Aromatic substitution , substituent effect

II. Substituent effects in substituted aromatic rings (Sections 16.4 - 16.6). 

Bromination of Aromatic Rings 593 Other Aromatic Substitutions 597 Alkylation of Aromatic Rings The Friedel-Crafts Reaction Acylation of Aromatic Rings 604 Substituent Effects in Substituted Aromatic Rings 605 An Explanation of Substituent Effects 610 Trisubstituted Benzenes Additivity of Effects... 

A wide variety of electrophiles can effect aromatic substitution. Usually, it is a substitution of some other group for hydrogen that is of interest, but this is not always the case. Eor example, both silicon and mercury substituents can be replaced by electrophiles. Scheme 9.1 lists some of the specific electrophiles that are capable of carrying out substitution of hydrogen. Some indication of the relative reactivity of the electrophiles is given as well. Many of these electrophiles are not treated in detail until Part B. Nevertheless, it is important to recognize the very broad scope of electrophilic aromatic substitution. 

The historical introduction in Reference 5 summarizes some early work on directing effects of substituents in aromatic substitution. 

Substituent Effects in Substituted Aromatic Rings An Explanation of Substituent Effects... 

Excluding the phenomenon of hyperconjugation, the only other means by which electronic effects can be transmitted within saturated molecules, or exerted by inductive substituents in aromatic molecules, is by direct electrostatic interaction, the direct field effect. In early discussions of substitution this was usually neglected for qualitative purposes since it would operate in the same direction (though it would be expected to diminish in the order ortho > meta > para) as the cr-inductive effect and assessment of the relative importance of each is difficult however, the field effect was recognised as having quantitative significance. ... 

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. 

The applicability of the two-parameter equation and the constants devised by Brown to electrophilic aromatic substitutions was tested by plotting values of the partial rate factors for a reaction against the appropriate substituent constants. It was maintained that such comparisons yielded satisfactory linear correlations for the results of many electrophilic substitutions, the slopes of the correlations giving the values of the reaction constants. If the existence of linear free energy relationships in electrophilic aromatic substitutions were not in dispute, the above procedure would suffice, and the precision of the correlation would measure the usefulness of the p+cr+ equation. However, a point at issue was whether the effect of a substituent could be represented by a constant, or whether its nature depended on the specific reaction. To investigate the effect of a particular substituent in different reactions, the values for the various reactions of the logarithms of the partial rate factors for the substituent were plotted against the p+ values of the reactions. This procedure should show more readily whether the effect of a substituent depends on the reaction, in which case deviations from a hnear relationship would occur. It was concluded that any variation in substituent effects was random, and not a function of electron demand by the electrophile. ... 

In addition to benzene and naphthalene derivatives, heteroaromatic compounds such as ferrocene[232, furan, thiophene, selenophene[233,234], and cyclobutadiene iron carbonyl complexpSS] react with alkenes to give vinyl heterocydes. The ease of the reaction of styrene with sub.stituted benzenes to give stilbene derivatives 260 increases in the order benzene < naphthalene < ferrocene < furan. The effect of substituents in this reaction is similar to that in the electrophilic aromatic substitution reactions[236]. 

As is broadly true for aromatic compounds, the a- or benzylic position of alkyl substituents exhibits special reactivity. This includes susceptibility to radical reactions, because of the. stabilization provided the radical intermediates. In indole derivatives, the reactivity of a-substituents towards nucleophilic substitution is greatly enhanced by participation of the indole nitrogen. This effect is strongest at C3, but is also present at C2 and to some extent in the carbocyclic ring. The effect is enhanced by N-deprotonation. 

What IS the effect of a substituent on the rate of electrophilic aromatic substitution s... 

These relative rate data per position are experimentally determined and are known as partial rate factors They offer a convenient way to express substituent effects m elec trophilic aromatic substitution reactions... 

SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION ACTIVATING SUBSTITUENTS... 

Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents... 

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