Benzene Benzaldehyde

O

H

Toluene

CH3

aromatic: usually fragrant ; benzaldehyde: from cherries, peaches, almonds; benzene: coal distillate; toluene: Tolu balsam

Aromatic; benzene-like, six-membered ring with three double bonds

Ch.15 Benzene and Aromaticity

O

HO

CH3

H

H H

Estrone

aromatic compounds; benzene-like structure

N

N

Cl

OH3C

Diazepam(tranquilizer)

O NCH3

HO

H HHO

H

Morphine

benzene: toxic, cause bone-marrow depression prolonged exposure → leukopenia (lowered white blood cell count)

- come from coal and petroleum- coal: complex mixture, arrays of benzene-like rings- thermal breakdown of coal at 1000oC in the absence of air boils off volatile products coal tar- fractional distillation of coal tar yields benzene, toluene, xylene, naphthalene, etc.

15.1 Sources of Aromatic Hydrocarbons

- petroleum: contains few aromatic compounds, consists largely of alkanesbut, during petroleum refining, aromatic molecules are formed (500oC, high pressure); eg) heptane is converted to toluene by dehydrogenation and cyclization

Benzene Toluene

CH3

Xylenes

CH3H3C

Indene

Naphthalene Biphenyl Anthracene Phenanthracene

Some aromatic hydrocarbons found in coal tar

CH3

OH

NH2

C

CHO

COOH

CN

CH3

CH3

O

CH3

Toluene

Phenol

Aniline

Acetophenone

Benzaldehyde

Benzoic acid

Benzonitrile

ortho-Xylene

Cumene Styrene

15.2 Nomenclature- a lot of common names are used

Br NO2

Bromobenzene Nitrobenzene

Et

Ethylbenzene

Monosubstituted benzenes:- benzene as parent name- as a substituent: C6H5- (phenyl), C6H5CH2- (benzyl),

A phenyl group 2-Phenylheptane

2

1

A benzyl group

Cl Me

ortho-Dichlorobenzene meta-Xylene

Cl

Me CHO

Cl

para-Chlorobenzaldehyde

Disubstituted benzenes:- ortho (o), meta (m), para (p)

X

ortho

meta

para

meta

ortho

Br CH3

CH31

24

4-Bromo-1,2-dimethylbenzene

Benzenes with more than two substitents- numbering: lowest numbers possible, alphabetically

O2N Cl

NO21

24

2-Chloro-1,4-dinitrobenzene

Common names as parent names: with the principal substituent assumed to be on carbon 1

O2N NO21

2

4

2,4,6-Trinitrotoluene

CH3

NO2

Br Br12

2,6-Dibromophenol

OH

Cl COOH

m-Chlorobenzoic acid

H

HH

HH

H Iron

Br2

Br

HH

HH

H

H

HH

BrH

Br

all six hydrogens are equivalent

Not formed

H

H

+ HBr

15.3 Structure and Stability of Benzene

- benzene is relatively unreactive toward most reagents that react with alkenes

- benzene reacts with Br2 in the presence of iron catalyst: substitution product but not addition product

OsO4

H3O+

HBr

OH

OH

OH

Br

Benzene does not undergo electrophilic addition reactions.

OsO4

H3O+

HBr

No reaction

No reaction

No reaction

Why is benzene unreactive compared with other alkenes?Why does benzene give a substitution product rather than an addition product on reaction with Br2?

-118 kJ/mol

-230 kJ/mol-356 kJ/mol(expected)

150 kJ/mol(difference)

-206 kJ/mol(actual)

Heat of hydrogenations

- benzene has 150 kJ/mol extra stability

H

HH

HH

H

H

HH

H H

H

1.5 bonds average

The Resonance Proposal

- resonance stabilization in benzene: more stable, less reactive than normal isolated alkenes

bond length: C-C (154 pm), C=C (134 pm)but benzene: 139 pm (all C-C bonds are equivalent)

15.4 Molecular Orbital Description of Benzene

structural properties of benzene: - flat, symmetrical, regular hexagon- bond angle: 120o, sp2

- six orbitals are equivalent

C

C

C C

C

H

H H

C H

HH

ψ2 ψ3

ψ1

ψ4* ψ5

*

ψ6*

Antibonding

Bonding

Nonbonding

six p- atomic orbitals

six benzene molecular orbitals

Benzene π molecular orbitals

Degenerated: orbitals with same energy level (ψ2 and ψ3, ψ4∗ and ψ5∗)

15.5 Aromaticity and the Hückel 4n + 2 Rule

review of benzene: - a cyclic conjugated molecule- unusually stable- planar, regular hexagon- substitution reaction (not addition reaction)- resonance hybrid

Hückel 4n + 2 Rule: needed to complete a description of aromaticity

For a planar, monocyclic system of conjugation;- aromatic: 4n + 2 electrons, n = 0, 1, 2, 3... (2, 6, 10, 14...), stabilized- antiaromatic: 4n electrons, (4, 8, 12, 16...), unstabilized

Cyclobutadiene: 4 π electrons, antiaromatic, highly reactive

-78oC

Diels-Alder

Benzene: 6 π electrons, aromatic

Cyclooctatetraene: 8 π electrons, antiaromatic

H 147 pm

134 pm

5.7 ppm- like alkene, reactive to Br2, KMnO4, HCl- not planar, tube shaped

15.6 Aromatic Ions

Hückel 4n + 2 Rule: applicable to ionic molecules

cyclopentadienyl anion cycloheptatrienyl cation

6 π electrons: aromatic cation, anion

HHpKa = 16

Cyclopentadiene: acidic C-H bond

cyclopentadienylanion

cyclopentadienylradical

cyclopentadienylcation

6π electrons 5π electrons 4π electrons

aromatic antiaromatic(highly reactive)

cycloheptatrienylanion

cycloheptatrienylradical

cycloheptatrienylcation

8π electrons 7π electrons 6π electronsaromaticantiaromatic

(highly reactive)

