OChem Lecture 8 1 Chapter 15 benzene and aromaticity I Naming and drawing benzenes 15 1 1 Drawing 2 Naming A Monosubstituted pre x benzene B Disubstituted Ortho 1 2 relationship Meta 1 3 relationship Para 1 4 relationship C Tri and poly substituted D Common names toluene styrene phenol aniline II Aromaticity 1 De ne 15 6 A Huckel s Rule electon count of a cyclic conjugated system must t the formula 4n 2 n 0 1 2 B 4n are antiaromatic unstable C Systems that do not apply are called nonaromatic D Charged molecules Lecture 1 10 2 Stability of aromatics benzene 15 2 A Three pi bonds unreactive B Electron cloud pi overlap C Heats of hydrogenation 3 Polycyclic benzeniods 4 Aromatic transitions states p 15 3 III General Reaction of Benzene electrophilic aromatic substitution EAS 15 8 1 Benzene is stable therefore need more reactive electrophile 2 Benzene is electron rich it is a Nucleophile attacking electrophile 3 Mechanism is a net substitution IV Speci c Reactions of Benzene 1 Halogenation 15 9 A Catalyst is needed halogens are weak electrophiles B Electrophile generation is unique to each reaction C Mechanism D Chlorination Cl2 FeCl3 is possible but F2 and I2 are not 2 Nitration 15 10 A Active electrophile is a nitronium ion B Mechanism 3 Sulfonation A Active electrophile is sulfur trioxide SO3 fuming H2SO4 about 8 SO3 B Mechanism C Sulfonate on is reversible 4 Friedel Crafts alkylation 15 11 A Active electrophile is a metallated alkyl halide B Mechanism C Limitations 15 12 i Polyalkylation ii Rearrangements D More useful systems intramolecular reactions E Carbocations 5 Friedel Crafts acylations 15 13 A More useful deactivate the ring monoaddition B Active electrophile is an acylium ion C Mechanism D Aqueous work up required E Can use carboxylate anhydrides Chapter 16 Substituted Benzene I Activation Deactivation of Benzene 16 1 1 Electron density alters the reaction rate rate of EAS increases with more electrons 2 Inductive E ects A Donators activators donate electron density R groups B Acceptors deactivators withdraw electron density 3 Resonance E ects A Will almost always trump induction B Donors are functional groups with a lone pair directly on the ring Qualify as inductive deactivators C Acceptors contain polarized pi bonds directly on the ring 4 Reaction rates II Directing E ects in benzene substituents 1 Standard Alkyl Groups R groups 16 2 A No resonance weak activators through induction therefore ortho para directors B Sterics dictate o p ratio 2 halogenated alkyl groups CF3 etc A No resonance weak strong deactivators induction meta directors 3 Strong electron donors resonance 16 3 A Strong activation through resonance weak deactivation through induction o p directors c Aniline and phenol are superactivated A Withdrawal induction and resonance meta directors 4 Strong deactivators resonance 5 Halogens exception to the rule A Deactivating through induction very weakly activating resonance o p directors III Reactions of polysubstituted benzenes 16 4 1 Competing directors ght for position strongest activator wins e ects are additive Sterics will dictate similar positions IV Synthetic Strategies 16 5 1 Change directing nature 2 F C Reagents DO NOT attack highly deactivated benzenes 3 Blocking Strategies sulfonation is reversible 4 Reducing the activity of FGs A Some FGs can be protected to reduce activity 5 Installing alkyl groups acylation reduction V EAS reactions on benzenoid structures 16 6 1 Naphthalene will raft at C1 why C1 A Attack at C1 will maximize aromatic resonance structures B Activating substituents will e ect the ring that they are attached to o p rules still apply C Deactivating substituents will add at C5 and C8 C5 and C8 are C1 positions on the other ring 2 Larger benzenoid reaction sites are determined by resonance structures Exam Questions What s wrong with the reaction 3 main functional groups that render a ring too deactivated to do F C