STRUCTURAL EFFECTS ON THE INTRINSIC BASICITIES OF ALPHA,BETA-UNSATURATED LACTONES AND KETONES

The proton affinities of 2(5H)-furanone, 1 (836 kJ/mol), 5,6-dihydro-2 H-pyran-2-one, 2 (862 kJ/ mol), cyclopentenone, 3 (857 kJ/mol), and cy clohexenone, 4 (863 kJ/mol), have been measured by Fourier transform i on cyclotron resonance techniques. A comparison is made with (reexamin ed) data concerning saturated cyclic and unsaturated aliphatic analogs . Three general observations are made. First, the basicity is found to increase with the size of the ring. Second, unsaturated lactones are more basic than their corresponding aliphatic unsaturated esters. Thir d, unsaturated and saturated lactones have almost identical gas-phase basicities, while unsaturated and saturated lactones have almost ident ical gas-phase basicities, while unsaturated cyclic ketones are more b asic than their saturated analogs. All these experimental findings hav e been rationalized by means of ab initio calculations up to the G2(MP 2,SVP) level. The basicity trends along the series are the result of t wo main factors: the different hybridization pattern of the carbonyl c arbon as the size of the ring changes and, in the case of lactones, th e nonbonding interaction between the proton attached to the carbonyl g roup and the ether-like oxygen which contributes to the enhanced stabi lity of the protonated form. For unsaturated ketones the C=C double bo nd participates fully in the change in charge distribution induced by the protonation, while for unsaturated lactones the existence of an ox ygen atom within the ring impedes this shift of the electron density.