Ortho and para directing substituents

The hydroxyl group is a strongly activating, ortho- and para-directing substituent in electrophilic aromatic substitution reactions (Section 16.4). As a result, phenols are highly reactive substrates for electrophilic halogenation, nitration, sulfonation, and lTiedel-Crafts reactions. 

It has been shown that such substitution of hydrogen is probably electrophilic in nature since complex formation is more favorable with ortho- and para-directing substituents on the azobenzene nucleus. Ustynyuk and Barinov (233, 234) have studied the effects of substitution in azobenzene on the reaction with nickelocene, and thereby confirmed the electrophilic nature of the substitution. o-Halogen atoms may undergo substitution as well. 

Although electron-donating substituents activate the ring towards electrophilic attack, they are both ortho and para directing, and an electrophilic... 

Aromatic substituents that increase the rate relative to hydrogen direct the electrophile predominantly to the ortho and para positions. Substituents that... 

Substituents which are electron donating, and can stabilize a positive charge, activate the ring towards electrophilic attack and direct further substitution to the ortho and para positions. Substituents which are electron withdrawing deactivate the ring towards electrophilic attack and direct further substitution to the meta position. 

Substituents on the benzene ring affect both the reactivity of the rii toward further substitution and the orientation of that substitution. Grouj can be classified as ortho- and para-directing activators, ortho- an para-directing deactivators, or meta-directing deactivators. Sul stituents influence aromatic rings by a combination of resonance and indu five effects. Resonance effects are transmitted through -ir bonds indu tive effects are transmitted throu [Pg.654]

---