Solvation of ethanol

A. Pohorille and M. A. Wilson, Adsorption and solvation of ethanol at the water liquid-vapor interface A molecular dynamics study, J. Phys. Chem. B 101 (1997)3130. 

Figure 6.15 shows two examples of PS in the systems of water-methanol and water-ethanol. In the first case, both the water and the ethanol are preferentially solvated by water. In the second case, water is preferentially solvated by water throughout the entire range of compositions. On the other hand, the preferential solvation of ethanol switches from water to ethanol as the concentration of water increases. The various curves correspond to different choices of correlation radius. The curve numbered 1 corresponds roughly to the first coordination spheres, etc. Usually, after 3-5 coordination spheres, the deviation of the PS curve from the diagonal line is negligible. This roughly corresponds to the limit of the correlation distance, which is of the order of a few molecular diameters. 

The well defined contact geometry and the ionic structure of the mica surface favours observation of structural and solvation forces. Besides a monotonic entropic repulsion one may observe superimposed periodic force modulations. It is commonly believed that these modulations are due to a metastable layering at surface separations below some 3-10 molecular diameters. These diflftise layers are very difficult to observe with other teclmiques [92]. The periodicity of these oscillatory forces is regularly found to correspond to the characteristic molecular diameter. Figure Bl.20.7 shows a typical measurement of solvation forces in the case of ethanol between mica. 

Figure Bl.20.7. The solvation force of ethanol between mica surface. The inset shows the fiill scale of the experimental data. With pennission from [75].

To the crude product there was added 100 ml of ethanol with warming until a clear solution was obtained. Then 150 ml ethyl acetate was added and the resultant filtered through a glass mat and the filtrate adjusted to pH 1 by the addition of saturated ethanolic HCI. Crystallization soon occurred. The resultant was allowed to stand at 0°C for 18 hours and then filtered through a sintered glass mat. The solid was dried under vacuum at 60°C for 18 hours yielding 35 g, a 67% yield of 7(S)-chloro-7-deoxylincomycin hydrochloride as an ethanol solvate. 

To a slightly warm suspension of 3-acetoxy-7-chloro-5-(o-chlorophenyl)-1 3-dihydro-2H-1,4-benzodiazepin-2-one thus obtained was added 4N sodium hydroxide solution with stirring. All the solid dissolved and soon a thick white solid precipitated out. The solid was filtered, washed well with water and recrystallized from ethanol. The product was isolated as a solvate with 1 mol of ethanol. When heated it loses the ethanol of solvation and melts at 166°C to 168 C. 

Protic solvents, such as methanol and ethanol, slow down SN2 reactions by solvation of the reactant nucleophile. The solvent molecules hydrogen bond to the nucleophile and form a "cage" around it, thereby lowering its energy and reactivity. 

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... 

US patent 6,821,990, Ethanol solvate of (—)-cA-2-(2-chlorophenyl)-5,7-dihy-droxy-8 [4R-(3S-hydroxy-l-M ethyl) piperidinyl]-4H-l-benzopyran-4-one [118], Disclosed in this invention is an ethanol solvate of (—)-cA-2-(2-chlorophenyl)-5,7-dihydroxy-8[4R-(3S-hydroxy-l-methyl)piperid inyl]-4H-l-benzopyran-4-one hydrochloride (identified as Form II), a method of making Form II, and a composition comprising Form II. 

The explanation for the above is twofold. Firstly there is the effect of increasing cavita-tional collapse energy via a lowering in vapour pressure as the temperature is reduced (see above). This does not adequately explain the effect of the change in solvent. The primary process is unlikely to occur inside the cavitation bubbles and a radical pathway should be discarded. The most likely explanation is that the disruption induced by cavitation bubble collapse in the aqueous ethanolic media is able to break the weak intermolecular forces in the solvents. This will alter the solvation of the reactive species present. Significantly the maximum effect is found in 50 % w/w solvent composition - the solvent composition very close to the maximum hydrogen bonded structure. 

The cadmium electrodeposition on the cadmium electrode from water-ethanol [222, 223], water-DMSO [224], and water-acetonitrile mixtures [225-229] was studied intensively. It was found that promotion of Cd(II) electrodeposition [222] was caused by the formation of unstable solvates of Cd(II) ions with adsorbed alcohol molecules or by interaction with adsorbed perchlorate anions. In the presence of 1 anions, the formation of activated Cd(II)-I complex in adsorbed layer accelerated the electrode reaction [223]. 

Finally, in the crystal structures of the chloroform and ethanol solvates of [(dppmSe)2Au]Cl [91] the ions are connected into chains by C-H - Cl hydrogen bonds, whereby the chloride accepts two hydrogen bonds. Additionally, there is a C-H - Cl contact from a solvent molecule in the chloroform solvate (Figure 5.56b) and an O- Cl contact involving the solvent in the ethanol solvate. 

On most occasions —H - interactions are not the only secondary bonds present in the supramolecular structure, but can afford higher dimensionality. In the ethanol and methanol solvates of the thiolate complex [Au(2-Hmba) P(o-Tol)3 [ (Hmba = 2-mercaptobenzoate) [34], two molecules are hydrogen bonded via two alcohol molecules about a center of inversion and centrosymmetrically related pairs aggregate via... 

We have accordingly made a number of measurements on pyridinium salts. In agreement with Lord and Merhifteld [8] we find the broad structureless band at about 2460 cm 1 in the spectrum of pyridine hydrochloride in chloroform solution, which they attributed to Nh— Ho., Cl" bonding. In addition we find this raised to about 2570 cm 3 in ethanol solution. The solid perchlorate, sulphate and chloride (in KBr discs) have practically identical spectra (to 4 tu.) in which the broad band now occurs centred on 2740 cm 1. Pyridiniain perchlorate dissolved in pyridine shows a similar band at 2530 cm 3. This is presumably due to solvation of the pyridinium ion through —H N... 

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