Aprotic dipolar dissolvent

The aromatic oligoesterketones can be produced by means of interaction between 4,4 -dichlorbisphenylketone and 1, l-dichlor-2,2-di(3,5-dibrom- -oxy-phenyl)ethylene in aprotic dipolar dissolvent (dimethylsulfoxide) at 140 °C in inert gas [ 413]. The copolyesterketones of increased thermo-stability, heat- and fire-resistance can be synthesized on the basis of obtained oligomers. 

Melamine is soluble only in water and has low solubility in dimethylsulfoxide (DMSO) and in other aprotic dipolar solvents (9% at 120 °C), in glycerol or ethylene glycol (10% at 140 °C). In the majority of other usual solvents it is insoluble. Kucharski and Lubczak discovered a new class of reactive solvents for melamine [36] poly (hydroxymethyl) derivatives of cyclohexanone, acetone, nitromethane which are able to dissolve 50-60% melamine. Melamine can be totally propoxylated or ethoxylated at lower temperatures (70-90 °C), in aprotic dipolar solvents (for example DMSO, dimethylformamide, N-methyl pyrrolidone and so on), in the presence of quaternary ammonium hydroxides as catalysts [for example tetrabutyl ammonium hydroxide (TBAH)], at a low reaction rate (reaction 15.35), for a very long reaction time (40-50 hours) [31, 37]. The resulting hexafunctional polyols give very thermostable rigid PU (up 200 °C). 

Other solvents can be divided into several classes. In hydrogen bond-breaking solvents (dipolar aprotics), the simple amino, hydroxy and mercapto heterocycles all dissolve. In the hydrophobic solvents, hydrogen bonding substituents greatly decrease the solubility. Ethanol and other alcohols take up a position intermediate between water and the hydro-phobic solvents (63PMH 1)177). 

In the field of soluble conducting polymers new data have been published on poly(3-alkylthiophenes " l They show that the solubility of undoped polymers increases with increasing chain length of the substituent in the order n-butyl > ethyl methyl. But, on the other hand, it has turned out that in the doped state the electro-chemically synthesized polymers cannot be dissolved in reasonable concentrations In a very recent paper Feldhues et al. have reported that some poly(3-alkoxythio-phenes) electropolymerized under special experimental conditions are completely soluble in dipolar aprotic solvents in both the undoped and doped states. The molecular weights were determined in the undoped state by a combination of gel-permeation chromatography (GPC), mass spectroscopy and UV/VIS spectroscopy. It was established that the usual chain length of soluble poly(3-methoxthythiophene) consists of six monomer units. 

A difficulty that occasionally arises when carrying out nucleophilic substitution reactions is that the reactants do not mix. For a reaction to take place the reacting molecules must collide. In nucleophilic substitutions the substrate is usually insoluble in water and other polar solvents, while the nucleophile is often an anion, which is soluble in water but not in the substrate or other organic solvents. Consequently, when the two reactants are brought together, their concentrations in the same phase are too low for convenient reaction rates. One way to overcome this difficulty is to use a solvent that will dissolve both species. As we saw on page 450, a dipolar aprotic solvent may serve this purpose. Another way, which is used very often, is phase-transfer catalysis ... 

Both of the above-mentioned catalyst types get the anions into the organic phase, but there is another factor as well. There is evidence that sodium and potassium salts of many anions, even if they could be dissolved in organic solvents, would undergo reactions very slowly (dipolar aprotic solvents are exceptions) because in these solvents the anions exist as ion pairs with Na or and are not free to attack the substrate (p. 443). Fortunately, ion pairing is usually much less with the quaternary ions and with the positive cryptate ions, so the anions in these cases are quite free to attack. Such anions are sometimes referred to as naked anions. 

Use of less hazardous solvents - Since the reaction is two phase, simple benign solvents can often be used since PTC avoids the need to find a solvent that will dissolve all reactants, e.g. dipolar aprotic solvents such as dimethyl formamide. In some cases an organic solvent may not be required at all, the substrate forming the second phase. 

Like the carbodiimide method, the mixed anhydride method results in an amide complex (Table 5, Figure 17). The acid-containing hapten is dissolved in a dry, inert, dipolar, aprotic solvent such as p-dioxane, and isobutyl chloroformate is added with an amine catalyst. The activated mixed anhydride is chemically stable and can be isolated and characterized. The aqueous protein solution is added to the activated acid and the pH is maintained at around 8.5. A low temperature (around 10 °C) is necessary during the reaction to minimize side reactions. 

Although water is an excellent solvent and the most popular, it has somewhat anomalous properties that come from the hydrogen bonding ability of water to form three-dimensional networks (Fig. 1.2, Section 1.1.3). Large molecules and ions are often difficult to dissolve in water, unless they have hydrophilic site(s). Therefore, water is not suitable as a medium for reactions involving large hydro-phobic molecules or ions. In contrast, most dipolar aprotic solvents are non-struc-tured or only weakly structured and can dissolve many large molecules and ions. This is another major reason why dipolar aprotic solvents are often used instead of water. 

In other words, the developing positive charges in the transition states are stabilised by such solvents.) Dipolar aprotic solvents, especially DMSO and DMF, dissolve mercuric halides very readily, and also co-ordinate strongly to the PhHg+ cation. Reutov and co-workers57 have determined the equilibrium constants for the reaction... 

In the particular case of sodamide containing complex bases where the base to be activated is insoluble, alcoholates can complex sodamide with help of two ionic sites (ROs and Nas+). This must be more favorable for dissolution of NaNH2 than simple solvation of the cation by dipolar aprotic solvent (solvation of anions being low). However, in solution because of the double complexation the basic power of complex bases must be lower than the basic power of NaNH2 in a dipolar aprotic solvent if this latter was able to dissolve it substantially. 

The formation of solvent shells around molecules is essential to prevent self-association of the solute species and to allow solution to take place. Solvents other than water which have high dielectric constants, and including some of the dipolar aprotic solvents (defined below), dissolve ionic species by separating and solvating the ions. 

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