Created byProfessor William Tam & Dr. Phillis

Chang Ch. 14 - 1

Chapter 14

Aromatic Compounds

Ch. 14 - 2

About The Authors

These PowerPoint Lecture Slides were created and prepared by Professor William Tam and his wife, Dr. Phillis Chang.

Professor William Tam received his B.Sc. at the University of Hong Kong in 1990 and his Ph.D. at the University of Toronto (Canada) in 1995. He was an NSERC postdoctoral fellow at the Imperial College (UK) and at Harvard University (USA). He joined the Department of Chemistry at the University of Guelph (Ontario, Canada) in 1998 and is currently a Full Professor and Associate Chair in the department. Professor Tam has received several awards in research and teaching, and according to Essential Science Indicators, he is currently ranked as the Top 1% most cited Chemists worldwide. He has published four books and over 80 scientific papers in top international journals such as J. Am. Chem. Soc., Angew. Chem., Org. Lett., and J. Org. Chem.

Dr. Phillis Chang received her B.Sc. at New York University (USA) in 1994, her M.Sc. and Ph.D. in 1997 and 2001 at the University of Guelph (Canada). She lives in Guelph with her husband, William, and their son, Matthew.

Ch. 14 - 3

1. The Discovery of Benzene

Benzene:

In 1825, Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil

or

Ch. 14 - 4

In 1834, a German chemist, Eilhardt Mitscherlich, synthesized benzene by heating benzoic acid with calcium oxide

COOH

+ CaOheat

+ CaCO3

Ch. 14 - 5

In 19th century, organic compounds were classified as being either aliphatic or aromatic

Aliphatic●The chemical behavior of a

compound was “fatlike” Aromatic

●The compound had a low hydrogen-to-carbon ratio and it was “fragrant”

Ch. 14 - 6

2. Nomenclature of BenzeneDerivatives

Naming monosubstituted benzenes● In many simple compounds,

benzene is the parent name and the substituent is simply indicated by a prefixF Cl Br NO2

Fluorobenzene Chlorobenzene Bromobenzene Nitrobenzene

Ch. 14 - 7

● For other simple and common compounds, the substituent and the benzene ring taken together may form a commonly accepted parent name

CH3 O N SO3H

Toluene Phenol Aniline Benzene-sulfonic acid

H H H

OH

O O

O

Anisole

CH3

Benzoic acid Acetophenone

Ch. 14 - 8

Naming disubstituted benzenes● When two substituents are present,

their relative positions are indicated by the prefixes ortho-, meta-, and para- (abbreviated o-, m-, and p-) or by the use of numbers

1,2-Dibromobenzene(o-dibromobenzene)

ortho

Br

Br

Br

1,3-Dibromobenzene(m-dibromobenzene)

meta

1,4-Dibromobenzene(p-dibromobenzene)

para

Br

Br

Br

Ch. 14 - 9

●Other examples

2-Nitrobenzoic acid(o-Nitrobenzoic acid)

NO2

CH3

OH

3-Methylphenol(m-Methylphenol)

4-Chlorotoluene(p-Chlorotoluene)

(1-Chloro-4-methyl-benzene)

CH3

Cl

COOH

Ch. 14 - 10

●The dimethylbenzenes are often called xylenes

1,2-Dimethylbenzene(o-xylene)

CH3

CH3

CH3

1,3-Dimethylbenzene(m-xylene)

1,4-Dimethylbenzene(p-xylene)

CH3

H3C

CH3

Ch. 14 - 11

Naming benzene rings with more than two groups● If more than two groups are

present on the benzene ring, their positions must be indicated by the use of numbers

● The benzene ring is numbered so as to give the lowest possible numbers to the substituents

1,2,3-Trichlorobenzene

1,2,4-Tribromobenzene(not 1,3,4-Tribromobenzene)

Cl

Cl

Cl

12

3

4

5

6

Br

Br12

3

4

5

6

Br

Ch. 14 - 12

●When more than two substituents are present and the substituents are different, they are listed in alphabetical order Cl

F12

3

4

5

6

Br

4-Bromo-1-chloro-2-fluorobenzene

Ch. 14 - 13

●When a substituent is one that, together with the benzene ring gives a new base name, that substituent is assumed to be in position 1 and the new parent name is usedCl

