Ortho-para direction, and reactivity

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... 

Biphenyl is two benzene rings joined by a single bond. The site of substitution for a biphenyl is determined by (1) which phenyl ring is more activated (or less deactivated), and (2) which position on that ring is most reactive, using the fact that a phenyl substituent is activating and ortho, para-directing. 

The effect of monofluorination on alkene or aromatic reactivity toward electrophiles IS more difficult to predict Although a-fluonne stabilizes a carbocation relative to hydrogen, its opposing inductive effort makes olefins and aromatics more electron deficient. Fluorine therefore is activating only fur electrophilic reactions with very late transition states where its resonance stabilization is maximized The faster rate of addition of trifluoroacetic acid and sulfuric acid to 2-fluoropropene vs propene is an example [775,116], but cases of such enhanced fluoroalkene reactivity in solution are quite rare [127] By contrast, there are many examples where the ortho-para-directing fluorme substituent is also activating in electrophilic aromatic substitutions [72 ]... 

---