Apolar solvent

Dramatic rate accelerations of [4 + 2]cycloadditions were observed in an inert, extremely polar solvent, namely in5 M solutions oflithium perchlorate in diethyl ether(s 532 g LiC104 per litre ). Diels-Alder additions requiring several days, 10—20 kbar of pressure, and/ or elevated temperatures in apolar solvents are achieved in high yields in some hours at ambient pressure and temperature in this solvent (P.A. Grieco, 1990). Also several other reactions, e.g, allylic rearrangements and Michael additions, can be drastically accelerated by this magic solvent. The diastereoselectivities of the reactions in apolar solvents and in LiClO EtjO are often different or even complementary and become thus steerable. 

Liquid membranes can be constituted by liquid chiral selectors used directly [170] or by solutions of the chiral molecules in polar or apolar solvents. This later possibility can also be an advantage since it allows the modulation of the separation con-... 

The loss of sulfur from substituted 4-phenyl-1-benzothiepins 7 can be achieved by heating in cyclohexane or carbon tetrachloride.90 In a similar way, but under mild conditions,14 the elimination of sulfur monoxide occurs from the corresponding 1-benzothiepin 1-oxides, reflecting the lower thermal stability of the sulfoxides in apolar solvents.85... 

The conductometric results of Meerwein et al. (1957 b) mentioned above demonstrate that, in contrast to other products of the coupling of nucleophiles to arenediazonium ions, the diazosulfones are characterized by a relatively weak and polarized covalent bond between the p-nitrogen and the nucleophilic atom of the nucleophile. This also becomes evident in the ambidentate solvent effects found in the thermal decomposition of methyl benzenediazosulfone by Kice and Gabrielson (1970). In apolar solvents such as benzene or diphenylmethane, they were able to isolate decomposition products arising via a mechanism involving homolytic dissociation of the N — S bond. In a polar, aprotic solvent (acetonitrile), however, the primary product was acetanilide. The latter is thought to arise via an initial hetero-lytic dissociation and reaction of the diazonium ion with the solvent (Scheme 6-11). 

Diffusion-controlled one-electron transfers can be observed in sulfolane (Elofson and Gadallah, 1967), in nitromethane (Bottcher et al., 1973), and in some other apolar solvents (Janderka and Cejpek, 1989), but unfortunately not in aqueous acid, where four electrons are involved (Elofson, 1958). For that process Orange et al. (1981) proposed a complex mechanism ending in arenehydra-zinium ions. 

Two-shot techniques for acyclic diene metathesis, 435-445 for polyamides, 149-164 for polyimides, 287-300 for polyurethanes, 241-246 for transition metal coupling, 483-490 Anionic deactivation, 360 Anionic polymerization, 149, 174 of lactam, 177-178 Apolar solvents, 90 Aprotic polar solvents, 185, 338 Aprotic solvents, low-temperature condensation in, 302 Aqueous coating formulations, 235 Aqueous polyoxymethylene glycol, depolymerization of, 377 Aqueous systems, 206 Ardel, 20, 22... 

The diastereoselectivity of the cycloaddition of cyclopentadiene with methyl acrylate in SC-CO2 at 40 °C and subcritical liquid CO2 at 22 °C is practically the same endojexo = 75 25 and 76 24 respectively) and is comparable to that found in hydrocarbon solvents (73 27 and 75 25 in heptane and cyclohexane, respectively). This shows that CO2, in these states, behaves like an apolar solvent with very low polarizability [82]. 

The effect of pressure on the rate constant of the Diels-Alder reaction between maleic anhydride and isoprene was investigated in SC-CO2 at 35 °C and at pressures ranging from 90 to 193 bar. For comparison purposes, the reaction was also carried out in an apolar solvent such as propane under... 

First of all, the reaction pathways shown in Scheme 1 involve the formation of charge transfer complexes (CTC) between olefin and Br2- The formation of molecular complexes during olefin bromination had been hypothesized often (ref. 2), but until 1985, when we published a work on this subject (ref. 3), complexes of this type had been observed only in a very limited number of circumstances, all of which have in common a highly reduced reactivity of the olefm-halogen system, i.e. strongly deactivated olefins (ref. 4), or completely apolar solvents (ref. 5) or very low temperatures (ref 6). 

A) than the apolar solvents C-E). All three eorrelation graphs demonstrate that careful drying drives off solvents and excess water, and in the process improves purity. 

Johansson, J., Szyperski, T, Curstedt, T., and Wuthrich, K. The NMR structure of the pulmonary surfactant-associated polypeptide SP-C in an apolar solvent contains a valyl-rich alpha-helix. Biochemistry 1994, 33, 6015-6023. 

Interestingly, 8-aminoxy acids which are homologs of y-amino acids have also been found to promote the formation of turns and helices. In apolar solvent and in the solid state, model diamides consisting of /9 -aminoxy add residues adopt a novel N-O turn stabilized by both a nine-membered H-bonded ring between C=0 and NHj+2, and a six-membered ring formed between N-O and NH +1. The X-ray crystal structure of a corresponding triamide revealed two consecutive C9 N-O turns suggesting a novel 1.79-helical fold [279]. 

On the other hand, polar molecules create a force field around them that is attractive or repulsive, depending on the relative orientation of the neighboring polar molecule. In this case, the spectrum of molecular arrangements actually explored by an ensemble of strongly polar molecules is severely restricted. It follows that these molecules display a more marked tendency to give a dimensionally unlimited ordered molecular arrangement and a limited mutual solubility with apolar solvents. 

