Salt forms of amides - protonation of acetanilide

Jaconelli, Harry S. and Kennedy, Alan R. (2024) Salt forms of amides - protonation of acetanilide. Acta Crystallographica Section C: Structural Chemistry. ISSN 2053-2296 (In Press)

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Abstract

Treating the amide acetanilide (C8H9NO) with aqueous strong acids allowed the structures of five hemi-protonated salt forms of acetanilide to be elucidated. [(C8H9NO)2H][Cl], [(C8H9NO)2H][Br], [(C8H9NO)2H][I3], [(C8H9NO)2H][BF4] and [(C8H9NO)2H][I2Br].0.5I2 all feature dimeric units linked by centrosymmetric O-H…O hydrogen bonds that extend into 1-dimensional hydrogen bonded chains through N-H…X interactions, where X = halide atom of the anion. Protonation occurs at the amide O atom and results in systematic lengthening of the C=O bond and 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 co-planar with the phenyl ring or twisted out of plane.

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