Lactones. 3. A comparison of the basicities of lactones and esters

Article abstract of DOI:10.1021/ja00020a037

The basicity of lactones and esters toward triethyloxonium ion was studied via NMR spectroscopy. The equilibrium constants were and the order of decreasing basicity was found to be δ-valerolactone > γ-butyrelactone > diethyl carbonate > ethyl acetate > ethyl propionate. The rates of reaction were parallel to the equilibrium constants. The origin of the differences in basicity was via ab initio calculations of structures and energies of ions formed by adding H⁺ or CH₃⁺ to methyl acetate. The calculated proton affinity of the Z ester rotamer agreed with the experimental data and was found to be considerably smaller than that for the E rotamer. The relative energies of the four ions that could be formed by adding a proton to methyl acetate were related to the orientation of the dipole components at oxygen. The methyl cation affinities of the methyl acetate conformers were found to parallel the proton affinities and gave the same preferred direction of addition. The proton affinity of valerolactone was calculated to be greater than that for butyrolactone in good accord with the experimental results.