Hydrophobic molecules in water

For carbon-carbon bond-formation purposes, S 2 nucleophilic substitutions are frequently used. Simple S 2 nucleophilic substitution reactions are generally slower in aqueous conditions than in aprotic organic solvents. This has been attributed to the solvation of nucleophiles in water. As previously mentioned in Section 5.2, Breslow and co-workers have found that cosolvents such as ethanol increase the solubility of hydrophobic molecules in water and provide interesting results for nucleophilic substitutions (Scheme 6.1). In alkylations of phenoxide ions by benzylic chlorides, S/y2 substitutions can occur both at the phenoxide oxygen and at the ortho and para positions of the ring. In fact, carbon alkylation occurs in water but not in nonpolar organic solvents and it is observed only when the phenoxide has at least one methyl substituent ortho, meta, or para). The effects of phenol substituents and of cosolvents on the rates of the competing alkylation processes... 

Breslow and co-workers have found that cosolvents such as ethanol increase the solubility of hydrophobic molecules in water and provide interesting results for nucleophilic substitutions of phenoxide ions by benzylic chlorides carbon alkylation occurs in water but not in nonpolar organic solvents, and it is observed only when the phenoxide has at least one methyl substituent (ortho, meta, or para). This has been discussed in Chapter 6 (Section 6.4.2). 

Thayumanavan s group reported dendrimers containing polar and apolar groups at every repeat unit that inverts from a normal to inverted micellar-type conformation going from aqueous to organic media (Fig. 11.9 Basu et al. 2004 Aathimanikandan et al. 2005 Klaikherd et al. 2006). This solvent-sensitive conformational behavior affords a unique capability to solubilize hydrophobic molecules in water and hydrophilhc molecules in lipophilic media. 

Another feature of the hydrophobic interactions is the self-assembling of hydrophobic molecules in water. At infinite dilution, the value of limj , o3 A can serve as a measure of self-aggregation of molecule i at infinite... 

This is a hydrophobic solute. A large decrease in entropy (increase in order) of the water occurs when a hydrophobic solute is added to pure water. Since entropy tends to increase, hydrophobic molecules in water will tend to aggregate so as to minimize the surface area exposed to the water molecules. 

If we envisage placing hydrophobic molecules in water the transfer of such molecules from a pure state to an aqueous solution is an unfavorable process due to the large decrease of entropy which results from the reorganization of the water molecules surrounding the solute. The region separating an immiscible liquid from water is perceived as an ill-defined border where hydrophilic species are forced to meet hydrophobic partners. The relative insolubility of reactants conditions the feasibility of the aqueous reaction in which they are involved. If one substrate is completely insoluble the reaction does not occur. In contrast, if all the substrates are totally soluble there is no extra kinetic effect except the normal effect due to the... 

Micelles self-assembled from amphiphilic block copolymers have attracted interest because of their ability to solubilize hydrophobic molecules in water. " " Interest is particularly keen in the context of drug solubilization. 

Hydrophobic Molecules in Water. When dissolved in water, organic molecules and macromolecules tend to aggregate because of hydrophobic forces, which are not tme attractive interactions bnt arise from the fact that hydrophobic entities become surrounded in the aqneous environment by ordered cages of water molecules, which have an energetic cost and... 

To understand the hydrophobic interaction more completely we need to know more about the energetics of the interaction of hydrophobic groups and water. Experiments indicate that the dissolution of a largely hydrophobic molecule in water is commonly endothermic > 0) but that the entropy change is... 

The major driving force in protein folding is the hydrophobic effect. This is the tendency for hydrophobic molecules to isolate themselves from contact with water. As a consequence during protein folding the hydrophobic side chains become buried in the interior of the protein. The exact physical explanation of the behavior of hydrophobic molecules in water is complex and can best be described... 

Lannibois H, Hasmy A, Botet R, Chariol OA, Cabane B (1997) Surfactant limited aggregation of hydrophobic molecules in water. J Phys 11 7(2) 319-342... 

The hydrophobic force is related to the hydrophobic effect [1174, 1175]. Nonpolar molecules such as hydro- and fluorocarbons and nonpolar gases poorly dissolve in water (Table 10.2). If, for example, liquid octane is in contact with water and the system is allowed to equilibrate, only 5.4 [xM octane dissolves in water. Rather than being dissolved as individual molecules hydro- and fluorocarbons attract each other and tend to form intermolecular aggregates in an aqueous medium. This aggregation effect is called hydrophobic effect. The apparent attraction between hydrophobic molecules in water is sometimes referred to as hydrophobic interaction. At the macroscopic level, the hydrophobic effect is apparent when oil and water are mixed together and form separate phases. On solid, hydrophobic surfaces, water forms a high contact angle (Table 10.1). At the molecular level, the hydrophobic effect is... 

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