THE META VERSUS PARA SUBSTITUENT EFFECT IN THE GAS-PHASE - SEPARATION OF INDUCTIVE AND RESONANCE COMPONENTS

Substituent 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, Substituents considered were NO2, CN, SO2CH3, SO2F, CP3, CH2Cl, COCH3 and COOCE(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: mefa 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 NO2 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 accepters, usually less represented in the sample.