Anion solvation, solution acidity

An acid is defined as a proton donor within the Lowry-Brpnsted theory (see Chapter 6). Molecules of acid ionize in aqueous solution to form an anion and a proton, both of which are solvated. An acid such as ethanoic acid (VI) is said to be weak if the extent to which it dissociates is incomplete we call it strong if ionization is complete (see Section 6.2). 

Statistically, we find fewer ethanoate anions than trichloroethanoate anions in the respective solutions of the two acids. And if there are fewer ethanoate anions in solution per mole of ethanoic acid, then there will be fewer solvated protons. In other words, the extent to which ethanoic acid dissociates is less than the corresponding extent for trichloroethanoic acid. I is a weak acid and VIII is strong dipping a simple pH electrode into a solution of each of the two acids rapidly demonstrates this truth. 

Extractive alkylation has a reaction scheme identical with that for the previous procedure. The substrate with a carboxyl group reacts in an aqueous solution with a quaternary base and is extracted in the form of an ion pair into a polar solvent of low solvation capacity (dichloromethane) that contains alkyl halide. Low solvation of the anion of the acid and high solvation of the reaction product lead to increased reactivity of the anion and to a rapid reaction with the alkylation agent in the organic phase. Methyl iodide [27] is used to prepare methyl esters and pentafluorobenzyl bromide [28] is used for the preparation of esters providing a high ECD response. 

The data show good correlation with hard and soft behaviour of the ions as a Lewis acid or base. The values consist of the energies of the frontier orbitals and the changes in solvation energy after the electron transfer or covalent bond formation. In the case of Al3+, it is the de-solvation energy that makes it a hard acid. It is to be noted that all cations would become softer in less polar solvents. In the gas phase they would become softest. Similarly soft anions in solution would become hard in the gas phase. 

Anion solvation leads to reactions with the solvent molecules producing a variety of anions possibly with reversed charge. This influences the pH for the aqueous solution of the hydrolyzed ions (see below). The pH concept is, however, traditionally only dealt with for the ions of the (IVB-) VB-VIIB groups producing what is called acids, although it is well known that metal cations are also acidic when dissolved in water. We may think of the acids as formed through the dissolution and hydrolysis of the acidic (anionic) element ... 

The solutions of HCl and HOSO3CH3 in aliphatic alcohol (i.e., C H2 +iOH) - normal alcohol C1-C5 and isomeric alcohol C3-C5 have been studied. If the components taking part in resolvation process are capable of H-bonding, the anion solvation by these components cannot be neglected. The differenees in K s values for both acids in different solvents may be explained as follows. 

Rustad JR, Hay BP (1995) A molecular-dynamics study of solvated orthosilicic acid and orthosilicate anion using parameterized potentials.. Geochim Cosmochim Acta 59 1251-1257 Rustad JR, Hay BP, Halley JW (1995) Molecular dynamics simulation of iron(III) and its hydrolysis products in aqueous solution. J Chem Phys 102 427-431 Rustad JR, Wasserman E, Felmy AR (1999b) Molecular modeling of the surface charging of hematite - II Optimal proton distribution and simulation of surface charge versus pH relationships. Surf Sci 424 28-35... 

Amines. Although liquid ammonia is used widely for certain chemical reductions (which hinge on the solubility of alkali-metals therein), amines are not widely used. Ammonia has (3 1) functionality, ethylamine (2 1), etc. They are quite basic via the nitrogen lone-pair, but only very weakly acidic, and many N-H groups are essentially free in the pure solvents and in aqueous solutions. Thus they are good cation, but poor anion, solvators. 

The aqueous solution of CIO2 contains just solvated CIO2 molecules, but the gas disproportionates with alkalis to generate the anions of chlorous acid, HCIO2, and chloric acid, HCIO3 (Figure 6.15) ... 

A comparison of phenol acidity in DMSO versus the gas phase also shows an attenuation of substituent effects, but not nearly as much as in water. Whereas the effect of ubstituents on AG for deprotonation in aqueous solution is about one-sixth that in the gas phase, the ratio for DMSO is about one-third. This result points to hydrogen bonding of the phenolate anion by water as the major difference in the solvating properties of water and DMSO. ... 

This is opposite from the order in solution as revealed by the pK data in water and DMSO shown in Table 4.14. These changes in relative acidity can again be traced to solvation effects. In the gas phase, any substituent effect can be analyzed directly in terms of its stabilizing or destabilizing effect on the anion. Replacement of hydrogen by alkyl substituents normally increases electron density at the site of substitution, but this effect cannot be the dominant one, because it would lead to an ordering of gas-phase acidity opposite to that observed. The dominant effect is believed to be polarizability. The methyl... 

In the discussion of the relative acidity of carboxylic acids in Chapter 1, the thermodynamic acidity, expressed as the acid dissociation constant, was taken as the measure of acidity. It is straightforward to determine dissociation constants of such adds in aqueous solution by measurement of the titration curve with a pH-sensitive electrode (pH meter). Determination of the acidity of carbon acids is more difficult. Because most are very weak acids, very strong bases are required to cause deprotonation. Water and alcohols are far more acidic than most hydrocarbons and are unsuitable solvents for generation of hydrocarbon anions. Any strong base will deprotonate the solvent rather than the hydrocarbon. For synthetic purposes, aprotic solvents such as ether, tetrahydrofuran (THF), and dimethoxyethane (DME) are used, but for equilibrium measurements solvents that promote dissociation of ion pairs and ion clusters are preferred. Weakly acidic solvents such as DMSO and cyclohexylamine are used in the preparation of strongly basic carbanions. The high polarity and cation-solvating ability of DMSO facilitate dissociation... 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

Extractive alkylation is used to derivatize acids, phenols, alcohols or amides in aqueous solution [435,441,448,502]. The pH of the aqueous phase is adjusted to ensure complete ionization of the acidic substance which is then extracted as an ion pair with a tetraalkylammonium hydroxide into a suitable immiscible organic solvent. In the poorly solvating organic medium, the substrate anion possesses high reactivity and the nucleophilic displacement reaction with an alkyl halide occurs under favorable conditions. 

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