Water hydrophobic effect

Occasionally, however, the ions are deviants and associate preferentially with the nonelectrolyte solute, shunning the water (hydrophobic effects). In the rare instances where these deviants appear, there is a rapid departure of the nonelectrolyte from the parent lattice and the solubility of the former is enhanced rather than deaeased. The phenomenon is called salting in. 

Water-soluble globular proteins usually have an interior composed almost entirely of non polar, hydrophobic amino acids such as phenylalanine, tryptophan, valine and leucine witl polar and charged amino acids such as lysine and arginine located on the surface of thi molecule. This packing of hydrophobic residues is a consequence of the hydrophobic effeci which is the most important factor that contributes to protein stability. The molecula basis for the hydrophobic effect continues to be the subject of some debate but is general considered to be entropic in origin. Moreover, it is the entropy change of the solvent that i... 

The hydrophobic effect. Water molecules around a non-polar solute form a cage-like structure, which ices the entropy. When two non-polar groups associate, water molecules are liberated, increasing the entropy. 

Jorgensen W L, J Gao and C Ravimohan 1985. Monte Carlo Simulations of Alkanes in Water Hydratior Numbers and the Hydrophobic Effect. Journal of Physical Chemistry 89 3470-3473. 

Hydrophobic effects include two distinct processes hydrophobic hydration and hydrophobic interaction. Hydrophobic hydration denotes the way in which nonpolar solutes affect the organisation of the water molecules in their immediate vicinity. The hydrophobic interaction describes the tendency of nonpolar molecules or parts thereof to stick together in aqueous media " . A related frequently encountered term is hydrophobicity . This term is essentially not correct since overall attractive interactions exist between water and compounds commonly referred to as... 

The ideas of Frank, Evans and Kauzmann had a profound influence on the way chemists thought about hydrophobic effects in the decades that followed However, after the study of the hydrophobic hydration shell through computer simulations became feasible, the ideas about the hydrophobic hydration gradually changed. It became apparent that the hydrogen bonds in the hydrophobic hydration shell are nof or only to a minor extent, stronger than in normal water which is not compatible with an iceberg character of the hydration shell. 

Breslow immediately grasped the significance of his observation. He interpreted this discovery in terms of a hydrophobic effect Since in the Diels-Alder reaction. .. the transition state. .. brings together two nonpolar groups, one might expect that in water this reaction could be accelerated by hydrophobic interactions ". ... 

Three years after the Breslow report on the large effects of water on the rate of the Diels-Alder reaction, he also demonstrated tliat the endo-exo selectivity of this reaction benefits markedly from employing aqueous media . Based on the influence of salting-in and saltirg-out agents, Breslow pinpoints hydrophobic effects as the most important contributor to the enhanced endo-exo... 

Apart from the thoroughly studied aqueous Diels-Alder reaction, a limited number of other transformations have been reported to benefit considerably from the use of water. These include the aldol condensation , the benzoin condensation , the Baylis-Hillman reaction (tertiary-amine catalysed coupling of aldehydes with acrylic acid derivatives) and pericyclic reactions like the 1,3-dipolar cycloaddition and the Qaisen rearrangement (see below). These reactions have one thing in common a negative volume of activation. This observation has tempted many authors to propose hydrophobic effects as primary cause of ftie observed rate enhancements. 

Hydrophobic effects. Implications for organic reactivity in water... 

Throughout this thesis reference has been made to hydrophobic effects. Enforced hydrophobic interactions are an important contributor to the acceleration of uncatalysed and also of the Lewis-acid catalysed Diels-Alder reactions which are described in this thesis. Moreover, they are likely to be involved in the beneficial effect of water on the enantioselectivity of the Lewis-acid catalysed Diels-Alder reaction, as described in Chapter 3. Because arguments related to hydrophobic effects are spread over nearly all chapters, and ideas have developed simultaneously, we summarise our insights at the end of this thesis. 

In the case of the retro Diels-Alder reaction, the nature of the activated complex plays a key role. In the activation process of this transformation, the reaction centre undergoes changes, mainly in the electron distributions, that cause a lowering of the chemical potential of the surrounding water molecules. Most likely, the latter is a consequence of an increased interaction between the reaction centre and the water molecules. Since the enforced hydrophobic effect is entropic in origin, this implies that the orientational constraints of the water molecules in the hydrophobic hydration shell are relieved in the activation process. Hence, it almost seems as if in the activated complex, the hydrocarbon part of the reaction centre is involved in hydrogen bonding interactions. Note that the... 

Also the arene-arene interactions, as encountered in Chapter 3, are partly due to hydrophobic effects, which can be ranked among enforced hydrophobic interactions. Simultaneous coordination of an aromatic oc amino acid ligand and the dienophile to the central copper(II) ion offers the possibility of a reduction of the number of water molecules involved in hydrophobic hydration, leading to a strengthening of the arene-arene interaction. Hence, hydrophobic effects can have a beneficial influence on the enantioselectivity of organic reactions. This effect is anticipated to extend well beyond the Diels-Alder reaction. 

Higher alcohols become more hydrocarbon like and less water soluble 1 Octanol for example dissolves to the extent of only 1 mL m 2000 mL of water As the alkyl chain gets longer the hydrophobic effect (Section 2 17) becomes more important to the point that It more than hydrogen bonding governs the solubility of alcohols... 

Modem understanding of the hydrophobic effect attributes it primarily to a decrease in the number of hydrogen bonds that can be achieved by the water molecules when they are near a nonpolar surface. This view is confirmed by computer simulations of nonpolar solutes in water [15]. To a first approximation, the magnimde of the free energy associated with the nonpolar contribution can thus be considered to be proportional to the number of solvent molecules in the first solvation shell. This idea leads to a convenient and attractive approximation that is used extensively in biophysical applications [9,16-18]. It consists in assuming that the nonpolar free energy contribution is directly related to the SASA [9],... 

Hydrophobicity ( water-hate ) can dominate the behavior of nonpolar solutes in water. The key observations are (1) that very nonpolar solutes (such as saturated hydrocarbons) are nearly insoluble in water and (2) that nonpolar solutes in water tend to form molecular aggregates. Some authors refer to item 1 as the hydrophobic effect and to item 2 as the hydrophobic interaction. Two extreme points of view have been taken to account for these observations. 

The hemiacetal 54 adsorbed on water-saturated silica gel gives, by MW irradiation, a 1 1 mixture of cycloadducts isolated as silyl derivatives 55 and 56. The water is probably necessary for the success of the reaction because (i) it is an efficient generator of heat in the MW process, (ii) it accelerates the cycloaddition by hydrophobic effect, and (iii) it facilitates the hemiacetal-hydroxyketone equilibrium which furnishes the dienophile moiety for the cycloaddition [42]. 

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