The structure of 2-keto and 2-aldehydobenzoic acids (spectroscopic studies)

Abstract

1H NMR (with particular reference to the acidic protons) and 13C NMR examination of the reaction products from phthalic anhydride and dialkylcadmiums, formally represented as 2-acylbenzoic acids (open chain structure), show that they are 3-hydroxy-3-alkylphthalides (the ring structure) in the solid state and in solution. Similarly, so-called phthalaldehydic acid has been confirmed to be 3-hydroxy phthalide. Only in aqueous alkaline solution do the preceeding compounds exist as 2-acylbenzoates and a 2-formylbenzoate respectively. By contrast, 2-benzoylbenzoic acid has the open chain structure in the solid state and in solution. Mass, i.r. and u.v. absorption spectra support the results found by NMR investigation. In appropriately nuclear-substituted acetobenzoic acids, namely 2-aceto-3-hydroxy and 2-aceto-3-amino benzoic acids the existence of intramolecular hydrogen bonding may well be the reason for the absence of the 3-hydroxy-3-methylphthalide structure.