benzene and aromaticity

Benzene and Aromaticity

Bonding in Benzene

Proposed Benzene Structures

Dewar benzene Ladenburg benzene

Kekule benzene

C6H6 reacts with Br2 to form one isomer of C6H5Br

Ladenburg benzene

Kekule benzene

Br2, cat.1 unique C6H5Br Br2, cat.

3 unique C6H4Br2(+ HBr) (+ HBr)

C6H6

C6H6

Br2, cat.

Br

Br2, cat.1,2- 1,3- & 1,4-dibromo

+ HBr

Other Conjugated Hydrocarbon RingsAnnulenes

Hhyd Data Illustrates Stability

Benzene Representations

Molecular Orbitals of Benzene

+

+

+

+

+

+

+++ +

+

+

++

+

+

+

+

+

--

- -

-

+

-

-

-

-

-

-

-

* *

*

M.O.’s of Cyclobutadiene

+

++

+

+

+

_

_

+

_

+

+

__

1 3

1

4

6 Electrons is Stable4 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 AcidpKa = 16

Cyclopentadienyl ion:One is Aromatic, one is Antiaromatic

Resonance Depictions of the Ions

Tropylium Ion

Aromatic?

c)b)a)NH

BHO

Aromatic, Anti-Aromatic or Nonaromatic?

c)

N

N

H

H

b) a)

Some Well-known Aromatic Compounds

CO2HOCCH3

O

acetyl salicylic acid

HO2Cibuprofin

N

NH

CH3HO2C

lysergic acid

N

S

CO2H

CH2CNH

O

penicillin G

O OH

OH

NCH3

morphine

Designer Drugs

O OH

OH

NCH3

morphine

O OH

OCH3

NCH3

codeine

OOCH3

NCH3

O

oxycodoneoxycontin (slow release)

percocet (w/ acetominophen)

O OAc

OAc

NCH3

heroin

Nomenclature

Br NO2

bromobenzene nitrobenzene ethylbenzene

CH2CH3

Disubstitutionortho, meta and para

X

X

X

X

X

Xortho meta para

Disubstitution Nomenclature

Br

CH2CH2CH3

ortho bromopropylbenzene

NO2

NO2

meta dinitrobenzene

Cl

CH2CH3

para chloroethylbenzene

Aromatic Template Names

toluene xylene mesitylene styrene

CH=CH2CH3

CH3CH3

CH3

CH3

CH3

OH NH2OCH3CH(CH3)2

cumene phenol anisole aniline

More Common Names

CCH3

O

O

acetophenone benzophenone

SO3H CHO CO2H CN

benzene sulfonic acid benzaldehyde benzoic acid benzonitrile

Aromatic Ring as a Substituent

5,7-dimethyl-4-phenyl-1-octene

CH2

Br

trans 1-benzyl-2-bromocycloheptane

Name These

d)c)

Cl

Cl

CH3

b)

NO2

CO2H

a)

OCH3

I

CH=CH2

d)c)

Cl

Cl

CH3

b)

NO2

CO2H

a)

OCH3

I

CH=CH2

meta nitrobenzoic acid 3,5-dichlorotoluene

4-iodo-2-propylstyrene ortho isopropylanisole