18.12
Directing Effect of Substituents: ortho–para-Directing Groups
Ortho–para directors are substituent groups attached to the benzene ring and direct the addition of an electrophile to the positions ortho or para to…
Substituents present on the benzene ring that direct the incoming group to a position ortho or para to itself during electrophilic substitution are ortho–para directors.
These substituents work by donating electrons to the benzene ring, thereby increasing the reactivity of the ring towards the electrophile.
Groups with unshared electron pairs on the atom adjacent to the ring donate electrons through resonance.
For example, nitration of phenol occurs more rapidly at the ortho–para positions than at meta because ortho and para carbocation intermediates are more stable. They have more resonance forms, including one stabilized by the oxygen's nonbonding electrons.
Groups without unshared electron pairs donate electrons through the inductive effect.
For instance, during the nitration of toluene, the carbocation intermediates involved in the ortho–para substitution are more stable than meta. The positive charge on the tertiary carbon is stabilized by the electron-donating inductive effect of the methyl group.
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Q1: What are ortho-para directors and how do they work?
Ortho-para directors are electron-donating substituents attached to a benzene ring that direct incoming electrophiles to the ortho or para positions. These groups donate electrons to the ring, increasing its electron density and reactivity toward electrophiles. By stabilizing the carbocation intermediates formed at ortho and para positions through resonance or inductive effects, they make these positions more favorable for substitution.
Q2: How do substituents with lone pairs donate electrons to the benzene ring?
Substituents containing unshared electron pairs on the atom adjacent to the ring, such as amino, hydroxy, or alkoxy groups, donate electrons through resonance. During electrophilic substitution, the nonbonding electrons stabilize the carbocation intermediate by creating additional resonance forms. For example, in phenol nitration, the oxygen's nonbonding electrons stabilize ortho and para carbocation intermediates, making them more stable than meta intermediates.
Q3: Why is phenol nitration faster at ortho and para positions?
Phenol nitration occurs more rapidly at ortho and para positions because the carbocation intermediates formed at these positions are more stable than the meta intermediate. The ortho and para intermediates have more resonance forms, including one particularly favorable form where the oxygen's nonbonding electrons directly stabilize the positive charge on the ring carbon.
Q4: How do alkyl groups function as ortho-para directors?
Alkyl substituents like methyl groups function as ortho-para directors through the inductive effect rather than resonance. They donate electron density to the benzene ring, stabilizing carbocation intermediates at ortho and para positions. At these positions, the positive charge forms on a tertiary carbon, which is effectively stabilized by the electron-donating inductive effect of the alkyl group.
Q5: What is the difference between resonance and inductive electron donation in directing effects?
Resonance donation occurs when substituents have unshared electron pairs that can delocalize into the ring through pi bonding, creating multiple stabilized carbocation forms. Inductive donation occurs when alkyl groups withdraw electron density from the ring through sigma bonds, stabilizing positive charges at adjacent carbons. Both mechanisms make ortho and para positions more reactive toward electrophilic aromatic substitution.
Q6: Why are ortho and para carbocation intermediates more stable than meta intermediates?
Ortho and para carbocation intermediates are more stable because they have more resonance forms than meta intermediates. In ortho and para positions, the positive charge can be delocalized to the atom bearing the electron-donating substituent, creating an especially favorable resonance form. Meta intermediates lack this stabilization option, making them less stable and less likely to form.
Q7: How does toluene nitration demonstrate the inductive effect of alkyl directors?
During toluene nitration, the methyl group stabilizes carbocation intermediates at ortho and para positions through its electron-donating inductive effect. The positive charge on the tertiary carbon adjacent to the methyl group is stabilized more effectively than at the meta position. This makes ortho and para substitution products form preferentially over meta products.