Solvents, nonpolar

More striking is the use of nonpolar solvents (e.g. xylene, toluene, carbon tetrachloride, hydrocarbons), because these are transparent to, and only weakly absorb microwaves. They can, therefore, be used to enable specific absorption by the reactants. If these reactants are polar, energy transfer from the reactants to the solvent occurs and the results may be different under the action of MW and A. This effect seems to be clearly dependent on the reaction and was therefore the subject of controversy. In xylene under reflux, for example, no MW-specific effect was observed 

These examples will be discussed and explained later during discussion of the dependence of MW effects on reaction mechanism. 

The effect of solvent clearly seems to be very important with regard to the possibility of MW-specific effects. These decrease when the polarity of the solvent is increased. This effect was shown in at least two studies by Berlan et al. [33] and, later, by Bogdal [57]. In the first study, the acceleration under the action of MW was much more apparent in xylene fi = 1 Debye) than in the more polar dibutyl ether (/ = 4 Debye) for the Diels-Alder reaction of 2,3-dimethylbutadiene with methylvinylketone (Fig. 4.7). 

In the second investigation [57], involving synthesis of a coumarin by Knoevena-gel condensation, supported by rate constant measurements and activation energy calculations, it was found that the effect of MW was more important when the reaction was conducted in xylene whereas it was noticeably reduced in ethanol (Eq. 

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The traditional association colloid is of the M R" type where R" is the surfactant ion, studied in aqueous solution. Such salts also form micelles in nonaqueous and nonpolar solvents. These structures, termed inverse micelles, have the polar groups inward if some water is present [198] however, the presence of water may prevent the observation of a well-deflned CMC [198,199]. Very complex structures may be formed in nearly anhydrous media (see Ref. 200). 

AG and AH can be expressed as a multiplicative function of hydrogen bonding in different polar and nonpolar solvents by means of enthalpy acceptor factors E - enthalpy donor factors free energy acceptor factors Q, and free energy donor factors Q (Eqs. (32) and (33), where kj, 2- 3 [kcal/mol] are regression coefficients). 

Allylic amines are coupled to halides giving either allylic amines or enamines depending on the reaction condition. Reaction of steroidal dienyl triflate with Boc-diprotected allylamine affords allylamine. Use of AcOK as a base is crucial for the clean coupling[102]. The tert-allylic amine 123 reacts with an aryl halide to give the enamine 125 in DMF and allylic amine 124 in nonpolar solvents[103]. 

Tautomerism of the A-2-thiazoline-5-thiones has not been investigated intensively. A recent report shows that 2-phenylthiazo e-5-thiols exist in the thiol form in both polar and nonpolar solvents (563). This behavior is in contrast with that of corresponding thiazolones. Addition reactions involve only the exocyclic sulfur atom, and thiazole-5-thiols behave as typical heteroaromatic thiols towards unsaturated systems, giving sulfides (1533) (Scheme 80) (563),... 

A-2-Thiazoline-4-ones are usually obtained by the heterocydization method (38b-388). 2 Alkylthio-4(5)-thiazolones (162) are obtained by alkylation at sulfur of rhodanine (160) in nonpolar solvent (Scheme 85). 

The free-radical reactivity of thiazoles has been well studied with various radicals such as methyl, phenyl, substituted phenyl, cyclohexyl, and aromatic-heterocyclic, in nonpolar solvent or strong acids (180-182). 

Solvent Effects on the Rate of Substitution by the S 2 Mechanism Polar solvents are required m typical bimolecular substitutions because ionic substances such as the sodium and potassium salts cited earlier m Table 8 1 are not sufficiently soluble m nonpolar solvents to give a high enough concentration of the nucleophile to allow the reaction to occur at a rapid rate Other than the requirement that the solvent be polar enough to dis solve ionic compounds however the effect of solvent polarity on the rate of 8 2 reactions IS small What is most important is whether or not the polar solvent is protic or aprotic Water (HOH) alcohols (ROH) and carboxylic acids (RCO2H) are classified as polar protic solvents they all have OH groups that allow them to form hydrogen bonds... 

Ethers form Lewis acid Lewis base complexes with metal ions Certain cyclic polyethers called crown ethers, are particularly effective m coor dinatmg with Na" and K" and salts of these cations can be dissolved m nonpolar solvents when crown ethers are present Under these conditions the rates of many reactions that involve anions are accelerated... 

Phase transfer catalysis succeeds for two reasons First it provides a mechanism for introducing an anion into the medium that contains the reactive substrate More important the anion is introduced m a weakly solvated highly reactive state You ve already seen phase transfer catalysis m another form m Section 16 4 where the metal complexmg properties of crown ethers were described Crown ethers permit metal salts to dissolve m nonpolar solvents by surrounding the cation with a lipophilic cloak leav mg the anion free to react without the encumbrance of strong solvation forces... 

A hydroxyl group is a very powerful activating substituent and electrophilic aro matic substitution m phenols occurs far faster and under milder conditions than m ben zene The hrst entry m Table 24 4 for example shows the monobrommation of phenol m high yield at low temperature and m the absence of any catalyst In this case the reac tion was carried out m the nonpolar solvent 1 2 dichloroethane In polar solvents such as water it is difficult to limit the brommation of phenols to monosubstitution In the fol lowing example all three positions that are ortho or para to the hydroxyl undergo rapid substitution... 

