Ortho, para deactivators

Classify the substituent—ortho, para activating, ortho, para deactivating, or meta deactivating— and draw the products. 

Why IS there such a marked difference between methyl and trifluoromethyl substituents m their influence on electrophilic aromatic substitution s Methyl is activating and ortho para directing trifluoromethyl is deactivating and meta directing The first point to remember is that the regioselectivity of substitution is set once the cyclohexadienyl cation intermediate is formed If we can explain why... 

Our analysis of substituent effects has so far centered on two groups methyl and triflu oromethyl We have seen that a methyl substituent is activating and ortho para directing A trifluoromethyl group is strongly deactivating and meta directing What about other substituents ... 

Halogen substituents are slightly deactivating but are ortho para directing... 

Activating Standard of comparison Deactivating —R —Ar —CH=CR —H —X (X = F Cl —CH2X (alkyl) (aryl) 2 (alkenyl) (hydrogen) (halogen) Br 1) (halomethyl) Ortho para directing Ortho para directing... 

Because 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... 

The mix of inductive and resonance effects varies from one halogen to another but the net result is that fluorine chlorine bromine and iodine are weakly deactivating ortho para directing substituents... 

Electrophilic aromatic substitution (Section 12 14) Halo gen substituents are slightly deactivating and ortho para directing Br... 

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. 

Activating Standard of comparison Deactivating —R (alkyl) —Ar (aryl) —CH=CR2 (alkenyl) —H (hydrogen) —X (halogen) (X = F, Cl, Br, I) —CH2X (halomethyl) Ortho, para-directing Ortho, para-directing... 

Direct deactivation in the methoxychloropyridines can be para (cf. 151) or ortho (cf. 152). It is to be expected that ortho direct deactivation of the more reactive 4-chloro-3-methoxypyridine will leave... 

F, -Cl -Br. -I 1 Deactivating Ortho, para Strong withdrawing Weak donating... 

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