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The meta versus para substituent effect in the gas phase: Separation of inductive and resonance components
Author(s) -
Decouzon Michèle,
Exner Otto,
Gal JeanFrancois,
Maria PierreCharles
Publication year - 1994
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.610071105
Subject(s) - substituent , chemistry , inductive effect , benzoates , resonance (particle physics) , acceptor , gas phase , phase (matter) , group (periodic table) , medicinal chemistry , stereochemistry , computational chemistry , organic chemistry , atomic physics , quantum mechanics , physics
Substitutent effects of acceptor groups were measured for the gas‐phase basicities of some substituted benzonitriles, methyl benzoates and acetophenones, and for gas‐phase acidities of some substituted benzoic acids. Substitutents considered were NO 2 , CN, SO 2 CH 3 , SO 2 F, CF 3 , CH 2 Cl, COCH 3 and COOCH 3 , always in the meta and para positions. By combination with the literature data, a general conclusion is drawn that the substituent effects are proportional in the two positions and almost equal (the para:meta ratio is 1·06). No sign of any resonance effect was detected within the accuracy of the approach. The substituent effects in solution were recalculated statistically and the result was similar with a higher ratio, 1·09–1·19. It follows that the conjugation of acceptor groups, particularly of NO 2 as a typical example, has been overestimated by current theories when compared with the much stronger conjugation of typical donors. Moreover, the inductive effect is propagated more effectively from the para than from the meta position. Many correlations in the literature, neglecting this situation, have been in fact controlled by the more complex, and more striking, behaviour of donors, which has obscured the simpler and more monotonous effect of acceptors, usually less represented in the sample.