Other Electrophilic Aromatic Substitutions

Scheme 11.5. Other Electrophilic Aromatic Substitutions Related to Friedel-Crafts... 

The isopropyl group is ortho, para-directing, and the nitro group is meta-directing. In this case their orientation effects reinforce each other. Electrophilic aromatic substitution takes place ... 

Unlike most other electrophilic aromatic substitutions, sulfonation is often reversible (see Section 17-4). When one sample of toluene is sulfonated at 0 °C and another sample is sulfonated at 100 °C, the following ratios of substitution products result ... 

Pertinent examples of zeolite-catalyzed reactions in organic synthesis include Friedel-Crafts alkylations and acylations and other electrophilic aromatic substitutions, additions and eliminations, cyclizations, rearrangements and isomeriza-tions, and condensations. 

A more interesting problem than the influence of substituents in the electrophilic reagent of azo coupling is the extremely high selectivity of the C-coupling reactions, relative to other electrophilic aromatic substitutions. Unsubstituted benzene does not react with any arenediazonium ion, 1,3,5-trimethoxybenzene reacts very slowly with strongly electrophilic diazonium ions only aromatic amines (e.g. N,N-dimethyl-aniline) or phenolate ions react very fast, in some cases close to diffusion control. 

In contrast to aromatic nitration and other electrophilic aromatic substitutions aromatic sulfonation is reversible. Sulfonation takes place in strong acidic conditions and desulfonation is the mode of action in a hot aqueous acid. 

The Hammett equation usually predicts substituent effects with reasonable accuracy, provided the correct choice is made between o and o. a values are based on a system in which there is little change in the degree of conjugation between the substituent and the remainder of the molecule on moving from one side of the transition state to the other. Electrophilic aromatic substitutions pass through a positively charged complex, the Wheland intermediate (I), in which the electronic distribution is different from that of either reactants or products. A generalised sequence is shown in Equation (10). The standard reaction on... 

This pattern is also followed for other electrophilic aromatic substitutions— chlorination, bromination, sulfonation, and so on. Toluene undergoes mainly ortho,para substitution, whereas nitrobenzene undergoes meta substitution. 

A variety of other electrophilic aromatic substitution reactions involve very strong electrophiles reacting with the weakly nudeophilic aromatic tt cloud to form an intermediate resonance-stabilized cation on the ring that loses a proton to give the substituted arene. 

Other electrophilic aromatic substitutions of flavan-3-ols have been examined, primarily regarding the use of proanthocyanidins in adhesives. Hillis and Urbach (157-158) studied the reactions of catechin and condensed tannins with formaldehyde early. Later studies (147) through H-NMR spectra and GPC profiles of... 

Other electrophilic aromatic substitutions related to Friedel-Crafts reactions... 

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