From Cherries, Peaches, Almonds ; Benzene: Coal Distillate ; Toluene: Tolu Balsam

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

O CH3 H

Benzene Benzaldehyde Toluene

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

H3C HO O O N CH3

H N O N Cl HH H CH3 H H HO HO

Estrone Morphine Diazepam (tranquilizer)

benzene: toxic, cause bone-marrow depression prolonged exposure → leukopenia (lowered white blood cell count) 15.1 Sources of Aromatic Hydrocarbons - 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.

- petroleum: contains few aromatic compounds, consists largely of alkanes but, during petroleum refining, aromatic molecules are formed (500oC, high pressure) ; eg) heptane is converted to toluene by dehydrogenation and cyclization Some aromatic hydrocarbons found in coal tar

CH3 CH3 H3C

Benzene Toluene Xylenes Indene

Naphthalene Biphenyl Anthracene Phenanthracene 15.2 Nomenclature - a lot of common names are used

CH3 CHO Toluene Benzaldehyde

OH COOH Phenol Benzoic acid

NH2 CN Aniline Benzonitrile

O CH3 C Acetophenone ortho-Xylene CH3 CH3

Cumene Styrene Monosubstituted benzenes: -benzeneas parent name

- as a substituent: C6H5-(phenyl), C6H5CH2-(benzyl),

Br NO2 Et

Bromobenzene Nitrobenzene Ethylbenzene

A benzyl group A phenyl group 2-Phenylheptane Disubstituted benzenes: - ortho (o), meta (m), para (p)

X ortho ortho meta meta

para

ClMe Me CHO

Cl Cl

ortho-Dichlorobenzene meta-Xylene para-Chlorobenzaldehyde Benzenes with more than two substitents - numbering: lowest numbers possible, alphabetically

4 2 Br CH3

CH 1 3

4-Bromo-1,2-dimethylbenzene

4 2 O2NCl

NO 1 2

2-Chloro-1,4-dinitrobenzene Common names as parent names: with the principal substituent assumed to be on carbon 1

CH3 1 OH O2N NO2 2 Br 1 Br 2 4 NO2 2,6-Dibromophenol 2,4,6-Trinitrotoluene

Cl COOH

m-Chlorobenzoic acid 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

H Br H H H H H H Br2 H Br

Iron H H H H H Br H H H H

all six hydrogens + HBr Not formed are equivalent Benzene does not undergo electrophilic addition reactions.

OsO OH OsO4 4 No reaction OH + H O+ H3O 3 No reaction OH

HBr HBr No reaction Br

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

150 kJ/mol (difference)

-356 kJ/mol -230 kJ/mol (expected) -206 kJ/mol (actual) -118 kJ/mol

- benzene has 150 kJ/mol extra stability The Resonance Proposal

H H H H HH

H H H H H H

1.5 bonds average

bond length: C-C (154 pm), C=C (134 pm) but benzene: 139 pm (all C-C bonds are equivalent) - resonance stabilization in benzene: more stable, less reactive than normal isolated alkenes 15.4 Molecular Orbital Description of Benzene

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

HH C C

H C CH CC H H Benzene π molecular orbitals

Antibonding * ψ6

* * ψ4 ψ5 Nonbonding

six p- atomic orbitals

ψ2 ψ3

Bonding

ψ1 six benzene 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

- like alkene, reactive to Br2, KMnO4, HCl 5.7 ppm - not planar, tube shaped H 147 pm

134 pm 15.6 Aromatic Ions Hückel 4n + 2 Rule: applicable to ionic molecules

6 π electrons: aromatic cation, anion

cyclopentadienyl anion cycloheptatrienyl cation Cyclopentadiene: acidic C-H bond

cyclopentadienyl cyclopentadienyl cyclopentadienyl anion radical cation 6π electrons 5π electrons 4π electrons aromatic antiaromatic (highly reactive)

H H pKa = 16 Cycloheptatrienyl cation: aromatic, 6 π electrons

cycloheptatrienyl cycloheptatrienyl cycloheptatrienyl anion radical cation 8π electrons 7π electrons 6π electrons antiaromatic aromatic (highly reactive) HH H Br-