3

2

1

6

5

4

OHCl

3,5-Dichlorophenol

COOH1

6

54

3

2

Br

5-Bromo-2-methylbenzoic acid

H3C

Ch. 14 - 14

●When the C6H5 group is named as a substituent, it is called a phenyl group

●A hydrocarbon composed of one saturated chain and one benzene ring is usually named as a derivative of the larger structural unit. However, if the chain is unsaturated, the compound may be named as a derivative of that chain, regardless of ring size

Ch. 14 - 15

●Examples

Butylbenzene Isopropylbenzene

3

2

1 6 8

4

trans-1-Phenyl-1-butene (R)-3-Phenyloctane

31 752 4

Ch. 14 - 16

●Benzyl is an alternative name for the phenylmethyl group. It is sometimes abbreviated Bn

The benzyl group(the phenylmethyl group)

Benzyl chloride(phenylmethyl chloride

or BnCl)

Cl

Ch. 14 - 17

3. Reactions of Benzene

Br2

CCl4

Br

Br

Br2

CCl4No Reaction

Ch. 14 - 18

OH

OH

No Reaction

1. OsO4

2. NaHSO3

1. OsO4

2. NaHSO3

Ch. 14 - 19

OH

No Reaction

H+

H2O

H+

H2O

Ch. 14 - 20

H2/Ni

25oC, 1 atm

H2/Ni

high temperatureand pressure

Ch. 14 - 21

Benzene undergoes substitution but not addition

Br2

CCl4

Br

Br

(C6H10) (C6H10Br2)

FeBr3(a Lewis acid)

Br2

(C6H6)

H Br

(C6H5Br)

(an addition)

(a substitution)

Ch. 14 - 22

4. The Kekulé Structure for Benzene

C

CC

C

CCH

H

H

H

H

H

or

The Kekulé formula for benzene

Ch. 14 - 23

and

Br

Br

Br

Br

1

23

4

5

61

23

4

5

6

Br

Br

Br

Br

1

23

4

5

61

23

4

5

6

X

These 1,2-dibromobenzenes do not exist as isomers

There is no such equilibrium between benzene ring bond isomers

Ch. 14 - 24

No ReactionBr2

Br2Br

Br

Ch. 14 - 25

5. The Thermodynamic Stabilityof Benzene

Since p bonds are formed from side-way overlap of p orbitals, p electron clouds are above & below the plane of the double bond

p-electrons above and below ring

Ch. 14 - 26

Ch. 14 - 27

6. Modern Theories of the Structureof Benzene

All bond lengths the same (1.39 Å) (compare with C–C single bond 1.54 Å, C=C double bond 1.34 Å)

Extra stabilization due to resonance aromatic

6A.The Resonance Explanation of theStructure of Benzene

C C

Ch. 14 - 28

3-D structure

p-electrons above and below ring

●Planar structure●All carbons sp2 hybridized

Ch. 14 - 29

6B.The Molecular Orbital Explanationof the Structure of Benzene

Ch. 14 - 30

Ch. 14 - 31

7. Hückel’s Rule: The 4n + 2 p Electron Rule

Hückel’s rule is concerned with compounds containing one planar ring in which each atom has a p orbital as in benzene

Planar monocyclic rings containing 4n + 2 p electrons, where n = 0, 1, 2, 3, and so on (i.e., rings containing 2, 6, 10, 14 . . . etc. p electrons), have closed shells of delocalized electrons like benzene and have substantial resonance energies

Ch. 14 - 32

Hückel’s rule states that planar monocyclic rings with 2, 6, 10, 14 . . . delocalized electrons should be aromatic

Ch. 14 - 33

7A.How To Diagram the Relative Energies of p Molecular Orbitals inMonocyclic Systems Based on Hückel’s Rule

Polygon in circle Energy levels of MOs Type of orbital

antibonding orbitals

bonding orbitals

nonbonding orbital

Ch. 14 - 34

The p molecular orbitals that cyclooctatetraene would have if it were planar. Notice that, unlike benzene, this molecule is predicted to have two nonbonding orbitals, and because it has eight p electrons, it would have an unpaired electron in each of the two nonbonding orbitals. Such a system would not be expected to be aromatic.

