Trisubstituted Benzenes Additivity of Effects

if the directing effects of the two groups reinforce each other, the situation is straighitforward. In p-nitrotoiuene, for example, both the methyl and the nitro group direct further substitution to the same position (ortho to the methyl = meta to the nitre). A single product is thus formed on electrophilic. substitution. 

If the directing effects of the two grouj )s oppo.se each other, the more powerful activating group has the dominant influence, hut mixtures of products often result. For example, bromination ofp-methylphenol yields primarily 2-bromo-4-niethylphenol because —OH is a more powerful activator than —CH3. 

Further substitution rarely occurs between the two groups in a meta-disubstituted compound because this site is too hindered. Aromatic rings with three adjacent substituents must therefore be prepared by some other route, usually by substitution of an ortho-disubstituted compound. 

What product would you expect from bromiiiation of /j-methylbenzoic acid  

Problem 16.15 At what position would you expect electrophilic substitution to occur in each of the 

Electrophilic substitution of a disubstituted benzene ring is governed by the same resonance and inductive effects that affect monosubstituted rings. The only difference is that it s now necessary to consider the additive effects of two different groups. Tn practice, this isn t as difficult as it sounds three rules are usually sufficient. 

CHAPTER 16 Chemistry of Benzene Electrophilic Aromatic Substitution 

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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... 

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