Intramolecular and intermolecular hydrogen bonds with participation of the enol proton of the 2-acetylcyclopentane-1,3-dione molecule

Abstract

A study has been made of the IR spectra of 2-acetylcyclopentane-1,3-dione (ACPD) in various solvents. This is a compound with an exocyclic enol group bonded to one of the β-carbonyl groups by an intramolecular hydrogen bond (IAHB). The IAHB remains in the ACPD when it is dissolved in a nonpolar solvents such as CCl4 or cyclohexane, or when a weak proton acceptor such as CH3CN or dioxane is present; in polar basic-type solvents (pyridine, isoquinoline, triethylamine), the enol proton dissociates, forming an ion pair. In the presence of a strong proton acceptor such as hexametapol (HMP) or tetrabutylammonium bromide (TBAB), an equilibrium exists between the ACPD and its open form with an opened chelate ring in which the OH group is connected by an intermolecular hydrogen bond (IEHB) to the acceptor. According to13C NMR spectroscopic data, dissolution of ACPD in CD3COOD also gives the open form of ACPD, which is stabilized primarily by an IEHB between the COOD groups of the solvent and the C=O groups of the ACPD. By the method of competing equilibria in the IR spectroscopic version, the energies of the H-bonds involving the ACPD have been determined (values in kcal/ mole): ΔHIEHB with HMP=−11.7 ± 0.3; ΔHIEHB with TBAB=−10.9 ± 0.3; ΔHIAHB in ACPD=−6.6 ± 0.4.