Ch. 14 - 35

The bonds of cyclooctatetraene are known to be alternately long and short; X-ray studies indicate that they are 1.48 and 1.34 Å, respectively

Ch. 14 - 36

7B.The Annulenes Hückel’s rule predicts that

annulenes will be aromatic if their molecules have 4n + 2 p electrons and have a planar carbon skeleton

Ch. 14 - 37

(4n + 2) planar annulenes:

[14]Annulene(aromatic)

Benzene[6]Annulene

[18]Annulene(aromatic)

All these (4n + 2)p, planar annulenes are aromatic

Ch. 14 - 38

HH

4 5 6

[10]Annulenes(None are aromatic

because none are planar)

Non-planar (4n + 2)p annulenes are antiaromatic

Ch. 14 - 39

[16]AnnuleneCyclobutadiene[4]Annulene

[8]Annulene

(4n)p non-planar annulenes are antiaromatic

Ch. 14 - 40

7C. NMR Spectroscopy: Evidence forElectron Delocalization inAromatic Compounds

The 1H NMR spectrum of benzene consists of a single unsplit signal at d 7.27

The signal occurs at relatively high frequency, which is compelling evidence for the assertion that the p electrons of benzene are delocalized

Ch. 14 - 41

The circulation of p electrons in benzene creates an induced magnetic field that, at the position of the protons, reinforces the applied magnetic field. This reinforcement causes the protons to be strongly deshielded and to have a relatively high frequency (d ~ 7) absorption

Ch. 14 - 42

Ch. 14 - 43

H

H

H

H

H

H

H H

H

H

H

H

H H

H

H

H

H

(d -3.0)

(d 9.3)

Ch. 14 - 44

7D.Aromatic Ions

H HH H

pka = 16pka = 36

Ch. 14 - 45

Bu Li

(a strong base)

H H H

H

HH

strong

base

sp3 sp2

6 p electrons aromatic

Ch. 14 - 46

H H

- H+

8 electrons

H H

- H

6 electrons(aromatic)

Ch. 14 - 47

7E. Aromatic, Antiaromatic, andNonaromatic Compounds

An aromatic compound has its p electrons delocalized over the entire ring and it is stabilized by the p-electron delocalization

Ch. 14 - 48

One way to evaluate whether a cyclic compound is stabilized by delocalization of p electrons through its ring is to compare it with an open-chain compound having the same number of p electrons

Based on sound calculations or experiments

● If the ring has lower p-electron energy, then the ring is aromatic

● If the ring and the chain have the same p-electron energy, then the ring is nonaromatic

● If the ring has greater p-electron energy than the open chain, then the ring is antiaromatic

Ch. 14 - 49

Cyclobutadiene-electron

energy increases+ H2

1,3-Butadiene4 electrons

Cyclobutadiene4 electrons(antiaromatic)

-electron

energy decreases+ H2

1,3,5-Hexatriene6 electrons

Benzene6 electrons(aromatic)

Benzene

Ch. 14 - 50

8. Other Aromatic Compounds

Benzenoid polycyclic aromatic hydrocarbons consist of molecules having two or more benzene rings fused together

8A.Benzenoid Aromatic Compounds

PhenanthreneC14H10

NaphthaleneC10H8

AnthraceneC14H10

PyreneC16H10

1

2

3

45

6

7

8 1

2

3

45

6

7

8 9

10

1

2

3

4

5

6

7

8

9

10

1

2

3 4

5

6

7

89

10

Ch. 14 - 51

8B.Nonbenzenoid AromaticCompounds

(Azulene)

Ch. 14 - 52

8C. Fullerenes

Ch. 14 - 53

9. Heterocyclic Aromatic Compounds

Cyclic compounds that include an element other than carbon are called heterocyclic compounds

Pyridine(electronically

related tobenzene)

N1

2

3

4

1

6

5

4

5

N

H

3

2

1

4

5

O

3

2

1

4

5

S

3

2

Pyrrole

(electronically related tocyclopentadienyl anion)

