Dear Antonio,

As aprotic solvents include the hydrocarbons and their halogen derivatives, which undergo no reaction with added acids or bases. Acid–base equilibrium in these solvents can be investigated only when a second acid–base system is added; the usual reaction A1 + B2 ⇄ B1 + A2 then takes place. Most such investigations have employed an indicator as one of the reacting systems, but the results are often difficult to interpret because of association of both ions and molecules in these media of low dielectric constant.

The term aprotic has been extended recently to include solvents that are unable to lose a proton, although they may have weakly basic properties. Some of these aprotic solvents have high dielectric constants (for example, N, N-dimethylformamide, dimethyl sulfoxide, and nitrobenzene) and are good solvents for a variety of substances. They have a powerful differentiating effect on the properties of acids and bases. In particular, basic anions are poorly solvated in these solvents and thus behave as very strong bases; for example, it has been estimated that sodium methoxide dissolved in dimethyl sulfoxide gives a solution 109 times as basic as in methanol.

Dilute solutions of strong acids—for example, hydrochloric, sulfuric, and perchloric (HCl, H2SO4, HClO4)—in water behave essentially as solutions of the ion H3O+, and their acidity increases in proportion to their concentration. At concentrations greater than about one molar (that is, one mole of acid per litre of solution), however, the acidity, as measured by action on indicators or by catalytic ability, increases much more rapidly than the concentration. For example, a 10 molar solution of any strong acid is about 1,000 times as acidic as a 1 molar solution. This behaviour is undoubtedly largely due to the depletion of water with increasing concentration of acid; the hydronium ion, H3O+, is known to have a strong tendency to further hydration, probably mainly to the ion H3O+(H2O)3 (that is, H9O4+), and a decrease in water content increases the proton-donating power of the solution. The acidity of these concentrated solutions is commonly measured by the acidity function, H0, a quantity measured by the effect of the solvent on a basic indicator I. It is defined by H0 + pKIH+ − log10 [IH+]/[I] and becomes equal to the pH in dilute solution. The acidity function H0 frequently is found to be independent of the nature of the indicator and to give an approximate measure of the catalytic power of the acid solution. Mixtures of sulfuric acid and water ranging from 10 to 100 percent sulfuric acid have H0 values between −0.3 and −11.1, which corresponds to an acidity range of nearly 11 powers of 10.

Regards

Gaurav Pandey, thanks for the explanation!

My reaction produces HCl and I'm in need to neutralize it in order to keep the reaction going (the same HCl may react with the reagents and lower the final yield). These side reactions are exclusively related to the acidic character from the HCl, so I need some way to neutralize it as it forms (it cannot generate water because it also reacts with the reagents nor can it possess an alcoholic or phenolic hydroxil).

That's why I had the idea of using a salt that, when protonated, would become insoluble and remove the HCl from the solvent. Before I've found out that fatty acids are highly soluble in DMF, I thought of using their salts so, as the HCl is generated, they would be desaponified and separated out of the solution.

Is there anything like that that I could use? Basically, some sort of a basic reagent that, when protonated, would separate from the DMF system (without generating water)?

Dear all, if there is no harm you can use an acid scavenger such as pyridine. This another strategy to avoid free HCl. My Regards