Cycloheptatrienyl cation: aromatic, 6 π electrons

H H H

+ HBrBr2

Br-

Cycloheptatrienyl cation: all 7 carbons are equally charged

Dianion aromatic

2 K2-

2K+ 2-

10 π electronsaromatic

15.7 Aromatic Heterocycles: Pyridine and Pyrrole

6 π electrons: heteroaromatic rings

C

C

C C

C

H

H H

N

HHN1

2

34 lone pair in

sp2 orbital

sp2 hybridPyridine

6 π electrons: heteroaromatic rings

NH

N H

6 π electrons

lone pair in p orbital

sp2 hybridPyrrole

other heteroaromatic rings

S

6 π electrons

lone pair in p orbital

sp2 hybrid

SThiophene

OFuran

O

6 π electrons

lone pair in p orbital

sp2 hybrid

15.8 Why 4n + 2?

- energy levels of 6 π electron system

ψ2 ψ3

ψ1

ψ4* ψ5

*

ψ6*

six p- atomic orbitals

Ener

gy

degenerate energy levels need 4 πelectrons

need 2 πelectrons

- energy levels of cyclopentadienyl molecular orbitals

ψ2 ψ3

ψ1

ψ4* ψ5

*

five p- atomic orbitals

Ener

gy

ψ2 ψ3

ψ1

ψ4* ψ5

*

ψ2 ψ3

ψ1

ψ4* ψ5

*

cation radical anion

- Increase by 4 electrons to fill the bonding orbitals for extended cyclic aromatic systems

bonding orbitals

antibonding orbitals

10 π electrons

14 π electrons

15.9 Naphthalene: A Polycyclic Aromatic Compound

Naphthalene Benz[a]pyreneAnthracene Coronenecancer suspect agent

many different resonance forms

C60 Fullerene and Carbon Nanotube

Br2

Feheat

Br

+ HBr

1

electrophilic aromatic substitution

C

C

C C

C

H

H H

C

H

C C

CH

C H

HH

10 π electrons: aromatic, fully delocalized throughout both rings

Azulene

15.10 Spectroscopy of Aromatic CompoundsIR Spectroscopy

810-840 cm-1p-Disubstituted735-770 cm-1o-Disubstituted

690-710 cm-1

810-850 cm-1m-Disubstituted690-710 cm-1

730-770 cm-1Monosubstituted

- strong690-900 cm-1

- complex motion of ring - 1500, 1600 bands are intense

1450-1600 cm-1

- weak1660-2000 cm-1C-H out of plane stretching

- low intensity- occurs left of typical saturated C-H band

3030 cm-1

C-H stretching

UV Spectroscopy

255-275 nmless intense205 nmintense

NMR Spectroscopy

110-140 ppm13C NMR6.5-8.0 ppm1H NMR

ring current: low field shifts (vinylic protons, 4.5-6.5 ppm)

Aromatic ring current: aromatic protons are deshielded by induced magnetic field caused by delocalized π electrons circulating in the molecular orbitals of the aromatic ring

π systems: C=C, C=O

induced magnetic field

proton deshielded by induced field

H0

Inside protons of aromatic ring: shielded, upfield shift

HH

HHH

H

HH

H

H

HH H

H

H

H

HH

[18]Annulene Inside H: δ -3.0

outside H: δ 9.3

O O

HH

δ 4.41

- ring current is characteristic of all Hückel aromatic molecules and is a good test of aromaticity

H 147 pm

134 pm

5.7 ppmH

HH

HH

H

7.37 ppm

H

HH

HH

CH2

2.3-3.0 ppm

R

- benzylic position: downfield shift

CH3 Cl

128.4

21.3137.7

129.3128.5

138.8127.6128.4

125.6 125.4

133.7 128.1

126.0

13C NMR

110-140 ppm13C NMR6.5-8.0 ppm1H NMR

- intense C-H out-of-plane bending600-900

- benzylic protons- aryl protons

2.3-3.06.5-8.0

1H NMR (δ)

- aromatic ring carbons110-14013C NMR (δ)

- weak255-275

- two absorptions due to ring motions1500, 1600

- intense205UV (nm)

- aryl C-H stretch3030

IR (cm-1)

Summary of Spectroscopic Information on Aromatic Compounds

CH2OH

OH

Salicyl alcohol

COOH

OH

Salicylic acid

COOH

O

Aspirin

CO

CH3

analgesic, anti-flammatorynonsteroidal anti-inflammatory drug (NSAID)

Aspirin, NSAID's, and COX-2 Inhibotors

• Aspirin: toxic side effects; stomach bleeding, long-term allergic reactionsReye's syndrome: fatal, often seen in children recovering from the flu

Chemistry @ Work

CH3

Naproxen(Naprosyn, Aleve)

CH3

CO2HH

Ibuprofen(Advil, Mortin, Nuprin)

MeO

COOH

Aspirin, NSAID's, and COX-2 Inhibotors

Chemistry @ Work

Chapter 15

Problem Sets

18, 23, 32, 37, 44