Additional Considerations Regarding Substituent Effects

When a benzene ring with an ortho-para-directing substituent undergoes an electrophilic aromatic substitution reaction, what percentage of the product is the ortho isomer and what percentage is the para isomer  

Solely on the basis of probabihty, more of the ortho product is expected because two ortho positions are available to the incoming electrophile but only one para position is available. The ortho positions, however, are stericaUy hindered by the substituent on the ring, whereas the para position is not. Consequently, the para isomer will be formed preferentially if either the substituent on the ring or the incoming electrophile is large. The following nitration reactions illustrate the decrease in the ortho-para ratio as the size of the substituent on the ring increases  

Fortunately, the physical properties of the ortho- and para-substituted isomers differ enough to allow them to be easily separated. Consequently, electrophilic aromatic substitution reactions that lead to both ortho and para isomers are useful in syntheses because the desired product can be easily separated from the reaction mixture. 

If the Lewis acid catalyst and excess bromine are used, the tribromo compound is obtained. 

Friedel-Crafts reactions are the slowest of the electrophilic aromatic substitution reactions. Therefore, if a benzene ring has been moderately or strongly deactivated— that is, if it has a meta-directing substituent—it will be too unreactive to undergo either Friedel-Crafts acylation or Friedel-Crafts alkylation. In fact, nitrobenzene is so unreactive that it is often used as a solvent for Friedel-Crafts reactions. 

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Section 16.7 Additional Considerations Regarding Substituent Effects... 

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