Chemists and biochemists And it convenient to divide the principal organic substances present m cells into four mam groups carbohydrates proteins nucleic acids and lipids Structural differences separate carbo hydrates from proteins and both of these are structurally distinct from nucleic acids Lipids on the other hand are characterized by a physical property their solubility m nonpolar solvents rather than by their structure In this chapter we have examined lipid molecules that share a common biosynthetic origin m that all their carbons are derived from acetic acid (acetate) The form m which acetate occurs m many of these processes is a thioester called acetyl coenzyme A... 

Critical micelle concentration (Section 19 5) Concentration above which substances such as salts of fatty acids aggre gate to form micelles in aqueous solution Crown ether (Section 16 4) A cyclic polyether that via lon-dipole attractive forces forms stable complexes with metal 10ns Such complexes along with their accompany mg anion are soluble in nonpolar solvents C terminus (Section 27 7) The amino acid at the end of a pep tide or protein chain that has its carboxyl group intact—that IS in which the carboxyl group is not part of a peptide bond Cumulated diene (Section 10 5) Diene of the type C=C=C in which a single carbon atom participates in double bonds with two others... 

Primary aliphatic alcohols 3640-3630 (s) Only in very dilute solutions in nonpolar solvents... 

Microreticular Resins. Microreticular resins, by contrast, are elastic gels that, in the dry state, avidly absorb water and other polar solvents in which they are immersed. While taking up solvent, the gel structure expands until the retractile stresses of the distended polymer network balance the osmotic effect. In nonpolar solvents, little or no swelling occurs and diffusion is impaired. 

DMAC and nonpolar solvents form synergistic mixtures which dissolve high molecular weight vinyl chloride homopolymers. For example, a mixture of DMAC with an equal volume of carbon disulfide [75-15-0] a nonsolvent, dissolves 14 wt % of Geon 101 vinyl chloride homopolymer at room temperature, whereas the solubUity of Geon 101 ia DMAC alone is about 5 wt % (15). 

Bromination in polar solvents usually gives /n j -3,4-dibromo-2-methyl-3-buten-2-ol in nonpolar solvents, with incandescent light, the cis isomer is the principal product (194). Chlorine adds readily up to the tetrachloro stage, but yields are low because of side reactions (195). 

Solution Properties. Typically, if a polymer is soluble ia a solvent, it is soluble ia all proportions. As solvent evaporates from the solution, no phase separation or precipitation occurs. The solution viscosity iacreases continually until a coherent film is formed. The film is held together by molecular entanglements and secondary bonding forces. The solubiUty of the acrylate polymers is affected by the nature of the side group. Polymers that contain short side chaias are relatively polar and are soluble ia polar solvents such as ketones, esters, or ether alcohols. As the side chaia iacreases ia length the polymers are less polar and dissolve ia relatively nonpolar solvents, such as aromatic or aUphatic hydrocarbons. 

Bond dissociation energies (BDEs) for the oxygen—oxygen and oxygen— hydrogen bonds are 167—184 kj/mol (40.0—44.0 kcal/mol) and 375 kj/mol (89.6 kcal/mol), respectively (10,45). Heats of formation, entropies, andheat capacities of hydroperoxides have been summarized (9). Hydroperoxides exist as hydrogen-bonded dimers in nonpolar solvents and readily form hydrogen-bonded associations with ethers, alcohols, amines, ketones, sulfoxides, and carboxyhc acids (46). Other physical properties of hydroperoxides have been reported (46). 

Substituted Phenols. Phenol itself is used in the largest volume, but substituted phenols are used for specialty resins (Table 2). Substituted phenols are typically alkylated phenols made from phenol and a corresponding a-olefin with acid catalysts (13). Acidic catalysis is frequendy in the form of an ion-exchange resin (lER) and the reaction proceeds preferentially in the para position. For example, in the production of /-butylphenol using isobutylene, the product is >95% para-substituted. The incorporation of alkyl phenols into the resin reduces reactivity, hardness, cross-link density, and color formation, but increases solubiHty in nonpolar solvents, dexibiHty, and compatibiHty with natural oils. 

Polyethers prepared from propylene oxide are soluble in most organic solvents. The products with the highest hydroxyl number (lowest molecular weight) are soluble in water, not in nonpolar solvents such as hexane. The solubihty of 3000 molecular weight triols is high enough in solvents such as toluene, hexane, and methylene chloride that the triols can be purified by a solvent extraction process. 

Solubility. Cross-linking eliminates polymer solubiUty. Crystallinity sometimes acts like cross-linking because it ties individual chains together, at least well below T. Thus, there are no solvents for linear polyethylene at room temperature, but as it is heated toward its (135°C), it dissolves in a variety of aUphatic, aromatic, and chlorinated hydrocarbons. A rough guide to solubiUty is that like dissolves like, ie, polar solvents tend to dissolve polar polymers and nonpolar solvent dissolve nonpolar polymers. 

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