Substituent Effects in Electrophilic Aromatic Substitution Halogens

14 SUBSTITUENT EFFECTS IN ELECTROPHILIC AROMATIC SUBSTITUTION HALOGENS 

Returning to Table 12.2, notice that halogen substituents direct an incoming electrophile to the ortho and para positions but deactivate the ring toward substitution. Nitration of chlorobenzene is a typical example of electrophilic aromatic substitution in a haloben-zene its rate is some 30 times slower than the corresponding nitration of benzene. The major products are o-chloronitrobenzene and p-chloronitrobenzene. 

PROBLEM 12.16 Reaction of chlorobenzene with 4-chlorobenzyl chloride and aluminum chloride gave a mixture of two products in good yield (76%). What were these two products  

Since we have come to associate activating substituents with ortho, para-directing effects and deactivating substituents with meta, the properties of the halogen substituents appear on initial inspection to be unusual. 

This seeming inconsistency between regioselectivity and rate can be understood by analyzing the two ways that a halogen substituent can affect the stability of a cyclo-hexadienyl cation. First, halogens are electronegative, and their inductive effect is to draw 

14 Substituent Effects in Electrophilic Aromatic Substitution Halogens 

Rate and product studies of electrophilic aromatic substitution in halobenzenes reveal a fairly consistent pattern of reactivity. The partial rate factors for chlorination show that, with one exception, all the ring positions of fluoro-, chloro-, and bromoben-zene are deactivated. The exception is the para position of fluorobenzene, which is slightly more reactive than a single position of benzene. 

The range of reactivity is not large. Benzene undergoes chlorination only about 1.4 times faster than the most reactive of the group (fluorobenzene) and 14 times faster than the least reactive (bromobenzene). In each halobenzene the para position is the most reactive, followed by ortho. 

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Rate and Regioselectivity in the Nitration of (Trifluoromethyl)benzene 474 Substituent Effects in Electrophilic Aromatic Substitution Activating Substituents 476 Substituent Effects in Electrophilic Aromatic Substitution Strongly Deactivating Substituents 480 Substituent Effects in Electrophilic Aromatic Substitution Halogens 482 Multiple Substituent Effects 484 Retrosynthetic Analysis and the Synthesis of Substituted Benzenes 486 Substitution in Naphthalene 488 Substitution in Heterocyclic Aromatic Compounds 489... 

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