Substituents, electron withdrawing Mesomeric effects

With substituents such as OH and OMe that have unshared electron pairs, an electron-donating, i.e. base-strengthening, mesomeric effect can be exerted from the o- and p-, but not from the m-position, with the result that the p-substituted aniline is a stronger base than the corresponding w-compound. The m-compound is a weaker base than aniline itself, due to the electron-withdrawing inductive effect exerted by the oxygen atom in each case. As so often, the effect of the o-substituent remains somewhat anomalous, due to direct interaction with the NH2 group by both steric and polar effects. The substituted anilines are found to have related pAa values as follows ... 

Substituents exerting a mesomeric effects on the substrates often favor the ortho photocycloaddition. This reaction mode is frequently observed when the aromatic ring carries electron-withdrawing substituents and the alkene moiety carries electron donor groups as well as in the reversed case with... 

Chlorine and bromine with inductive electron withdrawal ( —I) > mesomeric electron donation (-I- M) direct boron to the a position, the degree of selectivity varying with structure " . The direction of addition to allylic derivatives is influenced by steric and inductive effects of the substituent . Electron-withdrawing groups direct the boron... 

Significant electron-withdrawal by a substituent to stabilise the anionic intermediate, e.g. (81), only occurs through a mesomeric effect, i.e. when the nitro group, for example, is o- and/or p- to the leaving group. Thus we observe the reactivity sequence ... 

The idea of steric hindrance to resonance starts from the principle that the mesomeric electron-withdrawing effect is larger when the involved orbitals present parallel directions than when they are twisted. Section II.E.4 reports a different point of view on the mechanism of the mesomeric electron-withdrawing effect of substituents. 

Conversely, if a. para substituent stabilizes the conjugate base of an acid-base pair rather more than it stabilizes the benzoate ion, more positive substituent constants are required to achieve linearity in Hammett plots. Examples of this are acid dissociations of phenols and anilinium ions, where mesomerically electron-withdrawing substituents (Y = —NO2, —C N) are more effective in enhancing acid strength than they are in benzoic acid, because charge delocalization of the type [15] is not possible in the benzoate anion. 

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