Free nonpolar compounds

We would like to reiterate our initial suggestion (153) that studies of the structures of these compounds with highly hydrophobic ligands, in nonaqueous media, may reveal interesting aggregation characteristics. Such aggregations may persist in the solid state, particularly if these compounds are recrystallized from water-free, nonpolar solvents. Very little is known about the coordination properties of the alkaline earth—sulfur ligand compounds. 

Extraction of nonpolar compounds using equal volumes of sample and the Folsch mixture (2 1, chloroform/MeOH) gives a very broad polarity cut. Everything from steroids to triglycerides is pulled down into the bottom chloroform-rich layer. Extraction with methylene chloride from a sample acidified with sulfuric acid is more specific, pulling in steroids, fat-soluble vitamins, and free fatty acids. The triglyceride fraction can be extracted using i-PrOH/ hexane (1 9) with little emulsification. 

The surface of newly made silica is hydrophilic. There are many applications where less hydrophilicity is required such as in adsorption separation of various kinds of polar, and nonpolar compounds, and in mixing silica with hydrophobic materials. In these cases, it is rather easy to modify the free silanol groups by substitution to other functional groups by heating at the ambient or high pressure. 

Masking the chelating carboxylic acid groups with acetoxymethyl (AM) esters, BAPTA and its fluorescent derivatives can be converted to nonpolar compounds that passively cross the plasma membrane. Once in the cell, non-specific esterases cleave the AM ester back to the free indicator and the resulting tetra or penta anion accumulates in the cytosol. Cells incubated in a very dilute dye solution (1-S /aM) still reach a useful concentration of indicator (>2S fiM). This is an extremely valuable technique, especially vfhen used with Ca indicators that exhibit a shift in excitation or emission on ion binding. By these means, researchers can estimate intracellular Ca changes in single cells or populations of cells (8). 

FIGURE 2.66 (a) Temperature dependence of unfrozen water content and (b) relationships between and changes in the Gibbs free energy of bound water for hydrated powders (C = 10 wt%) and aqueous suspensions (C =90 wt%) of different oxides. (Adapted from J. Colloid Interface Sci., 348, Gun ko, V.M., G.R. Yurchenko, Turov, V.V. et ak. Adsorption of polar and nonpolar compounds onto complex nanooxides with silica, alumina, and titania, 546-558, 2010e, Copyright 2010, with permission from Elsevier.)... 

SPME (Solid-phase microextraction) has recently been developed as a rapid, inexpensive, and solvent-free technique. This technique uses a fine fused silica fiber with a polymeric coating to extract organic compounds from their matrix. The main advantages of SPME are simplicity, high sensitivity, small sample volume, and lower cost per analysis. SPME techniques can be successfully applied for polar and nonpolar compounds in gas, liquid, and solid samples and can be easily coupled with various analytical instruments such as GC, GC-MS, HPLC, and LC-MS [22, 56]. 

A considerable amount of hydrobromic acid is consumed in the manufacture of inorganic bromides, as well as in the synthesis of alkyl bromides from alcohols. The acid can also be used to hydrobrominate olefins (qv). The addition can take place by an ionic mechanism, usually in a polar solvent, according to Markownikoff s rule to yield a secondary alkyl bromide. Under the influence of a free-radical catalyst, in aprotic, nonpolar solvents, dry hydrogen bromide reacts with an a-olefin to produce a primary alkyl bromide as the predominant product. Primary alkyl bromides are useful in synthesizing other compounds and are 40—60 times as reactive as the corresponding chlorides (6). 

The reduction of the stannyl radical (t-Bu2MeSi)3Sn with alkali metals produces a variety of structural modifications depending on the solvent used (Scheme 2.55). Thus, in nonpolar heptane, a dimeric stannyllithium species [58c Li ]2 (E = Sn) was formed, whereas in more polar benzene, the monomeric pyramidal structure 58c [Ti -Li (C6H5)] was produced. In the latter compound the Li+ ion was covalently bonded to the anionic Sn atom being at the same time n -coordinated to the benzene ring. A similar monomeric pyramidal CIP 58c [Li (thf)2] was prepared by reduction in polar THE the addition of [2.2.2]cryptand to this compound resulted in the isolation of the free stannyl anion 58c K+([2.2.2]cryptand), in which the ion lacked its bonding to the Sn atom. ... 

Factors that influence the retentive powers and selectivity of such bonded phases include the surface concentrations of hydrodartenaceous ligates and free silanol groups. The thermodynamic aspectitm solute interactions with the hydrocarbonaceous ligates at the surface, which are hydrophobic interactions in the case of aqueous eluents, are discussed later in this chapter within the framework of the solvophobic theory. In practice, however, solute interactions with surface silanol which may be termed silanophilic interactions can also contribute ]to retention (71, 75, 93), particularly in the case of amino compounds. Consequently the retention mechanism may be different from that which would be ol served with an ideal nonpolar phase. Therefore, increasing attention is paid to the estimation of the concentration of accessible sianols and to their elimination from the surface of bonded phases. 

---