Ortho/para directing

The ortho - para directing groups include those which are saturated or only weakly unsaturated at the point of the attachment of the ring. These are (—N(CHj)j, —NHj[ and —OH are by far the most powerful) —... 

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... 

Very strongly activating —NH2 (ammo) 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... 

The greater stability of the intermediates arising from attack at the ortho and para posi tions compared with those formed by attack at the position meta to the oxygen sub stituent explains the ortho para directing property of hydroxyl alkoxy and acyloxy groups... 

Alkyl groups are as we saw when we discussed the nitration of toluene in Sec tion 12 10 activating and ortho para directing substituents Aryl and alkenyl substituents resemble alkyl groups in this respect they too are activating and 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... 

Often the directing effects of substituents reinforce each other Brommation of p mtrotoluene for example takes place at the position that is ortho to the ortho para directing methyl group and meta to the meta directing nitro group... 

When the ortho para directing bromine is introduced first the major product is p bro moacetophenone (along with some of its ortho isomer from which it is separated by distillation)... 

Sometimes the orientation of two substituents m an aromatic compound precludes Its straightforward synthesis m Chloroethylbenzene for example has two ortho para directing groups m a meta relationship and so can t be prepared either from chloroben zene or ethylbenzene In cases such as this we couple electrophilic aromatic substitution with functional group manipulation to produce the desired compound... 

The reactivity of arylamines was noted m Section 12 12 where it was pointed out that —NH2 —NHR and —NR2 are ortho para directing and exceedingly powerful... 

Protecting the amino group of an arylamine in this way moderates its reactivity and per mits nitration of the ring to be achieved The acetamido group is activating toward elec trophilic aromatic substitution and is ortho para directing... 

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

Very strongly activating —NHj (amino) Ortho, para-directing... 

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... 

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. 

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