Substitution, electrophilic polycyclic aromatics

As described earlier, the S Ar involves the reaction of an electrophilic species with an arene nucleophile. There are several types of arenes common to the S Ar reactions substituted benzenes, polycyclic aromatic compounds, and heterocyclic compounds. Substituent effects largely control the chemistry of substituted benzenes and related compounds. This includes both activating and directing effects of substituents on the S Ar reaction. 

Electrophilic substitution of polycyclic aromatic compounds is generally faster than that of benzene, because they have lower aromatic stabilization. 

Polycyclic aromatic hydrocarbons undergo electrophilic aromatic substitution when treated with the same reagents that react with benzene In general polycyclic aromatic hydrocarbons are more reactive than benzene Most lack the symmetry of benzene how ever and mixtures of products may be formed even on monosubstitution Among poly cyclic aromatic hydrocarbons we will discuss only naphthalene and that only briefly Two sites are available for substitution m naphthalene C 1 and C 2 C 1 being normally the preferred site of electrophilic attack... 

Section 12 17 Polycyclic aromatic hydrocarbons undergo the same kind of electrophilic aromatic substitution reactions as benzene... 

This type of addition process is particularly likely to be observed when the electrophile attacks a position that is already substituted, since facile rearomatization by deprotonation is then blocked. Reaction at a substituted position is called ipso attack. Addition products have also been isolated, however, when initial electrophilic attack has occurred at an unsubstituted position. The extent of addition in competition with substitution tends to increase on going to naphthalene and the larger polycyclic aromatic ring systems. ... 

The polycyclic aromatic hydrocarbons such as naphthalene, anthracene, and phenan-threne undergo electrophilic aromatic substitution and are generally more reactive than benzene. One reason is that the activation energy for formation of the c-complex is lower than for benzene because more of the initial resonance stabilization is retained in intermediates that have a fused benzene ring. 

Naphthalene and other polycyclic aromatic hydrocarbons show many of the chemical properties associated with aromaticity. Thus, measurement of its heat of hydrogenation shows an aromatic stabilization energy of approximately 250 kj/mol (60 kcal/mol). Furthermore, naphthalene reacts slowly with electrophiles such as Br2 to give substitution products rather than double-bond addition products. 

Polycyclic aromatic compounds also undergo electrophilic aromatic substitution reactions. Because the aromatic resonance energy that is lost in forming the arenium ion is lower, these compounds tend to be more reactive than benzene. For example, the brotni-nation of naphthalene, like that of other reactive aromatic compounds, does not require a Lewis acid catalyst ... 

The fluorination of aromatic compounds with xenon difluoride has been extensively investigated.180- 188 The fluorination of benzene with xenon difluoride in the presence of hydrogen fluoride as a catalyst results in the formation of fluorobenzene in 68% yield.180 Monosub-stituted aromatic systems are reported to give high yields of monofluorinated compounds, the isomer distributions of which are similar to those observed in electrophilic substitution (Table 14).181 Alkylaromatics, benzocyclenes and polycyclic aromatics are all successfully fluorinated by xenon difluoride in the presence of hydrogen fluoride examples are the fluorination of 21,182 22,182 23,184 185 24,185 and 25.182... 

Electrophilic aromatic substitution is a general reaetion of all aromatie eompounds, including polycyclic aromatic hydrocarbons, heterocycles, and substituted benzene derivatives. A substituent affects two aspects of electrophilic aromatic substitution ... 

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