Weakly basic solvents, relative strengths

The relative strengths of weakly basic solvents are evaluated from the extent of protonation of hexamethylbenzene by trifluoro-methanesulfonic acid (TFMSA) in those solvents or from the effect of added base on the same protonation in solution in trifluoroacetic acid (TFA), the weakest base investigated. The basicity TFA < di-fluoroacetic acid < dichloroacetic acid (DCA) < chloroacetic acid < acetic acid parallels the nucleophilicity. 2-Nitropropane appears to be a significantly stronger base than DC A by the first approach, although in the second type of measurement, the two have essentially equal basicity. The discrepancy is due to an interaction, possible for hydroxylic solvents such as DC A, with the anion of TFMSA. This anion stabilization is a determining factor of carbocationic reactivity in chemical reactions, including solvolysis. A distinction is made between carbocation stability, determined by structure, and persistence (existence at equilibrium, e.g., in superacids), determined by environment, that is, by anion stabilization. 

Reactions of acids or bases with water are only one aspect of solvent effects. Any acid will react with a basic solvent and any base will react with an acidic solvent, with the extent of the reaction varying with their relative strengths. Eor example, acetic acid (a weak acid) will react with water to a very slight extent, but hydrochloric acid (a strong acid) reacts completely, both forming H3O, together with the acetate ion and chloride ion, respectively. 

Non-aqueous solvents that are good proton acceptors (e.g. NH3) encourage adds to ionize in them. Thus, in a basic solvent, aU acids are strong. The solvent is said to exhibit a levelling effect on the acid, since the strength of the dissolved acid carmot exceed that of the protonated solvent. For example, in aqueous solution, no addic spedes can exist that is a stronger add than [H30]. In an addic solvent (e.g. MeCOzH, H2SO4), ionization of bases is fadUtated most acids are relatively weak under these conditions, and some even ionize as bases. 

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