Hydrogen-bond acidity, solvation effects

The general or universal effects in intermolecular interactions are determined by the electronic polarizability of solvent (refraction index n0) and the molecular polarity (which results from the reorientation of solvent dipoles in solution) described by dielectric constant z. These parameters describe collective effects in solvate s shell. In contrast, specific interactions are produced by one or few neighboring molecules, and are determined by the specific chemical properties of both the solute and the solvent. Specific effects can be due to hydrogen bonding, preferential solvation, acid-base chemistry, or charge transfer interactions. 

The accurate determination of gas-phase basicities and gas-phase acidities opened the way to analyses of the effect of solvation on proton acidities, and on hydrogen-bond acidities and basicities, as well as on substituents effects. 

In the same way, Abraham has calculated, from partition coefficients, an effective or summation hydrogen-bonding acidity scale, half - However, while sot is obtained from one reference partition system (octanol/chloroform), with the hypothesis that a number of solute parameters acting on log P cancel out, Abraham determines the haf values by a back calculation procedure over numerous sets of partition systems, and uses linear solvation energy relationships with a more complete set of solute parameters. Table 7 shows that Ea > ea and af values do not differ much for simple mono hydrogen-bond donors. It is also important to note that the order of acidity ... 

It can be seen for the [N(Tf)2] ionic liquids, that the hydrogen-bond acidity does indeed vary with cation with [BMIM]+ being the most acidic followed by [BMPYJ+ and finally [BMMIMJ+ in both studies. However, changing to more basic anions leads to a dramatic drop in the acidity measurements in the solvation study, whereas it has only a limited effect in the Kamlet-Taft experiment. That is, the solvation measurement is anion dominated, whereas the Kamlet-Taft measurement is cation dominated. 

The strength of acids and bases is affected by molecular structure, steric effects, resonance, inductive effects, hydrogen bonding, and solvation. 

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. ... 

Recent studies have found enhanced substituent solvation assisted resonance effects in dipolar non-hydrogen bonding solvents131. For several +R substituents acidities of phenols in DMSO are well correlated with their gas-phase acidities. The substituents include m- and p-SOMe, m- and p-S02Me, m-S02CF3 and m-N02. But there is very considerable enhancement of the effect of p-S02CF3, p-N02 and various other para-substituents in DMSO solution. 

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