Differential substituent effects in 4-X-acetophenones and 4-X-2,6-dimethylacetophenones: basicity constants (pKBH+) and 17O chemical shifts

Abstract

The protonation equilibria of some 4-X-acetophenones (1c–h, j–l) and their 2,6-dimethyl derivatives (2c, d, f–h, l) have been investigated in sulfuric acid at 298.0 K. Correlations of the pK_BH⁺ values with the para-substituent constants evidence a markedly different sensitivity of the base strength of 1 and 2 to substituent effects [ρ⁺(1)= 1.20 ± 0.09, ρ(2)= 5.3 ± 0.5]. Solvation plays a major role in determining an attenuation (in 1) or an exaltation (in 2) of the substituent effect as suggested by the excellent correlation with gas phase basicities for the series 1 and by the behaviour of m* whose values are essentially constant for 1c–h, j–l(0.57 ± 0.03) but increase, in the series 2, as the base strength decreases (m* 0.48 for X = OMe, 1.08 for X = NO₂). In 2 the steric inhibition of through-conjugation between the 4-X substituent and the carbonyl group also contributes to the observed trends. The ¹⁷O NMR chemical shifts for 1a–l and 2a–d, f–i, l in CDCl₃ have also been collected: in 1 they appear to be related to the pK_BH⁺ values and then to the carbonyl oxygen electron densities, whereas in 2 they are strongly influenced by the spatial location of the oxygen atom and/or of the relevant p-orbitals.