DIFFERENTIAL SUBSTITUENT EFFECTS IN 4-X-ACETOPHENONES AND 4-X-2,6-DIMETHYLACETOPHENONES - BASICITY CONSTANTS (PK(BH)(-17 CHEMICAL-SHIFTS())AND O)

The protonation equilibria of some 4-X-acetophenones (1c-h, j-l) and t heir 2,6-dimethyl derivatives (2c, d, f-h, l) have been investigated i n sulfuric acid at 298.0 K. Correlations of the pK(BH+) values with th e para-substituent constants evidence a markedly different sensitivity of the base strength of 1 and 2 to substituent effects [rho(+)(1) = 1 .20 +/- 0.09, rho(2) = 5.3 +/- 0.5]. Solvation plays a major role in d etermining an attenuation (in 1) or an exaltation (in 2) of the substi tuent effect as suggested by the excellent correlation with gas phase basicities for the series 1 and by the behaviour of m whose values ar e 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, 1.0 8 for X = NO2). In 2 the steric-inhibition of through-conjugation betw een the 4-X substituent and the carbonyl group also contributes to the observed trends. The O-17 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.