With Low-boiling Solvents

Typically, reaction times were up to three orders of magnitude shorter than for literature preparations performed conventionally at lower temperatures under atmospheric pressure. Thus, procedures requiring hours or days at reflux or ambient temperature and pressure often were complete within minutes and with high selectivity on use of microwave heating. Other technologies or methods compatible with these systems, included solvent-free conditions with neat starting material. 

We have previously reported the intramolecular aldol reaction of hexane-2,5-dione to afford 3-methylcyclopent-2-en-l-one in highly dilute aqueous base [10]. 

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The study of the properties and applications of clusters requires them to be accessible on a preparative scale and this is often conditioned by the ease with which they can be separated from other reaction products. Because their molecular weights are not too high, several tetranuclear clusters are volatile and/or soluble in organic solvents, and therefore the first methods of separation to be considered are sublimation under high vacuum (if the compound is thermally stable) and crystallization. When the solubility is low, continuous extraction with low boiling solvents can be tried50. ... 

Solid samples are extracted with low-boiling solvents. As the polarity of the volatiles is different, a two-step extraction procedure is recommended, e.g. methylene chloride as the first solvent and diethyl ether as the second solvent [13]. The yield of the odorants is enhanced when the dry sample is soaked in water before the extraction procedure [14]. After filtration and drying, the extract is concentrated to approximately 50 mb and is then freed from the non-volatile material by using the solvent-assisted flavour evaporation (SAFE) method (Sect. 16.2.2.2). 

Anionic polymerization of (meth)acrylates with hindered ester functions can most likely be conducted at room temperature and above to remove the heat of polymerization with low boiling solvents. Polymerization of the important methyl and ethyl (meth)acrylate members of the family, however, are still plagued by chain termination at higher temperatures. The phosphorus based counterions have a stability advantage over tetraalkylammonium counterions which undergo Hoffman elimination. 

Separation is carried out using various techniques, depending on the nature of the compounds. For nonionic species the solubilities of the compounds are generally low and similar, and it has been necessary to use either fractionation by continuous extraction with low boiling solvents (36) or thin-layer chromatography (58, 60). The first method has been used with air-sensitive compounds, whereas the second has been applied only to air-stable substances. In both cases, it is possible to separate only limited amounts of compounds, whose characterization is, therefore, carried out using particular techniques such as mass spectroscopy (58-60). 

Traditional flavouring raw materials are produced under rather harsh conditions heat, distillation at high temperatures, concentration, extraction. These result in the destruction of all sensitive substances. Modem flavouring raw materials are produced under controlled and careful conditions low pressure, low temperature distillation, extraction with low boiling solvents or CO2, careful selection of fresh, high quality raw materials. Therefore sensitive substances survive the production process and influence the quality with all positive (flavour profile) and negative effects (stability). 

More efficient devices are commercially available. One of these is the Accelerated Solvent Extractor sold by DIONEX, " where the extraction takes place in a closed vessel in which temperatures of 100°C and pressures up to 2000 psi are reached even with low boiling solvents. 

Signiflcant advantages of microwave reactors designed for operating with low-boiling solvents at elevated temperature and pressure are summarized below ... 

Table 4.2 Extraction of DLTDP from liquid paraffin extractant with low-boiling solvents ...

Note 4—Pyridine should be used with low boiling solvents (430 F). 

In these polymers, the fractions which are extractable with low-boiling solvents (e.g., diethyl ether, pentane, hexane) may be conceived, from the microstructural viewpoint, as mixtures of enantiomorphic-site controlled isotactoid sequences and of chain-end controlled syndiotactoid sequences. 

These can be converted to their sodium salts by precipitation below 30° with aqueous 25% NaOH. The salt is then decomposed by addition of solid (powder ) carbon dioxide and extract with low-boiling petroleum ether. The solvent should be removed under reduced pressure below 20°. The manipulation should be adequately shielded at all times to guard against EXPLOSIONS for the safety of the operator. 