In the case of amphiphilic molecules, characterized by the coexistence of spatially separated apolar (alkyl chains) and polar moieties, both parts cooperate to drive the intermolecular aggregation. This simple but pivotal peculiarity makes amphiphilic molecules soluble in both polar and apolar solvents and able to realize, in suitable conditions, an impressive variety of molecular aggregates characterized by spatially separated apolar and polar domains, local order at short times and fluidity at long times, and differences in size, shape (linear or branched chains, cyclic or globular aggregates, extended fractal-like molecular networks), and lifetime. 

An enormous literature has been produced in recent decades in the field of molecular aggregation of amphiphilic molecules in liquid systems, emphasizing the extremely wide variety of accessible structures and dynamics. Among these molecular aggregates, in this chapter our attention will be restricted to those formed by some amphiphilic molecules (surfactants) in apolar solvents called reversed micelles [1]. 

The frequent breaking and reforming of the labile intermolecular interactions stabilizing the reversed micelles maintain in thermodynamic equilibrium a more or less wide spectrum of aggregates differing in size and/or shape whose relative populations are controlled by some internal (nature and shape of the polar group and of the apolar molecular moiety of the amphiphile, nature of the apolar solvent) and external parameters (concentration of the amphiphile, temperature, pressure) [11], The tendency of the surfactants to form reversed micelles is, obviously, more pronounced in less polar solvents. 

Highly monodisperse reversed micelles are formed by sodium bis(2-ethylhexyl) sul-fosuccinate (AOT) dissolved in hydrocarbons that are in equilibrium with monomers whose concentration (cmc) is 4 X 10 M, have a mean aggregation number of about 23, a radius of 15 A, exchange monomers with the bulk in a time scale of 10 s, and dissolve completely in a time scale of 10 s [1,2,4,14], Other very interesting surfactants able to form reversed micelles in a variety of apolar solvents have been derived from this salt by simple replacing the sodium counterion with many other cations [15,16],... 

The main peculiarity of solutions of reversed micelles is their ability to solubilize a wide class of ionic, polar, apolar, and amphiphilic substances. This is because in these systems a multiplicity of domains coexist apolar bulk solvent, the oriented alkyl chains of the surfactant, and the hydrophilic head group region of the reversed micelles. Ionic and polar substances are hosted in the micellar core, apolar substances are solubilized in the bulk apolar solvent, whereas amphiphilic substances are partitioned between the bulk apolar solvent and the domain comprising the alkyl chains and the surfactant polar heads, i.e., the so-called palisade layer [24],... 

Obviously, water, aqueous solutions of salts, and mixtures of highly hydrophilic solvents have also been found to be solubilized in the micellar core [13,44]. The maximum amount of such solubilizates that can be dissolved in reversed micelles varies widely, strongly depending on the nature of the surfactant and the apolar solvent, on the concentrations of surfactant and of additives, and on temperature [24,45-47]. 

The effects of the intramicellar confinement of polar and amphiphilic species in nanoscopic domains dispersed in an apolar solvent on their physicochemical properties (electronic structure, density, dielectric constant, phase diagram, reactivity, etc.) have received considerable attention [51,52]. hi particular, the properties of water confined in reversed micelles have been widely investigated, since it simulates water hydrating enzymes or encapsulated in biological environments [13,23,53-59]. 

Independent of the nature of the apolar solvent, nearly spherical and monodisperse water-containing reversed micelles are formed by AOT, whose size is quite independent of the surfactant concentration and regulated mainly by the molar ratio R(R = [water]/[sur-factant]) [5,84,85]. 

Nanoparticles of water-soluble compounds can be also obtained by simply solubilizing the solid compound in dry surfactant/apolar solvent solutions. Typical electronic micrographs of nanoparticles of Co(N03)2 and urea obtained using this methodology are shown in Figures 7 and 8 [41],... 

It has been proposed that the overlapping of the surfactant hydrocarbon tails is mainly responsible for the micelle-micelle interactions [247]. However, since tail-tail interactions are of the same order of magnitude as tail-apolar solvent interactions, it seems more reasonable to consider the overlapping of the surfactant hydrocarbon tails as an effect rather than the origin of the micelle-micelle interactions. 

To ensure that proton transfer takes place from the protonated catalyst 64-H and not from the acidic reagent itself, apolar solvents favoring contact rather than solvent separated ion pairs as well as a slow addition of the acidic substrate RX-H are required. In addition, it was sometimes found beneficial to lower the basicity of the catalyst, thus rendering the protonated species [catalyst-H" ] more acidic for the stereo-determining protonation of the enolate. This was accomplished by formally replacing NR2 by Me (see 64e, Fig. 36). 

At the present time, "interest in reversed micelles is intense for several reasons. The rates of several types of reactions in apolar solvents are strongly enhanced by certain amphiphiles, and this "micellar catalysis" has been regarded as a model for enzyme activity (. Aside from such "biomimetic" features, rate enhancement by these surfactants may be important for applications in synthetic chemistry. Lastly, the aqueous "pools" solubilized within reversed micelles may be spectrally probed to provide structural information on the otherwise elusive state of water in small clusters. 

This technique is used to extract effectively analytes that are polar in nature and strongly bound to soil. Typically, a solvent mixture containing a water-miscible solvent and an apolar solvent (e.g. methanol-dichloromethane) is used. A small aliquot of soil (10-30 g) is dried by mixing with sodium sulfate and refluxed for 8-16h to extract the residues. 

Enzymes are suspended in hydrated microemulsion surrounded by a monolayer of surfactant molecules dispersed in an apolar solvent [53-60,135] [Fig. 1(b)]. Micelles ( 2 nm sphere) are formed when lyophilized or aqueous preparation of enzymes are introduced with stirring or shaking into a solution of synthetic or natural surfactant in an organic solvent. 

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