Apolar and aprotic solvent

The intrinsic (measured in cyclohexane, apolar and aprotic solvent) and effective (measured in water or other liquids) acidity of series of mixed silica-niobia oxides have been determined by this method with the principal aim of disclosing relationships between the surface acid properties and catalytic activity [9]. Dispersed niobia... 

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). 

The sequence of the selectivities towards cations is also solvent dependent for dibenzo-18-crown-6 [11] the sequence is K+ > Na+ > Rb+ > Cs+ in water, methanol, dimethylformamide and dimethyl sulfoxide (Dechter and Zink, 1976 Srivanavit et al., 1977), whereas it is Na+ > K+ > Rb+ > Cs+ in acetonitrile (Hofmanova et al., 1978). A reversal of the K+/Na+ selectivity on going to apolar aprotic solvents was also observed for fluorenyl salts (Wong et al., 1970). Whereas for alkali cations the sequence of binding constants and enthalpies are the same in water (Izatt et al., 1976a), they differ considerably in methanol/water mixtures (Izatt et al., 1976b), dimethyl sulfoxide and acetone (Arnett and Moriarity, 1971). 

Classification of Solvents in Terms of Specific Solute-Solvent Interactions Parker divided solvents into two groups according to their specific interactions with anions and cations, namely dipolar aprotic solvents and protic solvents (Parker, 1969). The distinction lies principally in the dipolarity of the solvent molecules and their ability to form hydrogen bonds. It appears appropriate to add to these two groups a third one, namely, the apolar aprotic solvents. 

The model of the polymer derived from magnetic resonance and fluorescence spectroscopy studies has proved very useful in suggesting other types or reactions besides hydrolytic ones in which catalytic effects might be achieved. For example, it has been shown by Kemp and Paul46-47 that the decarboxylation of certain benzisoxazole carboxylic acids is very markedly accelerated in apolar, aprotic solvents, in contrast to water. Such an apolar solvent apparently lowers the energy of the charge-delocalized transition state in this decarboxylation reaction. Since... 

An apolar aprotic solvent is characterized by a low relative permittivity (sr < 15), a low dipole moment [ju < 8.3 10 Cm = 2.5 D), a low value ca. 0.0... 0.3) cf. Table A-1, Appendix), and the inability to act as a hydrogen-bond donor. Such solvents interact only slightly with the solute since only the non-specific directional, induction, and dispersion forces can operate. To this group belong aliphatic and aromatic hydrocarbons, their halogen derivatives, tertiary amines, and carbon disulfide. 

The equilibrium constants, measured by NMR spectroscopy, of ethyl acetoacetate and acetylacetone [47, 48, 134] (Table 4-2) indicate a higher enol content for these czx-enolizing 1,3-dicarbonyl compounds in apolar aprotic than in dipolar protic or dipolar aprotic solvents. 

Solvents may be classified according to their polarity into three groups apolar aprotic solvents, dipolar aprotic solvents, and (polar) protic solvents. Examples of these three classifications for some common laboratory solvents are listed in Table 16.1, in order of increasing polarity (indicated by dielectric constant), together with some other solvent properties. For information on the hazards and toxicity of solvents, see Chapter 11. 

Although the hydrolysis of alkyl halides to alcohols has been extensively investigated, an alternative two-step sequence involving substitution with carboxylate ion is more practical for the preparation of alcohols. Activation of the carboxylate anion prepared by the reaction of the acid with a base can be achieved (i) by use of a polar aprotic solvent and (ii) by use of aprotic apolar solvents under phase transfer catalysis, polymer conditions, or with crown ethers. 

Self-association of protic solvents, including anilines and aliphatic amines in apolar aprotic solvents, is an important process, because it involves a high percentage of stoichiometric amine as solute, depending on the solute structure, the solvent, the temperature and, obviously, the amine concentration. 

This graph gives a selection of 14 (out of approx. 360) usual solvents above the basis line and 7 exotic solvents (ionic liquids included) below. The 14 compounds include (from left to the right increasing solvent polarity) apolar, aprotic solvents (such as TMS, cyclohexene, or benzene), bipolar solvents (such as acetone, DMF, or DMSO), and eventually bipolar, protic solvents (cyclohexanol, ethanol, phenol, 2,2,2-trifluoroethanol). Using the Ej values numerous solvent-dependent processes may be correlated by far better than with the physical values of the solvents alone. This is true because the Ej values include specific cross interactions as well. 

The hetero-Michael addition of 2,4-dihydropyrazol-3-one 422a-h to trichloro-acetaldimine derivatives 423a-h takes place best in apolar aprotic solvents and the... 

The nature of the aggregates formed from amphiphiles in dipolar aprotic solvents is not well established, probably because these systems have been less thoroughly studied than the normal micelles in water and the reverse micelles in apolar solvents. Bile salts (Scheme 2) have one face apolar, (hydrophobic), and the other face is hydrophilic because of the hydroxyl groups, therefore substrates could associate with the hydrophihc face by hydrogen bonding which could influence reactivity. 

Point 2 above is understandable (Eq. I) since in apolar solvents the energy for charge separation is large and makes electron transfer slow. In the case of aromatic nitriles as acceptors, with which we are particularly concerned, there is obviously also a strong n-interaction that can occur between donor and acceptor, and irradiation in aprotic solvents usually causes no reaction but only quenching of the nitrile monomolecular fluorescence and appearance of a new red-shifted emission attributable to the exciplex (see Section 2.2). Independent solvation of the radical ions is also important. For example, the bichro-mophoric molecules 5 and 6 show strong exciplex emission and... 

Copper-mediated alkynylations of alkenyl- or aryl-substituted compounds should be considered as variations of the original Castro-Stephens reaction under catalytic reaction conditions. Copper(I) salts stabilized by aryl-substituted phosphines can mediate the coupling of aryl iodides in polar aprotic solvents using either conventional heating [87] or microwave irradiation [88]. More recently, weU-defined 1,10-phenanthroline-derived copper complexes were shown to be effective catalysts for the coupling of activated and deactivated aryl iodides at 10mol% catalyst loading in an apolar solvent (Scheme 6.29) [89]. 

DMC can be used in a large excess (10-30 molar excess), thus acting both as the methylating agent and the solvent, actually, it has also proved to be the better solvent for these reactions. No improvements in the reaction rate are observed using apolar (cyclohexane), protic polar (methanol) or aprotic polar (DMF) solvent in particular, the reaction rate is dramatically lowered in cyclohexane solvent (scarce solubility of K2CO3), while selectivity is decreased in DJ solvent (Table HI). 

---