Solvophobic bonds

In summary, solvents can influence Diels-Alder reactions through a multitude of different interactions, of which the contributions to fire overall rate uniquely depend on the particular solvent-diene-dienophile combination. Scientists usually feel uncomfortable about such a situation and try to extract generalities. When limited to the most extensively studied type A Diels-Alder reactions this approach seems feasible. These Diels-Alder reactions are dominated by hydrogen bonding interactions in combination with solvophobic interactions. This observation predicts a very special role of water as a solvent for type A Diels-Alder reactions, which is described in Section 1.4. 

Studies on solvent effects on the endo-exo selectivity of Diels-Alder reactions have revealed the importance of hydrogen bonding interactions besides the already mentioned solvophobic interactions and polarity effects. Further evidence of the significance of the former interactions comes from computer simulations" and the analogy with Lewis-acid catalysis which is known to enhance dramatically the endo-exo selectivity (Section 1.2.4). 

The mechanism of reversed-phase chromatography arises from the tendency of water molecules in the aqueous-organic mobile phase to self-associate by hydrogen bonding. This ordering is perturbed by the presence of nonpolar solute molecules. As a result, solute molecules tend to be excluded from the mobile phase and are bound by the hydrophobic stationary phase. This solvophobic... 

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. 

The solvophobic theory could be extended to the treatment of other special effects such as hydrogen bonding between eluite and species. present in the eluent. The predictive power of the theory may be improved by such extension. In addition a rigorous theory for treatment eluite interaction with surface silanols would be needed. 

In spite of the preceding observation that eluite retention in RPC with hydrocarbonaceous bonded phases may not occur by partitio ng of the eluite between two liquid phases, theoretical considerations based on the solvophobic treatment of solvent effects shows that it might be possible to relate the observed retention factors to partition coefficients between water and an organic solverit. Such a relationship would be quite useful in light of the scale developed by Hansch and his co-workers (2/12, 283) to characterize hydrophobic properties of drugs and other biologically active... 

The theoretical treatment of the hydrophobic effect is limited to pure aqueous systems. To describe chromatographic separations in RPC Horvath and Melander developed the solvophobic theory [47]. In this theory, no special assumptions are made about the properties of solute and solvent, and besides hydrophobic interaction electrostatic and other specific interactions are included. The theory has been valuable to describe the retention of nonpolar [48], polar [49], and ionizable [50] solutes in RPC. The modulation of selectivity via secondary equilibria (variation of pH, ion pair formation [51]) can also be described. On the other hand, it is not a problem to find examples of dispersive interactions in literature, e.g., separation of carotinoids with a long chain (C30) RP gives a higher selectivity compared to standard RP C18 cyclohexanols are preferentially retarded on cyclohexyl-bonded phases compared to phases with linear-bonded alkyl groups. 

LC Tan, PW Carr. Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography. II. Partitioning vs. adsorption mechanism in monomeric alkyl bonded phase supports. J Chromat A 775 1-12, 1997. 

Another subtle case, where specific interactions may obscure the effects of Coulombic criticality, is ethylammonium nitrate (EtNH3N03) +l-octanol (Tcs315K) [85], In contrast to all other known examples, the critical point is located in the salt-rich regime at a critical mole fraction of Xc = 0.77. Electrical conductance data indicate strong ion pairing, presumably caused by a hydrogen bond between the cation and anion which stabilizes the pairs in excess to what is expected from the Coulombic interactions [85]. This warns that, beyond the Coulombic/solvophobic dichotomy widely discussed in the literature, additional mechanisms may affect the phase separation [5]. 

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