Substituents in electrophilic aromatic

There were two schools of thought concerning attempts to extend Hammett s treatment of substituent effects to electrophilic substitutions. It was felt by some that the effects of substituents in electrophilic aromatic substitutions were particularly susceptible to the specific demands of the reagent, and that the variability of the polarizibility effects, or direct resonance interactions, would render impossible any attempted correlation using a two-parameter equation. - o This view was not universally accepted, for Pearson, Baxter and Martin suggested that, by choosing a different model reaction, in which the direct resonance effects of substituents participated, an equation, formally similar to Hammett s equation, might be devised to correlate the rates of electrophilic aromatic and electrophilic side chain reactions. We shall now consider attempts which have been made to do this. 

Classification of Substituents in Electrophilic Aromatic Substitution Reactions... 

A Summary of Substituent Effects in Aromatic Substitution A summary of the activating and directing effects of substituents in electrophilic aromatic substitution is shown in Table 16.2. 

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

These polarizations are seen to be in the opposite direction to those in aniline (3.133), so that higher pi density remains at the Ci (junction) and C3 and C5 (meta) positions. These polarity shifts are again consistent with the well-known m-directing effect of nitro substituents in electrophilic aromatic substitution reactions, and the results are again quite independent of which starting Kekule structure is selected for the localized analysis.63... 

The effects of substituents in electrophilic aromatic substitution processes,... 

In an analogous reaction, where anisole (0.5 mole), chloromethyl ether (0.1 mole), and REX catalyst (2 gm) were stirred for 14 hours at room temperature, a 28% yield of a mixture of 2,2 -dimethoxydiphenyl-methane, 2,4 -dimethoxydiphenylmethane, and 4,4 -dimethoxydi-phenylmethane in the ratio of 1 5.7 5.4 was obtained. No 1 1 adduct (chloromethyl- or methoxymethylanisole) was observed. The lower reaction temperatures required with anisole reflect the activating effect of an electron-donating substituent in electrophilic aromatic substitution. 

In studies concerning the effects of substituents in electrophilic aromatic substitution, particularly in orientational control of the entering group, in reaction rates, and most recently in details of the substitution process itself, steric and polar effects of the substituent and the reagent appear to be factors simultaneously affecting the reaction. 

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