Electrophilic aromatic substitution, influence

Electrophilic aromatic substitution, influence of sulphinyl and sulphonyl groups on 532, 533... 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

The azo coupling reaction proceeds by the electrophilic aromatic substitution mechanism. In the case of 4-chlorobenzenediazonium compound with l-naphthol-4-sulfonic acid [84-87-7] the reaction is not base-catalyzed, but that with l-naphthol-3-sulfonic acid and 2-naphthol-8-sulfonic acid [92-40-0] is moderately and strongly base-catalyzed, respectively. The different rates of reaction agree with kinetic studies of hydrogen isotope effects in coupling components. The magnitude of the isotope effect increases with increased steric hindrance at the coupler reaction site. The addition of bases, even if pH is not changed, can affect the reaction rate. In polar aprotic media, reaction rate is different with alkyl-ammonium ions. Cationic, anionic, and nonionic surfactants can also influence the reaction rate (27). 

Resonance effects are the primary influence on orientation and reactivity in electrophilic substitution. The common activating groups in electrophilic aromatic substitution, in approximate order of decreasing effectiveness, are —NR2, —NHR, —NH2, —OH, —OR, —NO, —NHCOR, —OCOR, alkyls, —F, —Cl, —Br, —1, aryls, —CH2COOH, and —CH=CH—COOH. Activating groups are ortho- and para-directing. Mixtures of ortho- and para-isomers are frequently produced the exact proportions are usually a function of steric effects and reaction conditions. 

Substituents which are not directly bound to the aromatic ring can also influence the course of electrophilic aromatic substitution. Several alkyl groups bearing electron-... 

Anticipating the products of electrophilic aromatic substitution can be more difficult when two or more substituents compete for control. For example, both methyl and methoxy groups are ortho para directors, and compete for control in electrophilic substitution of 2-methylanisole. The reaction product depends on which substituent has the stronger directing influence. 

More than just a few parameters have to be considered when modelling chemical reactivity in a broader perspective than for the well-defined but restricted reaction sets of the preceding section. Here, however, not enough statistically well-balanced, quantitative, experimental data are available to allow multilinear regression analysis (MLRA). An additional complicating factor derives from comparison of various reactions, where data of quite different types are encountered. For example, how can product distributions for electrophilic aromatic substitutions be compared with acidity constants of aliphatic carboxylic acids And on the side of the parameters how can the influence on chemical reactivity of both bond dissociation energies and bond polarities be simultaneously handled when only limited data are available ... 

Trifluoromethyl)benzene (benzotrifluoride, 15) was the first organic fluoride to incorporate a trifluoromethyl group. By a standard nitration process, it formed l-nitro-3-(trifluoromethyl)-benzene (16) which was reduced to the 1-amino derivative, 17. This we a-directive influence on electrophilic aromatic substitution contrasted with that for fluorobenzene, which gave 4-and 2-nitro products. 

Annelation of 2,5-disubstituted pyrroles provides a useful route to isoindoles as shown in reactions (140)-(142). These reactions can be formulated as electrophilic aromatic substitutions of the pyrrole ring and proceed under the influence of acid catalysts (66T2481, 68JCS(C)3036, 72JCS(P1)904). 

In nitration, an electrophilic aromatic substitution, the electrophile is a nitronium cation. It attacks the benzene ring in the place of the highest electron density. In o-fluorotoluene, there are several positions of high electron density ortho and para with respect to the methyl group, and ortho and para with respect to fluorine. Both substituents, methyl and fluorine, direct the entering electrophile to the activated positions. Whose influence is stronger, that of the methyl group, or that of fluorine ... 

Aromatic compounds undergo many reactions, but relatively few reactions that affect the bonds to the aromatic ring itself. Most of these reactions are unique to aromatic compounds. A large part of this chapter is devoted to electrophilic aromatic substitution, the most important mechanism involved in the reactions of aromatic compounds. Many reactions of benzene and its derivatives are explained by minor variations of electrophilic aromatic substitution. We will study several of these reactions and then consider how substituents on the ring influence its reactivity toward electrophilic aromatic substitution and the regiochemistry seen in the products. We will also study other reactions of aromatic compounds, including nucleophilic aromatic substitution, addition reactions, reactions of side chains, and special reactions of phenols. 

Predict the position(s) of electrophilic aromatic substitution on molecules containing substituents on one or more aromatic rings, and design syntheses that use the influence of substituents to generate the correct isomers. Problems 17-46,47, 48,51, 54, 5G, 60, and 69... 

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. 

TABLE 4.1 Influence of Substituents in Electrophilic Aromatic Substitution... 

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