Protic solvents have hydrogen bound directly to electronegative atoms, such as oxygen or nitrogen. They are characterized by their ability to form strong hydrogen bonds with suitable acceptors, particularly simple anions. They include alcohols, formamide and other primary and secondary amides, and formic acid. In methanol, dissociation constants of carboxylic acids, phenols, and protonated nitrogen bases show excellent correlations with corresponding values in water. The largest differences occur for carboxylic acids, which are typically 5 pK-units weaker than in water. Acids become increasingly weak in the higher alcohols, especially t-butanol, because of poorer ion solvation. Homohydrogen-bond formation is generally weak, but ion-pair formation becomes progressively stronger as the solvent polarity decreases. Formamide contains a polar carbonyl group in addition to the ability to hydrogen-bond to anions, and displays pKa-values close to those in water. Formic acid hydrogen-bonds strongly with anions, but poor solvation of the proton, which inhibits the dissociation of acids, normally prevails.
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