Gas-Phase and Solution Basicities of Some Alkyl 2,6-Dialkylphenyl Ketones: a Comparative Analysis

The gas-phase basicity of a number of alkyl 2,6-dialkylphenyl ketones (2,6-R2C6H3COR′) has been found to be almost insensitive to structural variations, as a result of a compensation of steric and electronic effects associated with the bulkiness and to the polarizability, respectively, of R and/or R′. On the contrary, the basicity in concentrated sulfuric acid undergoes, along the same series of compounds, a variation of nearly 8 pK units, as a consequence of steric inhibition of solvation of the protonated carbonyl as the main effect played by R and/or R′. The results in the condensed phase agree very nicely with recent findings relevant to some 4-substituted 2,6-dimethylacetophenones and 4-substituted acetophenones as well as with data for a large number of dialkyl or alkyl aryl ketones (either from the literature or determined herein) leading to an overall pKBH+ vs. m∗ correlation (slope=8.8, n=31, r=0.996; ) which appears to be of general significance and identifies steric inhibition of solvation as a common prevailing factor influencing the carbonyl basicity, at least in the absence of strong conjugative interactions.