Electrophilic substitution mesomerically stabilized

The best-known equation of the type mentioned is, of course, Hammett s equation. It correlates, with considerable precision, rate and equilibrium constants for a large number of reactions occurring in the side chains of m- and p-substituted aromatic compounds, but fails badly for electrophilic substitution into the aromatic ring (except at wi-positions) and for certain reactions in side chains in which there is considerable mesomeric interaction between the side chain and the ring during the course of reaction. This failure arises because Hammett s original model reaction (the ionization of substituted benzoic acids) does not take account of the direct resonance interactions between a substituent and the site of reaction. This sort of interaction in the electrophilic substitutions of anisole is depicted in the following resonance structures, which show the transition state to be stabilized by direct resonance with the substituent ... 

Electrophilic substitution of the carbonyl compounds occurs the more easily the less the carbonyl group is stabilized by mesomerism. It is increasingly difficult in the series ketone — acid chloride — acid anhydride — ester, carboxylic acid — carboxyamide.546 This gradation can be illustrated by, for instance, the bromination of levulic acid (4-oxovaleric acid) with 2 moles... 

Pyrrole reacts in SgAr reactions about 10 faster than furan under comparable conditions. This is in spite of the fact that the resonance energy of pyrrole is greater than that of furan pyrrole should, therefore, react more slowly. This discrepancy can be explained by considering the mechanism for electrophilic substitution (as formulated for furan on p. 63). In the case of pyrrole, it can be postulated that the a-complex for attack of the electrophile E" " in the 2-position is especially stabilized by a carbenium-iminium mesomerism ... 

In most SgAr processes, indole reacts more slowly than pyrrole, but faster than benzo(b]furan. In contrast to pyrrole, substitution occurs in the 3-position.This might be due to different a-complex stabilization Attack of the electrophile (E ) on the 3-position leads to a low-energy iminium structure 3 conserving the benzene ii-sextet, attack on the 2-position, however, leads to a a-complex 4, for which mesomeric delocalization is possible only by participation of an orthoquinonoid structure ... 

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