The choice of solvent

The selection of the solvent is an important issue. One will take into account expectations concerning growth rate, temperature range and electrical properties of the epitaxial layers. The main criteria are listed below  

High solubility of silicon in the solvent This parameter, or the evolution of solubility versus temperature during the growth has to be compatible with a reasonable time of obtainment of the active layer. 

Low solubility of solvent into the silicon During epitaxial process, atoms of the solvent are incorporated in the Si crystal. Their incorporation can modify the electronic properties of the layer. Actually, many metallic impurities act as recombination centre or as dopant and reduce the lifetime of minority carriers. Solvent purity is also an important parameter to avoid other impurities. 

Low toxicity and availability of the solvent For instance, materials like antinomy (Sb), gold (Au) or silver (Ag) offer high solubility of silicon. However, the first one is very toxic and the others are quite expensive. 

A combination of two or more solvents can also be used. This is mainly the case, when lowering the growth temperature is the main objective (see Sect. 9.6). 

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The choice of the solvent also has a profound influence on the observed sonochemistry. The effect of vapor pressure has already been mentioned. Other Hquid properties, such as surface tension and viscosity, wiU alter the threshold of cavitation, but this is generaUy a minor concern. The chemical reactivity of the solvent is often much more important. No solvent is inert under the high temperature conditions of cavitation (50). One may minimize this problem, however, by using robust solvents that have low vapor pressures so as to minimize their concentration in the vapor phase of the cavitation event. Alternatively, one may wish to take advantage of such secondary reactions, for example, by using halocarbons for sonochemical halogenations. With ultrasonic irradiations in water, the observed aqueous sonochemistry is dominated by secondary reactions of OH- and H- formed from the sonolysis of water vapor in the cavitation zone (51—53). 

Solution Polymerization. This method is not commercially important, although it is convenient and practical, because it provides viscous cements that are difficult to handle. Also, the choice of the solvent is a key parameter due to the high solvent chain-transfer constants for acrylates. 

The reaction of GTP must be strictly conducted under anhydrous conditions. The choice of the solvent... 

The enantioselective addition of organometallic reagents to, V-(trimethylsilyl)benzaldehyde imine (1) in the presence of enantiomerically pure modifiers has been investigated. The best result is obtained with butyllithium (the corresponding Grignard reagent affords both lower yield and selectivity, 1 fails to react with diethylzinc) and two equivalents of the enantiomerically pure diol 2 in diethyl ether. It should be noted that the choice of the solvent is crucial for the stereoselectivity of the reaction1 2 3 5 7 8 9. 

Electrochemical measurements are commonly carried out in a medium that consists of solvent containing a supporting electrolyte. The choice of the solvent is dictated primarily by the solubility of the analyte and its redox activity, and by solvent properties such as the electrical conductivity, electrochemical activity, and chemical reactivity. The solvent should not react with the analyte (or products) and should not undergo electrochemical reactions over a wide potential range. 

After the discovery of the remarkable acceleration of some Diels Alder reactions performed in water, a number of polar non-aqueous solvents and their salty solutions were investigated as reaction medium. This revolutionized the concept that the Diels-Alder reaction is quite insensitive to the effect of the medium and emphasized that a careful choice of the solvent is crucial for the success of the reaction. The polarity of the reaction medium is an important variable which also provides some insights into the mechanism of the reaction. If the reaction rate increases by using a polar medium, this means that the transition state probably has polar character, while the absence of a solvent effect is generally related to an uncharged transition state. 

K.—Ordinarily the intrinsic viscosity should depend on the molecular weight not only owing to the factor occurring in Eq. (26), but also as a result of the dependence of the factor on M. The influence of this expansion resulting from intramolecular interactions may be eliminated by suitable choice of the solvent and temperature. Specifically, in an ideal solvent, or -solvent, a = l and Eq. (26) reduces to... 

Nevertheless, in some cases, this criterion is not sufficient for the choice of the solvent. For instance, Kuo and Parkin [78], demonstrated that hydrophobicity of solvent in the presence of lipase also affect selectivity and partition of reactants in esterification reactions. On the other hand, in the presence of certain solvents, even in low concentration, enzyme can be activated [13]. 

The new ligands were also tested in the hydrogenation of methyl (Z)-a-(N-acetamido)-4-chlorocinnamate. The results are summarized in Table 24.2. The symmetrical bisphospholane ligand 3 delivered chiral N-acylaminoacid methyl-ester with excellent enantioselectivities in all solvents studied. The unsymmetrical ligands 4-6 were less selective and sensitive to the choice of the solvent. 

In both cases, either conventional FTIR transmission or diffuse reflection detection may be used. Because TLC and the postspectroscopic evaluation are not linked directly, few compromises have to be made with regard to the choice of the solvent system employed for separation. Chromatographic selectivity and efficiency are not influenced by the needs of the detector. The TLC plate allows the separation to be made in a different site from the laboratory where the separated analytes are evaluated. The fact that the sample is static on the plate, rather than moving with the flow of a mobile phase, also puts less demand on the spectrometer. The popularity of TLC-IR derives in part from its low cost. 