Br2 + HBr

Cycloheptatrienyl cation: all 7 carbons are equally charged Dianion aromatic

2- 2 K 2K+ 2-

10 π electrons aromatic 15.7 Aromatic Heterocycles: Pyridine and Pyrrole

6 π electrons: heteroaromatic rings

4 HH C C lone pair in 3 2 sp orbital H C N N 2 CC 1 H H Pyridine sp2 hybrid 6 π electrons: heteroaromatic rings

lone pair in p orbital

N H N H Pyrrole sp2 hybrid 6 π electrons other heteroaromatic rings

S O Thiophene Furan

lone pair in lone pair in p orbital p orbital

S O

2 sp2 hybrid sp hybrid 6 π electrons 6 π electrons 15.8 Why 4n + 2? - energy levels of 6 π electron system

* ψ6

* * ψ4 ψ5

degenerate Energy six p- atomic orbitals energy levels need 4 π ψ2 ψ3 electrons

need 2 π electrons ψ1 - energy levels of cyclopentadienyl molecular orbitals

* * * ψ * * ψ * ψ4 ψ5 ψ4 5 ψ4 5 Energy five p- atomic orbitals

ψ2 ψ3 ψ2 ψ3 ψ2 ψ3

ψ1 ψ1 ψ1 cation radical anion - Increase by 4 electrons to fill the bonding orbitals for extended cyclic aromatic systems

antibonding orbitals

bonding orbitals

10 π electrons

14 π electrons 15.9 Naphthalene: A Polycyclic Aromatic Compound

Naphthalene Anthracene Benz[a]pyrene Coronene cancer suspect agent many different resonance forms C60 Fullerene and Carbon Nanotube electrophilic aromatic substitution

Br 1 Br2 + HBr Fe heat

10 π electrons: aromatic, fully delocalized throughout both rings

HH C C H C C H C Azulene CC H CH CC H H 15.10 Spectroscopy of Aromatic Compounds IR Spectroscopy - low intensity 3030 cm-1 - occurs left of typical C-H stretching saturated C-H band - complex motion of ring 1450-1600 cm-1 - 1500, 1600 bands are intense C-H out of plane 1660-2000 cm-1 -weak stretching 690-900 cm-1 - strong

690-710 cm-1 690-710 cm-1 Monosubstituted m-Disubstituted 730-770 cm-1 810-850 cm-1 o-Disubstituted 735-770 cm-1 p-Disubstituted 810-840 cm-1 UV Spectroscopy

intense 205 nm less intense 255-275 nm

NMR Spectroscopy

1H NMR 6.5-8.0 ppm 13C NMR 110-140 ppm

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 Inside protons of aromatic ring: shielded, upfield shift

H H H H

H H H Inside H: δ -3.0 H H [18]Annulene H H H H H outside H: δ 9.3

H H HH δ 4.41

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

5.7 ppm H H H 7.37 ppm H 147 pm H H 134 pm H

- benzylic position: downfield shift

H H H 2.3-3.0 ppm

H CH2 R H 13C NMR 1H NMR 6.5-8.0 ppm 13C NMR 110-140 ppm

133.7 128.1 CH3 21.3 Cl 137.7 138.8 126.0 129.3 127.6 128.5 128.4 128.4 125.6 125.4 Summary of Spectroscopic Information on Aromatic Compounds

3030 - aryl C-H stretch

-1 IR (cm ) 1500, 1600 - two absorptions due to ring motions 600-900 - intense C-H out-of-plane bending 205 -intense UV (nm) 255-275 -weak 2.3-3.0 - benzylic protons 1H NMR (δ) 6.5-8.0 - aryl protons 13C NMR (δ) 110-140 - aromatic ring carbons Chemistry @ Work Aspirin, NSAID's, and COX-2 Inhibotors

analgesic, anti-flammatory nonsteroidal anti-inflammatory drug (NSAID)

COOH CH2OH COOH

OH OH O C CH3 O Salicyl alcohol Salicylic acid Aspirin

• Aspirin: toxic side effects; stomach bleeding, long-term allergic reactions Reye's syndrome: fatal, often seen in children recovering from the flu Chemistry @ Work Aspirin, NSAID's, and COX-2 Inhibotors

COOH H CO2H CH CH3 3 MeO

Ibuprofen Naproxen (Advil, Mortin, Nuprin) (Naprosyn, Aleve) Problem Sets

Chapter 15

18, 23, 32, 37, 44