HYDROGEN-BOND BASICITY PK(HB) SCALE OF ALDEHYDES AND KETONES

The thermodynamic hydrogen-bond basicity scale pK(HB) (logarithm of th e 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 a nd related compounds, acetophenones, benzophenones and various other c onjugated ketones, The relationship between pK(HB) and a spectroscopic scale of basicity is obscured by the presence of two stereoisomeric c omplexes. 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 elec tron-donating, In cycloalkanones the basicity sequence with ring size is 4 < 11 similar to 12 similar to 15 < 5 < 6 < 7 < 8. Quantitative st ructure-basicity relationships have been established in the aromatic 3 - and 4-XC6H4COMe and the aliphatic XCOMe series, Intramolecular hydro gen bonding causes a basicity decrease in acetylacetone, Hydrogen bond ing sites are discussed.