Sometimes the choice of the solvent can exert a great influence on ion selectivity, e.g., whereas calcium dialkylphosphate in dioctylphenyl phosphonate... 

A novel synthesis of 1,2,3-selenadiazoles 338 starts with the Michael addition of 2-nitropropane to a,P-unsaturated ketones 336 under basic conditions <06JHC149>. The resulting adducts are treated with semicarbazide hydrochloride to give semicarbazones 337, which are converted to 1,2,3-selenadiazoles 338 by reaction with selenium dioxide in THF (the choice of the solvent appears to be important in this case). 

By comparing the two adsorbents, it turns out that the non-polar character of the petroleum ether is exploited in the extraction from the strong nonspecific active sites of carbon black. On the other hand, the polar character of acetone makes the extraction from the highly strong specific active sites of the siliceous material of the soil possible. The retention time obtained with the mixture is intermediate between those two separate solvents, and the recovery, in turn, is higher. The proper choice of the solvent mixture plays a very important role on the size of the final volume of solution in which the pesticides are collected. 

On occasions it is not possible to satisfy all three of these criteria (because either the choice of the solvent or the supporting electrolyte has been determined by the substance under study or there are practical limits imposed on the construction of the electrode). 

An important aspect related to the choice of the solvent lies in its coordinating ability towards the redox species under study. This property is related to the polarity of the solvent (we will discuss this in... 

The choice of the solvent is critical, and both non-coordinating solvents or polar aprotic solvents such as DMF can lead to intractable product mixtures. Solvent effects and side-reactions in chromium carbene benzannulation reactions have been thoroughly investigated [207,333,334]. 

For a long time a proven concept for product separation following chemical reactions has been to cool down the homogeneous reaction mixture and allow the product to crystallize or solidify. Very often also the educts of the reaction are not soluble at lower temperature. Appropriate choice of the solvent for the reactions is the prerequisite for the success of that operation. Chemists developing procedures for production acquire a lot of expertise selecting the optimal solvent for a chemical reaction which should enable both optimal reaction conditions and the opportunity to separate the product in a clean form. 

If the non-polar solvent s2 is varied, the following tendenqr is observed the less polar s2 the more mediator s3 is required until a single-phase solution is formed. Alcohols (2-octanol, 2-nonanol and 1-dodecanol), toluene, p-xylene and cyclohexane were used as s2. With DMF as the mediator a clear solution is obtained at a weight ratio si 2-octanol DMF of 1 3 0.6, while in the system si toluene DMF a ratio of 1 3 2.5 is required. With the very nonpolar solvent cyclohexane the solvent mixture becomes homogeneous at a ratio si cyclohexane DMF =1 3 11. The temperature dependency of the solvent systems si s2 DMF is almost not affected by the choice of the solvent s2. 

Liquid-liquid extractions involve the separation of analytes from interferences by partitioning the sample between two immiscible solvents. In most cases, one of the liquids is an aqueous solvent and the other is an organic solvent. The selectivity and efficiency of the extraction process are governed by the choice of the solvent pair. In aqueous and organic solvent pairs, the more hydrophilic compounds prefer the aqueous phase and the more hydrophobic compounds will be found in the organic phase. 

Although the concentration of fluorine is the most important quantity in the control of the reaction rate and must be maintained within certain limits, in practice the stoichiometry, the molecular fluorine to substrate H-atom molar ratio, is used to determine the reaction parameters leading to a successful and efficient perfluorination. AF is most successful when sublimable solids are introduced into the hydrocarbon evaporator unit of the aerosol fluorinator as solutions by a syringe pump. This now common procedure emphasizes the individual molecule s isolation as it is fluorinated using AF. No intermolecular reactions between solute and solvent have been observed Choice of the solvent is important as it must not boil at a temperature below the melting point of the solute in order to prevent solid deposition in the tubes feeding the evaporator. It must also fluorinate to a material easily separable from the solid reactant after perfluorination. In most cases it has been found that aliphatic hydrochlorocarbons are excellent choices, but that carbon tetrachloride and chloroform and other radical-scavenging solvents are not (sec ref 6). 

For a given polymer with fixed X0 and Ax, Eqs. (3.15) and (3.16) describe a non-linear but monotonous variation of 0 with the composition of the mobile phase Cb (Fig. 10). The procedure of finding the critical conditions then becomes very simple it is necessary to find two solvents, in one of which (a) the adsorption and in the other one (b) the exclusion mode is operative and then, by changing their ratio, to find the point C where there is no retention volume dependence on the molecular weight of the polymer. The only requirement imposed on the choice of the solvents is that they both should be good solvents. In practice, however, it is... 

In water, however, the endoperoxide is exclusively transformed by SN2 reaction with the solvent into the corresponding ring-opened hydroperoxide (Eq. 55), which then undergoes intramolecular fragmentation to the desired product [12]. Limitations in the choice of the solvent may in turn restrict the choice of the sensitizer for reasons of solubility and photochemical (and thermal) stability. As a result, experimental conditions for an optimal chemical yield may call for a sensitizer exhibiting a relatively small d>A. 

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