Benzene activating ortho-, para-directing

Qualitatively, the effect of the electropositive heteroatom of arsabenzene appears to be comparable to that of an activating ortho-para directing substituent on benzene. Presumably, the electrophile attacks only at the 4- (and 2-)positions because only these positions allow efficient electronic stabilization of the intermediate cr-com-plex 64. 

Protodesilylation takes place faster in the a- than in the y-position. Arsabenzene behaves like a moderately activated ortho-, para-directing substituted benzene toward electrophilic attack. 

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. 

Any substituent whose atom attached to the benzene contains a lone pair of electrons is ortho-para directing (but not necessarily a ring activator). Substituents without a lone pair on the atom attached to the ring are likely meta directors (with the exception of alkyl groups and aromatic rings, which turn out to be ortho-para directors). 

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. 

OH groups on benzene rings are ortho, para-directing and activating... 

This last sequence suggests that the introduction of OMe, or of course OH, could be done by displacement of activated halide. That is simple enough in a pyridine but is also possible in benzenes as halogens are ortho, para-directing and sequences such as 1 to 72 do a similar job. 

---