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
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