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Methyl-substituted Acetophenones (methyl-substituted + acetophenone)
Selected AbstractsOrtho effect and steric inhibition of resonance: basicities of methyl-substituted acetophenones,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 10 2003Eva Otyepková Abstract The basicity of 12 methyl-substituted acetophenones was measured spectrophotometrically in 46,90 vol.% sulfuric acid. The acidity function was constructed and the pKa values were calculated by a new algorithm proposed by Pytela. The substituent effects were divided into polar and steric, assuming that the former are approximately equal in the ortho and para positions. Polar effects of the methyl group bring about stronger basicity as expected; the effect is more intense than the acid weakening in equally substituted benzoic acids. Steric effects of ortho methyl groups are base strengthening. This is not due to steric inhibition of resonance since the conformation remains planar in most derivatives. Two ortho methyl groups are necessary to distort the planarity; their steric effect is more than doubled compared with one methyl group. These results do not agree with the common idea of twisted conformations with gradually increasing twisting angle but are better rationalized by the existence of two groups of derivatives, planar and non-planar. Copyright © 2003 John Wiley & Sons, Ltd. [source] 13C and 1H nuclear magnetic resonance of methyl-substituted acetophenones and methyl benzoates: steric hindrance and inhibited conjugationMAGNETIC RESONANCE IN CHEMISTRY, Issue 10 2004ínský Abstract The 1H and 13C NMR spectra of 14 methyl-substituted acetophenones and 14 methyl-substituted methyl benzoates were assigned and interpreted with respect to the conformation of the Car,C(O) bond. The substituent effects are proportional in the two series and can be divided into polar and steric: each has different effects on the 13C SCS of the individual atoms. In the case of C atoms C(O), C(1) and CH3(CO), the steric effects were quantitatively separated by comparing SCS in the ortho and para positions. The steric effects are proportional for the individual C atoms and also to steric effects estimated from other physical quantities. However, they do not depend simply on the angle of torsion , of the functional group as anticipated hitherto. A better description distinguishes two classes of compounds: sterically not hindered or slightly hindered planar molecules and strongly sterically hindered, markedly non-planar. In order to confirm this reasoning without empirical correlations, the J(C,C) coupling constants were measured for three acetophenone derivatives labeled with 13C in the acetyl methyl group. The constants confirm unambiguously the conformation of 2-methylacetophenone; their zero values are in accord with the conformation of 2,6-dimethylacetophenone. The zero values in the unsubstituted acetophenone are at variance with previous erroneous report but all J(C,C) values are in accord with calculations at the B3LYP/6-311++G(2d,2p)//B3LYP/6,311+G(d,p) level. Copyright © 2004 John Wiley & Sons, Ltd. [source] | ||||||||||