Electron withdrawal by resonance

The <7 constants obtained for substituents electron-donating by resonance that are para to a reaction site that becomes electron-rich, and for substituents electron-withdrawing by resonance that are para to a reaction site that becomes electron-deficient, were also taken as crn values, since in these cases there would be no stabilization by resonance. 

The Hammett cr-values contain contributions from both inductive/field effects and the resonance effect. The cr-constant can be separated quantitatively into a resonance component R, which operates mainly in the para position, and an inductive component F, which is assumed to be equal in the meta and para positions. Hansch, Leo and Taft11 have calculated the F and R values of Me3Si to be 0.01 and —0.08, respectively, as quoted in Table 1. These values seem somewhat at odds with experimentally determined values12-18 for the inductive and resonance parameters, which give mean values of —0.08 and 0.06, respectively. These values confirm the generally accepted view that MesSi is electron-supplying by inductive effects and electron-withdrawing by resonance effects. 

LOWER LEFT QUADRANT ELECTRON LOWER RIGHT QUADRANT ELECTRON WITHDRAWING BY RESONANCE/... 

There are two ways substituents can donate electrons into a benzene ring inductive electron donation and electron donation by resonance. There are also two ways substituents can withdraw electrons from a benzene ring inductive electron withdrawal and electron withdrawal by resonance. 

By contrast, Op for MesSi has a value of -1-0.11, compared to —0.07 for Op. This is attributed to the MesSi group being electron-withdrawing by resonance, as pr icted by (p-d) 7t bonding or hyperconjugation. The SiPhs group also shows a greatly enhanced cTp value. 

We have aheady explained why the nitro group is so powerfully electron-withdrawing. The nitto group is electron-withdrawing by resonance and induction. 

The para nitro group is electron withdrawing by resonance, thus activating the carbonyl carbon with respect to nucleophilic attack, relative to a simple benzoate ... 

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