Substituted benzenes directing power

In an attempt to trap fulvene 2 by the Diels-Alder reaction with maleic anhydride, another new photoreaction was discovered/ A cycloadduct was obtained which was suggested to be formed by direct addition to the excited state of benzene. Since then, photoadditions of alkenes to benzene and substituted benzenes have been studied extensively and become a powerful tool for synthetic approaches to complex polycycHc compounds. - ... 

The second point is somewhat less obvious but is readily illustrated by the synthesis of 1,3,5-tribromobenzene. This particular- substitution pattern cannot be obtained by direct brornination of benzene because bromine is an ortho, para director. Instead, advantage is taken of the powerful activating and ortho, para-directing effects of the fflnino group in aniline. Brornination of aniline yields 2,4,6-tribromoaniline in quantitative yield. Diazotization of the resulting 2,4,6-tribromoaniline and reduction of the diazonium salt gives the desired 1,3,5-tribromobenzene. 

Even sulphonate esters 109 are powerful directing groups, competing well with tertiary amides. No substitution accompanies ortholithiation of ethyl or isopropyl benzene-sulphonate by BuLi. Hydrolysis and chlorination of the products 110 gives functionalized sulphonyl chlorides 111 (Scheme 48) °°. 

In benzimidazole, electrophiles preferentially substitute in the fused benzene ring in the 5(6)-position a powerful electron donor at C-5 will direct subsequent attack to C-4 electron-withdrawing groups lead to subsequent 4- or 6-substitution. Attack at C-2 is virtually unknown. 

Sulfonation is a bimolecular electrophilic substitution reaction (SE2) and the electrophile is sulfur trioxide.3a Sulfur trioxide is a powerful electrophile because of the electron-withdrawing effect of the three double-bonded oxygen atoms. Consequently, oleum (fuming sulfuric acid), which contains approximately 10% of excess sulfur trioxide, is a much more powerful sulfonating agent than concentrated sulfuric acid. Sulfur trioxide is a sufficiently powerful electrophile to attack benzene (23) directly. The mechanism of the sulfonation of benzene by hot concentrated sulfuric acid to give benzenesulfonic acid (24) is shown in Scheme 15.4a... 

Where a benzene ring bears a resonance acceptor group, substitution occurs at the mete-position and is slow compared with the reaction of benzene. We describe these substituents as mete-directing and deactivating. We can exemplify this by thinking about the reaction of nitrobenzene with electrophiles. The nitro group is a powerful electron withdrawer, both by resonance and induction (Figure 12.35). 

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