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 variation s, as a result of a compensation of steric and electronic effects associate d with the bulkiness and to the polarizability, respectively, of R and/or R '. On the contrary, the basicity in concentrated sulfuric acid undergoes, a long the same series of compounds, a variation of nearly 8 pK units, as a c onsequence 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 a gree very nicely with recent findings relevant to some 4-substituted 2,6-di methylacetophenones and 4-substituted acetophenones as well as with data fo r a large number of dialkyl or alkyl aryl ketones (either from the literatu re or determined herein) leading to an overall pK(BH+) vs. m* correlation ( slope=8.8, n=31, r=0.996; Eq. (3)) which appears to be of general significa nce and identifies steric inhibition of solvation as a common prevailing fa ctor influencing the carbonyl basicity, at least in the absence of strong c onjugative interactions. (C) 2000 Elsevier Science Ltd. All rights reserved .