Hydrogen-bond basicity pKHB scale of aldehydes and ketones

Abstract

The thermodynamic hydrogen-bond basicity scale pK_HB (logarithm of the formation constant of 4-fluorophenol–aldehyde or ketone complexes in CCl₄ at 298 K) has been determined for aldehydes, aliphatic ketones, cycloalkanones, diketones and quinones, halogenated ketones, pyrones and related compounds, acetophenones, benzophenones and various other conjugated ketones. The relationship between pK_HB and a spectroscopic scale of basicity is obscured by the presence of two stereoisomeric complexes. In the R¹COMe series the electronic and steric effects of the alkyl R¹ almost cancel out, whereas steric effects prevail in R¹COR². Among alkyl substituents the 1-adamantyl is the most electron-donating. In cycloalkanones the basicity sequence with ring size is 4 < 11 ∼ 12 ∼ 15 < 5 < 6 < 7 < 8. Quantitative structure–basicity relationships have been established in the aromatic 3- and 4-XC₆H₄COMe and the aliphatic XCOMe series. Intramolecular hydrogen bonding causes a basicity decrease in acetylacetone. Hydrogen bonding sites are discussed.