GAS-PHASE BASICITIES OF LACTONES

The gas-phase basicities of beta-propiolactone 1,beta-butyrolactone 2, gamma-butyrolactone 3, 4-methyl-gamma-butyrolactone 4, 3-methyl-gamma- butyrolactone 5, 2-methyl-gamma butyrolactone 6, delta-valerolactone7 and epsilon-caprolactone 8 were measured by Fourier transform ion cycl otron resonance techniques. The proton affinities values deduced from experiments are 796, 809, 834, 852, 848, 848, 863 and 865 kJ/mol for 1 -8, respectively, thus demonstrating a clear increase of basicity when passing from beta- to gamma-lactones and further to delta-lactone whi le this effect vanishes when going to the higher member of the series, epsilon-caprolactone. Methyl substitution causes an almost constant in crease of the gas phase basicity of ca. 15 kJ/mol. It is also observed that lactones are more basic than the corresponding aliphatic esters, with the only exception being beta-lactones. All these experimental f indings are nicely reproduced by ab initio molecular orbital calculati ons carried out at the MP2/6-31G() level on optimized structures obta ined at the same level of theory. The calculations demonstrate that pr otonation of the lactones preferentially takes place at the carbonyl o xygen. Two factors are responsible for the increase in basicity along the series of lactones: (i) the hybridization change undergone by the carbonyl carbon on going from a four-membered ring to larger rings; an d (ii) an efficient stabilizing interaction with the ether-like oxygen which enhances the basicity of larger rings. The enhanced basicity of lactones with regard to the corresponding aliphatic esters is explica ble in similar terms.