. Write a short on Ortho - Para - Directing Groups
Except for the alkyl and phenyl substituents, most of the ortho-para-directing groups in Table 2 are of the following general type:
All of these ortho-para directors have at least one lone pair of electrons on the atom attached to the benzene ring. This structural feature - an unshared electron pair on the atom attached to the ring - determines the orientation and influence reactivity in electrophilic substitution reactions.
The directive effect of these groups with an unshared pair is predominantly caused by an electron - releasing resonance effect. The resonance effect, moreover, operates primarily in the arenium ion and, consequently, in the transition state leading to it.
Except for the halogens, the primary effect on reactivity of these groups is also caused by an electron-releasing resonance effect. And, again, this effect also operates in the transition state leading to the arenium ion.
In order to understand these resonance effects let us begin by recalling the effect of the amino group on electrophilic aromatic substitution reactions. The amino group is not only a powerful activating group, it is also a powerful ortho–para director. Aniline reacts with bromine in aqueous solution at room temperature and in the absence of a catalyst to yield a product in which both ortho positions and the para position are substituted.
The inductive effect of the amino group makes it slightly electron withdrawing. Nitrogen, as we know, is more electronegative than carbon. The difference between the electronegativities of nitrogen and carbon in aniline is not large, however, the carbon of the benzene ring is sp2 hybridized and thus is somewhat more electronegative than it would be if it were sp3 hybridized.