Deactivating groups Substituents that, when

Deactivating substituent (Sections 12 11 and 12 13) A group that when present in place of hydrogen causes a particular reaction to occur more slowly The term is most often ap plied to the effect of substituents on the rate of electrophilic aromatic substitution... 

The entrance of a third or fourth substituent can be predicted by Beilstein s rule. If a substituent Z- enters into a compound C H XY, both X and Y exert an influence, but the group with the predominant influence directs Z- to the position it will occupy. Since all meta-directing groups are deactivating, it follows that ortho—para activating groups predominate when one of them is present on the benzene ring. 

Remember from Section 12.16 that it is the more activating substituent that determines the regioselectivity of electrophilic aromatic substitution when an arene bears two different substituents. Methoxy is a strongly activating substituent fluorine is slightly deactivating. Friedel-Crafts acylation takes place at the position para to the methoxy group. 

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