Comparative basicities of heterocyclic analogs of chalcone

Measurements are made of valence vibration frequency shifts of the hydroxyl group of phenol in carbon tetrachloride solution due to the effect of hydrogen bonding with the carbonyl group (u_OH) of chalcone, and its furan, thiophene, selenophene, and N-methylpyrrole analogs. All the compounds form rather stable hydrogen bonds with phenol; u_OH varies in the range 200–284 cm^–1. For propen-3-ones, where the carbonyl group is remote from the heterocyclic ring, a linear relationship is observed between u_OH and pK, previously determined in 100% sulfuric acid solution in glacial acetic acid. The absence of a linear relationship between these quantities for the other ketones is ascribed to steric factors. It is established that heterocyclic groups exhibit a positive conjugation effect as compared with phenol, decreasing in the order N -methyl-2-pyrryl > 2-furyl > 2-selenienyl > 2-thienyl.