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Ortho effect and steric inhibition of resonance: basicities of methyl‐substituted acetophenones
Author(s) -
Otyepková Eva,
Nevěčná Tat'jana,
Kulhánek Jiří,
Exner Otto
Publication year - 2003
Publication title -
journal of physical organic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.325
H-Index - 66
eISSN - 1099-1395
pISSN - 0894-3230
DOI - 10.1002/poc.642
Subject(s) - steric effects , chemistry , planarity testing , substituent , methyl group , resonance (particle physics) , polar , stereochemistry , deprotonation , group (periodic table) , medicinal chemistry , computational chemistry , crystallography , organic chemistry , ion , physics , particle physics , astronomy
The basicity of 12 methyl‐substituted acetophenones was measured spectrophotometrically in 46–90 vol.% sulfuric acid. The acidity function was constructed and the p K a 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.