Hydrogen bonding and structure of 2-hydroxy-N-acylanilines in the solid state and in solution

Abstract

In crystals of 2-hydroxy-N-benzoylaniline and 2-hydroxy-N-trifluoroacetylaniline, ribbons of molecules are held by intermolecular hydrogen bonds between the hydroxy and carbonyl groups of adjacent molecules. The O· · ·H· · ·O bond lengths are 2.65 and 2.72 Å, respectively, indicating the hydrogen bonds are moderately strong. The alkyl and aryl groups on the carbonyl carbon and the amide nitrogen have the usual trans configuration. In solution in CDCl₃ and Me₂SO the ¹H NMR spectra of N-acetyl- and N-benzoyl-anilines with substituents H, CH₃ and OCH₃ at the 2-position show downfield shifts of ca.δ 0.2–1.5 for the proton in the 6-position of the aniline ring. This is compatible with a trans configuration for these amides and strong deshielding of the 6-H by the nearby carbonyl group. N-Acetyl- and N-benzoyl-anilines with a 2-hydroxy substituent behave similarly in Me₂SO but in CDCl₃ solution no downfield shift is observed and this is explained by the formation of an intramolecular hydrogen bond between the 2-hydroxy substituent and the amide carbonyl in this solvent.