Hydrogen-bond basicity of secondary and tertiary amides, carbamates, ureas and lactams

The hydrogen-bond basicity scale pKHB(logarithm of the formation constant of 4-fluorophenol–base complexes in CCl4) has been measured for tertiary and secondary amides, carbonates, ureas and lactams. The hydrogen-bonding fixation site is the carbonyl group, even for the very hindered amide ButCON(C6H11)2. In the amides R1CONR2R3 the hydrogen-bond basicity is decreased more by bulky R1 substituents on the carbonyl carbon than by bulky R2 and R3 substituents on nitrogen. The field effect of X substituents operates more effectively on hydrogen-bond basicity than the resonance effect in the XCONMe2 series. The hydrogen-bond basicity is increased by six-membered cyclisation.