Another piece of glassware used in synthetic procedures is the cold finger condenser (Fig. A3.7). Used in the case where the reaction is carried out in a low-boiling solvent (for example, liquid ammonia), the condenser is filled with a suitable cooling mixture and fitted with a drying tube. 

Obviously, the use of a nonvolatile ionic liquid simplifies the distillative workup of volatile products, especially in comparison with the use of low-boiling solvents, where it may save the distillation of the solvent during product isolation. Moreover, common problems related to the formation of azeotropic mixtures of the volatile solvents and the product/by-products formed are avoided by use of a nonvolatile ionic liquid. In the Rh-catalyzed hydroformylation of 3-pentenoic acid methyl ester it was even found that the addition of ionic liquid was able to stabilize the homogeneous catalyst during the thermal stress of product distillation (Figure 5.2-1) [21]. This option may be especially attractive technically, due to the fact that the stabilizing effects could already be observed even with quite small amounts of added ionic liquid. 

Lussier [71] has given an overview of Uniroyal Chemical s approach to the analysis of compounded elastomers (Scheme 2.2). Uncured compounds are first extracted with ethanol to remove oils for subsequent analysis, whereas cured compounds are best extracted with ETA (ethanol/toluene azeotrope). Uncured compounds are then dissolved in a low-boiling solvent (chloroform, toluene), and filler and CB are removed by filtration. When the compound is cured, extended treatment in o-dichlorobenzene (ODCB) (b.p. 180 °C) will usually suffice to dissolve enough polymer to allow its separation from filler and CB via hot filtration. Polymer identification was based on IR spectroscopy (key role), CB analysis followed ASTM D 297, filler analysis (after direct ashing at 550-600 °C in air) by means of IR, AAS and XRD. Antioxidant analysis proceeded by IR examination of the nonpolymer ethanol or ETA organic extracts. For unknown AO systems (preparative) TLC was used with IR, NMR or MS identification. Alternatively GC-MS was applied directly to the preparative TLC eluent. 

Another problem results from high vapor pressures of relatively low-boiling solvents. With regard to the dependence on reactor geometry, it can take some time for the vapor pressure of the solvent to become established in the gas phase of the closed system. As this equilibration of vapor pressure provides a positive volume contribution (the pressure above the reaction solution increases in a closed system), measured gas consumptions can be considerably falsified not only as a function of time but also in respect of the overall balance One way to avoid this problem is to separate the gas phase above the reaction solution from the gas in the measuring burette by using a tempered bubble counter (cf. Fig. 10.3). 

If none (or very little) of the solid dissolved at room temperature, unstopper the tube and heat it (Careful—no flames ) and shake it and heat it and shake it. You may have to heat the solvent to a gentle boil (Careful Solvents with low boiling points often boil away). If it does not dissolve at all, then do not use this as a recrystallization solvent. 

N-( 1,3-Benzoxazinyl)aminomethylenemalonates (1529 and 1531) were obtained in 70-87% yields when AM 3- and 4-carbamoyl-4- and 3-hydroxy-phenyl)aminomethylenemalonates (1528 and 1530) were first treated with ethyl chloroformate dropwise in a mixture of pyridine and acetonitrile at 0-5°C. After refluxing the mixtures for 2 hr, the low-boiling solvent was removed under reduced pressure and the residues again were refluxed at about 125°C for 1 hr (75IJC1275). 

Solvent a liquid in which certain kinds of molecules dissolve. Although they typically are liquids with low boiling points, they may include high-boihng liquids, supercritical fluids, or gases. 

Apparently, the formation of the microporous structure within the PVdF—HFP copolymer was of critical importance to the success of Bellcore technology, and the ion conductivity was proportional to the uptake of the liquid electrolyte. To achieve the desired porosity of PVdF film, Bellcore researchers prepared the initial polymer blend of PVdF with a plasticizer dibutylphthalate (DBP), which was then extracted by low boiling solvents after film formation. Thus, a pore-memory would be left by the voids that were previously occupied by DBP. However, due to the incomplete dissolution of such high-melting DBP during the extraction process, the pore-memory could never be restored at 100% efficiency. Beside the total volume of pores thus created by the plasticizer. 

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