Hydrogen-bond basicity pKHB scale of aldehydes and ketones

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 (RCOMe)-C-1 series the electronic and steric effects of the alkyl R-1 almost cancel out, whereas steric effects prevail in (RCOR2)-C-1, Among alkyl substituents the 1-adamantyl is the most electron-donating, In cycloalkanones the basicity sequence with ring size is 4 < 11 similar to 12 similar to 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.