Lecture Organic chemistry - Chapter 16: Electrophilic attack on derivatives of benzene. This chapter presents the following content: Donors and acceptors, induction and resonance, induction, resonance, disubstituted benzenes- higher substitution, the strongest activator wins,.and other contents. | Electrophilic Attack on Derivatives of Benzene What are the effects of the substituent? Reactivity of the benzene ring b. Regiochemistry of EAS: ortho, meta, para Donors and Acceptors Donors activate Acceptors deactivate Activators: Send electrophile to the ortho and para positions Deactivators: Send electrophile to the meta positions Induction and Resonance Inductive effect: Occurs through the σ framework - Tapers off rapidly with distance -Governed by hyperconjugation and electronegativity of atoms and resulting polarization of bonds Both can operate, not always in the same direction, but resonance usually wins Resonance effect: Usually overrides inductive effect when in competition. -Takes place through π bonds -Longer range -Strong in charged systems Special deactivate A. Induction Same results with HNO3, SO3, Friedel-Crafts reagents: Mainly ortho and para substitution (less ortho, because of sterics). Why ortho-para ? 1. Donors: R = alkyl, activate by induction and . | Electrophilic Attack on Derivatives of Benzene What are the effects of the substituent? Reactivity of the benzene ring b. Regiochemistry of EAS: ortho, meta, para Donors and Acceptors Donors activate Acceptors deactivate Activators: Send electrophile to the ortho and para positions Deactivators: Send electrophile to the meta positions Induction and Resonance Inductive effect: Occurs through the σ framework - Tapers off rapidly with distance -Governed by hyperconjugation and electronegativity of atoms and resulting polarization of bonds Both can operate, not always in the same direction, but resonance usually wins Resonance effect: Usually overrides inductive effect when in competition. -Takes place through π bonds -Longer range -Strong in charged systems Special deactivate A. Induction Same results with HNO3, SO3, Friedel-Crafts reagents: Mainly ortho and para substitution (less ortho, because of sterics). Why ortho-para ? 1. Donors: R = alkyl, activate by induction and hyperconjugation, ortho/para directing Faster than benzene No further bromination, because Br deactivates Mechanism: Steric effect larger with large alkyl group: tert-Bu gives only para Friedel-Crafts reactions with deactivated arenes are too slow, but other E+ work. Why meta? 2. Acceptors: R = -CF3, -CCl3, -C(OR)3, deactivate by electronegativity, meta directing Slower than benzene Mechanism: Wins by default: Least bad option B. Resonance 1. Donors: R = -NH2, -NR’R”, -NHCR’, -OR, activate by resonating lone pairs, ortho/para-directing. Do not even need catalyst in halogenations! O Why ortho/para? Resonance wins over induction Mechanism: EASNH2 Octet!! Octet!! Reminder: Friedel-Crafts reagents too slow. Why meta? 2. Acceptors: R = COOH, CR, NO2, SO3H, C N, deactivate by resonance, meta-directing O Mechanism: EASCO2H 3. Exception: R = X (halogen), deactivate (slightly) by electronegativity. Here: Induction wins over resonance in the starting arene. Nevertheless: ortho/para-directing! Why? Resonance
