Benzene and Aromaticity

Benzene and Aromaticity

Bonding in Benzene

Proposed Benzene Structures Dewar benzene Ladenburg benzene Kekule benzene

C6H6 reacts with HBr to form one isomer of C6H5Br Br Br2, cat. 1,2- 1,3- & 1,4-dibromo HBr Kekule benzene C6H6 Br2, cat. Ladenburg benzene C6H6 Br2, cat. 1 unique C6H5Br ( HBr) Br2, cat. 3 unique C6H4Br2 ( HBr)

Other Conjugated Hydrocarbon Rings Annulenes

Hhyd Data Illustrates Stability

Benzene Representations

Molecular Orbitals of Benzene - - - - * * - - - - * - -

M.O.’s of Cyclobutadiene - - -

6 Electrons is Stable 4 or 8 Electrons is not

Huckel’s Rule: 4N 2

In Order to Exhibit Aromaticity a Compound Must - Cyclic Planar Conjugated throughout Has a “Huckel #” of electrons o 4N 2 where N is any integer or 0 hence, 2,6,10,14 e- naphthalene

Naphthalene

Pyridine, an Aromatic Heterocycle

Pyridine as a Base

Pyrrole

Pyrrole as a Base

Other Aromatic Heterocycles

Cyclopentadiene as an Acid pKa 16

Cyclopentadienyl ion: One is Aromatic, one is Antiaromatic

Resonance Depictions of the Ions

Tropylium Ion

Aromatic? a) BH NH b) c) O

Aromatic, Anti-Aromatic or Nonaromatic? H a) N b) c) N H

Some Well-known Aromatic Compounds CO 2H O OCCH3 acetyl salicylic acid HO 2C N ibuprofin HO2C CH3 O CH2CNH NH lysergic acid CH3 N O OH OH morphine S N penicillin G CO H 2

CH3 CH3 N N O OH O OH morphine OCH3 codeine CH3 CH3 N N O O OCH3 oxycodone oxycontin (slow release) percocet (w/ acetominophen) O OAc heroin Designer Drugs OH OAc

Nomenclature Br bromobenzene NO2 nitrobenzene CH2CH3 ethylbenzene

Disubstitution ortho, meta and para X X X X X ortho meta X para

Disubstitution Nomenclature ortho bromopropylbenzene meta dinitrobenzene para chloroethylbenzene NO2 CH2CH2CH3 Cl CH2CH3 Br NO2

Aromatic Template Names CH3 toluene CH(CH3)2 cumene CH3 CH3 xylene OH phenol CH3 CH3 CH CH2 CH3 mesitylene OCH3 anisole styrene NH2 aniline

More Common Names SO3H CHO benzene sulfonic acid O benzaldehyde CCH3 acetophenone CO2H benzoic acid O benzophenone CN benzonitrile

Aromatic Ring as a Substituent CH2 Br 5,7-dimethyl-4-phenyl-1-octene trans 1-benzyl-2-bromocycloheptane

Name These NO2 Cl b) a) CO 2H CH3 CH CH2 c) d) I Cl OCH3

NO2 Cl b) a) CO 2H CH3 meta nitrobenzoic acid 3,5-dichlorotoluene CH CH2 c) d) I 4-iodo-2-propylstyrene Cl OCH3 ortho isopropylanisole