Salt forms of amides: protonation of acetanilide

Treating the amide acetanilide (N-phenylacetamide, C₈H₉NO) with aqueous strong acids allowed the structures of five hemi-protonated salt forms of acetanilide to be elucidated. N-(1-Hydroxyethylidene)anilinium chloride-N-phenylacetamide (1/1), [(C₈H₉NO)₂H][Cl], and the bromide, [(C₈H₉NO)₂H][Br], triiodide, [(C₈H₉NO)₂H][I₃], tetrafluoroborate, [(C₈H₉NO)₂H][BF₄], and diiodobromide hemi(diiodine), [(C₈H₉NO)₂H][I₂Br]·0.5I₂, analogues all feature centrosymmetric dimeric units linked by O-H...O hydrogen bonds that extend into one-dimensional hydrogen-bonded chains through N-H...X interactions, where X is the halide atom of the anion. Protonation occurs at the amide O atom and results in systematic lengthening of the C=O bond and a corresponding shortening of the C-N bond. The size of these geometric changes is similar to those found for hemi-protonated paracetamol structures, but less than those in fully protonated paracetamol structures. The bond angles of the amide fragments are also found to change on protonation, but these angular changes are also influenced by conformation, namely, whether the amide group is coplanar with the phenyl ring or twisted out of plane.