Reaction methods solvent evaporation

Phenomena of purity drop in racemic resolution by preferential crystallization were surveyed and factors relevant to spontaneous crystallization and their significance were quantitatively summarized. It was empirically concluded that washing of seed crystals would be the best way to prevent such purity drop, although it could not completely suppress the occurrence. With additional experiments on the preparation method of seed crystals, adequate procedures were found. However, recrystallization under these conditions could only prolong the start of purity drop but could not prevent it. A combined process to produce optically pure enantiomers consisting of preferential crystallization, racemization reaction and solvent evaporation steps was proposed. 

To remove any metallic Au, the reaction mixture is evaporated to dryness and subsequently passed over a commercial terra infusorii column in a minimal volume of solvent. Crystallization may be performed by slow diffusion of a two-layer system. Yields are low (10-60%). The metal evaporation technique (see 8.2.1.2) gives higher yields than the sodium borohydride method. 

In addition to the formation of ceramic particles in the gas phase, particles can be formed in the liquid phase and consolidated via solvent evaporation to form useful products. Unlike slurry-based processes in which no liquid-phase reaction occurs, the processing of ceramics via the sol-gel method involves several important reactions. And like gas-phase reactions, the ceramics are formed from precursors that contain the component ions for the ceramic. 

The incorporation of a cationic azobenzene derivative, p-( a> -dimethyl-ethanolammonioethoxyj-azobenzene bromide, into nanoporous silica films and the photochemical reactions of the adsorbed dye were investigated. The nanoporous silica films were prepared from tetramethoxysilane and octadecyltrimethyl-ammonium chloride by the rapid solvent evaporation method which we have reported previously. The adsorption of the cationic azo dye was conducted by casting an ethanol solution of the dye onto the nanoporous silica films. Upon UV light irradiation, trans-azobenzene isomerized photochemically to the c/s-form and photochemically formed c/ s-form turned back to the frans-form upon visible light irradiation. The nanoporous silica films were proved to be an excellent reaction media to immobilize organic photocromic species. 

Cooling by solvent evaporation is an efficient method. On one hand, it is independent of the heat transfer at the reactor wall and, on the other hand, the condenser can be dimensioned independently of the reactor geometry. This reaches relatively high specific cooling powers. In case a reaction cannot be performed at boiling temperature, it is possible to work under partial vacuum to decrease the boiling... 

By using ether instead of benzene or petroleum ether, as specified in the older methods, the evaporation of the solvent after the reaction is facilitated. 

A monolayer or submonolayer is formed when chemisorption or chemical reaction occurs, or when a self-assembly method is applied, however, further layers can be developed on top of the primary layer. Usually multilayer films are formed when polymers are deposited on a conductor surface by -> electropolymerization, by solvent evaporation, or other methods. Further modification of a polymer film is also often used, e.g., by incorporation of enzymes. 

The traditional approach used in poly(imide-siloxane) synthesis is the reaction of aminopropyl-terminated dimethylsiloxane oligomers with aromatic dianhydrides and additional diamines (9-13). Typically, subambient temperatures and dipolar aprotic solvents are used. The resulting high-molecular-weight polyamic acid solution can be heated to effect imidization and solvent evaporation. This procedure is analogous to the synthetic method used to prepare conventional polyimides for films and coatings. 

As any method of anion analysis may be applied if isolation techniques such as evaporation, precipitation, ion exchange, or solvent extraction are employed, we shall limit the discussion to direct methods and admit isolation techniques only if they are simple and rapid. The methods apparently best suited to the direct analysis of trace amounts of anions therefore are limited to selective membrane potentiometric, atomic absorption, fluorescence, and spectrophotometric methods following oxidation-reduction or complexometric reactions, or solvent extraction. Most of the traditional analytical methods—gravimetric, titrimetric, emission spectrometric, and electrical methods involving oxidation and reduction are less suitable, as are most radioactive procedures including neutron activation analysis, except in special cases. 

It is now appropriate to differentiate the sol-gel method from MOD. In general, we consider a process sol-gel if it involves these two characteristic chemical reactions (1) hydrolysis of at least one precursor compound and (2) poly-condensation of the hydrolyzed precursor compound. A MOD process involves the thermal decompositions of all precursor compounds physically condensed onto substrates through rapid solvent evaporation. Therefore, methods using all alkoxide and partially alkoxide precursors are considered sol-gel processing, and those using precursors other than alkoxides are MOD. One last interesting point to emphasize is that one can use all alkoxide precursors in a MOD process. This is possible when neither of the two characteristic reactions for sol-gel is involved and the reactions used to form the oxide is simply thermal decomposition of the metal alkoxides. 

An alternative method that has been used to prepare ferrite particles in a single step is the evaporative decomposition of the solutions, or spray-roasting (Ruthner, 1977). The solutions were mixed and atomised, and the droplets fell through a reaction chamber at 900-1050 °C. The solvent evaporated and the salts decomposed to oxides. The process took 3-5 s. By using a roasting furnace for industrial production, agglomerates of 40-200 pm containing 1 pm ferrite particles were obtained. The furnace feedstock was an aqueous suspension of the oxides, carbonates or hydroxides of the desired composition. However, the residence time was insufficient and complete transformation to the desired ferrite was not achieved. 

Homophthalic anhydrides can be prepared by the dehydration of homoph-thalic acid with add chloride, acid anhydride, phosgene, thionyl chloride, benzene sulfonyl chloride, ketene, phosphorous pentoxide, dicyclohexyl-carbodiimide, or J -carbonyldiimidazole. However, these methods are not always effective for the acid-sensitive and/or unreactive homophthaUc acid derivatives. We developed a very mild and efficient method for obtaining homophthaUc anhydrides using (trimethylsilyl)ethoxyacetylene [9] (Scheme 1). Thus the treatment of homophthaUc acid 1 with (trimethylsi-lyl)ethoxyacetylene 2 in inert solvents such as methylene chloride, 1,2-di-chloroethane, and acetonitrile gave the homophthaUc anhydride 3 in a quantitative yield accompanied by ethyl trimethylsilyl acetate as the only side product. The high purity homophthaUc anhydride 3 could be obtained for the next cycloaddition reaction just by evaporation to remove the reaction solvent and the formed ethyl trimethylsilyl acetate. The method is quite useful, and was also applicable to the syntheses of hetero-homophthalic anhydrides 4. 

Thermoplastic fibers are typically fabricated via a melt spinning process wherein the melt is extruded through the many holes in a spinneret and cooled to harden. Other fiber materials can be dissolved in a solvent and then exhuded, with the sohd fibers precipitating from solution either due to a chemical reaction in a liquid bath or due to solvent evaporation in a gas. The latter method, dry spinning, is closest to the focus of this section— electro spinning of nanoflbers. 

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