preparation of stable nitrile oxides. It is usually carried out in a two-phase water-organic solvent system with methylene dichloride as the preferred solvent. 

The preferred solvent is supercritical CO2. The reasons for this choice are many and various. Firstly, the CO2 is not hot (CO2 first becomes critical at 31 °C and 73 atm pressure see Figure 5.5), so no charring of the coffee occurs during decaffeination. Furthermore, at such a low temperature, all the components within the coffee that impart the flavour and aroma remain within the solid coffee - try soaking coffee beans in cold water and see how the water tastes afterwards Caffeine is removed while retaining a full flavour. 

Glycosidation (13,302). Glycosidation of 2-acetamido-3,4,6-triacetyl-2-deoxy-a-D-glucopyranosyl chloride (1) mediated by Sn(OTf)2 provides exclusively P-glycosides. The most satisfactory base is 1,1,3,3-tetramethylurea, and CH2CI2 is the preferred solvent.2... 

Prepare treatment medium containing various concentrations of test compound 19.7 ml of Eagle s medium (without serum) plus 300 pi of stock concentration of compound in a preferred solvent (e.g., water, ethanol, DMSO, etc.). The final concentration of solvent other than water should not exceed 1% v/v. Normally a range of 0-5000 pg ml-1 (final concentration) is covered. For a sparingly soluble compound, the highest concentration will be the lowest at which visible precipitation occurs. Similarly, if a compound has a marked effect on osmolality, concentrations should not be used that exceed 500 milliosmoles (mosm) per kg. In addition, a pH range of 6.5-7.5 should be maintained. 

While ether is the common solvent for LiAlH4, in which it is soluble, hydroxylic solvents like water, methanol and ethanol are preferred for NaBH4, It is more soluble in methanol than in ethanol, but since it reacts with the former at an appreciable rate than the latter, hence ethanol is the preferred solvent. Isopropanol, in which NaBH4 is stable, is used for kinetic studies of the reduction of aldehydes and ketones. 

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