Furan Thiophene

Ch. 14 - 54

Examples of useful heterocyclic aromatic compounds

N

H

HO

NH2

Serotonin(neurotransmitter)

S

N

N

S

HOOC

COOHO

H

O

Penicillin(antibiotic)

OO2N

N N N

O

H

ONitrofurantoin(urinary antibacterial)

O

N

NN

N

S N

OH

O

O"Viagra"

N

Ch. 14 - 55

Aromaticity

X

X

N

H

X = O, S

N H

6 e : aromatic

Ch. 14 - 56

Aromaticity●Evidence: 1H NMR shift

Z

O

NH

S

H H Z

H H

(2.5 ppm) (3.4 ppm)

(ppm)

7.3

6.4

7.1

6.2

6.2

7.0

Z H

H

(5.5 ppm)

(7.4 ppm)

Ch. 14 - 57

Basicity of nitrogen-containing heterocycles

N

N

N NN

H HH

Order of Basicity: >> >

pKa of theconjugate acid: 11.2 7 5.2 0.4

(c.f. Et3N, pKa of the conjugate acid = 9.7)

Ch. 14 - 58

Basicity of nitrogen-containing heterocycles

N N NHH

H

H

H H(lost of aromaticity)

+ H+

(still aromatic)

Imidazole(a very common basein organic synthesis)

N

N

H

+ H+N

N

N

N

H

H

H

HN

N

H

H

Ch. 14 - 59

NN H1

23

45 (aromatic)

NN H XX H

NN H

H

+

(aromatic)

NN H XX H

NN

H+

(aromatic)6 electrons

H

(non-aromatic)4 electrons

basic nitrogen

Non-basic nitrogen

Ch. 14 - 60

10.Aromatic Compounds in Biochemistry

Two amino acids necessary for protein synthesis contain the benzene ring

Phenylalanine

O

O

NH3

Tyrosine

O

O

NH3HO

Ch. 14 - 61

N

N1

2 34

5N

NH

N

N

67

8

9

5

6 1 2

34

PyrimidinePurine

Derivatives of purine and pyrimidine are essential parts of DNA and RNA

Ch. 14 - 62

N

N

Nicotinamide NH2

N

N

O

POP

O

OO

O

O

OH OH

OH HO

N

H2N

O

Ribose

Ribose

Pyrophosphate

Adenine

Nicotinamide adenine dinucleotide, one of the most important coenzymes in biological oxidations and reductions, includes both a pyridine derivative (nicotinamide) and a purine derivative (adenine) in its structure

Ch. 14 - 63

11.Spectroscopy of AromaticCompounds

The ring hydrogens of benzene derivatives absorb downfield in the region between d 6.0 and d 9.5 ppm

11A. 1H NMR Spectra

The carbon atoms of benzene rings generally absorb in the d 100–170 ppm region of 13C NMR spectra

11B. 13C NMR Spectra

Ch. 14 - 64

Ch. 14 - 65

N

H O

(c)

(d)

N

H O

N

H O

N

H O

A B C D

(c)

(d)

Ch. 14 - 66

N

H O

(c)

(d)

N

H O

N

H O

N

H O

E F G H

(c)

(d)

Ch. 14 - 67

11C. Infrared Spectra of Substituted Benzenes

Ch. 14 - 68

11D. Ultraviolet–Visible Spectra of Aromatic Compounds

O

O

Me2N

O

MeO

O

Octyl-4-N-N-dimethylaminobenzoate(Padimate O)max 310 nm

2-Ethylhexyl 4-methoxycinnamate(Parsol MCX)max 310 nm

Ch. 14 - 69

NCO

O

2-Ethylhexyl 2-cyano-3,3-diphenylacrylate

(Octocrylene)max 310 nm

2-Hydroxy-4-methoxybenzophenone(Oxybenzone)

max 288 and 325 nm

Homomenthyl salicylate(Homosalate)max 309 nm

O OH

OMe

O

O

OHH

Ch. 14 - 70

11E. Mass Spectra of Aromatic Compounds

RCH2

m/2 = 91 m/2 = 91

Y

m/2 = 77

Ch. 14 - 71

END OF CHAPTER 14