In the 19 century, organic compounds Chapter 14home.konkuk.ac.kr/~parkyong/Classes/ch14.pdf · 2 2. Nomenclature of Benzene Derivatives Naming monosubstituted benzenes In many simple

1

Created byProfessor William Tam & Dr. Phillis Chang

Chapter 14

Aromatic Compounds

Copyright © 2014 by John Wiley & Sons, Inc. All rights reserved.

In the 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 was “fragrant”

© 2014 by John Wiley & Sons, Inc. All rights reserved.

© 2014 Pearson Education, Inc.

The Structure of Benzene

© 2014 Pearson Education, Inc.

Resonance ContributorsResonance Hybrid

2

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


● 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© 2014 by John Wiley & Sons, Inc. All rights reserved.

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


● 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


3

● 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


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)

ClCl

Cl

12

3

4

5

6

BrBr1

2

3

45

6

Br© 2014 by John Wiley & Sons, Inc. All rights reserved.

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

ClF1

2

3

45

6

Br

4-Bromo-1-chloro-2-fluorobenzene© 2014 by John Wiley & Sons, Inc. All rights reserved.

● 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

32

1

6

5

4

OHCl3,5-Dichlorophenol

COOH1

6

54

3

2

Br5-Bromo-2-methylbenzoic acid

H3C


4

● 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


● Examples

Butylbenzene Isopropylbenzene

3

2

1 6 8

4

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

31 752 4


● 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


3. Reactions of Benzene

Br2CCl4

Br

Br

Br2CCl4

No Reaction


5

OH

OH

No Reaction

1. OsO4

2. NaHSO3

1. OsO4

2. NaHSO3


OH

No Reaction

H+

H2O

H+

H2O


H2/Ni

25oC, 1 atm

H2/Ni

high temperatureand pressure


Benzene undergoes substitution, but not addition

Br2CCl4

Br

Br(C6H10) (C6H10Br2)

FeBr3(a Lewis acid)

Br2

(C6H6)

H Br

(C6H5Br)

(an addition)

(a substitution)


6

4. The Kekulé Structure for Benzene

CC

CCC

CH

HH

H

HH

or

The Kekulé formula for benzene


and

Br

Br

Br

Br

1

23

4

56

1

23

4

56

Br

Br

Br

Br

1

23

4

56

1

23

4

56

X

These 1,2-dibromobenzenes do not exist as isomers

There is no such equilibrium between benzene ring bond isomers


No ReactionBr2

Br2Br

Br


5. The Thermodynamic Stabilityof Benzene

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

-electrons aboveand below ring


7


6. Modern Theories of the Structure of Benzene

All C C 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


3-D structure

-electrons aboveand below ring

● Planar structure● All carbons sp2 hybridized


6B. The Molecular Orbital Explanationof the Structure of Benzene


8


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

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

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


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

© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 Pearson Education, Inc.

Examples of Compounds That are Not Aromatic

Cyclobutadiene has an even number of pairs of π electrons.

Cyclooctatetraene has an even number of pairs of π electrons andit is not planar.

9

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, and that the molecule has a tub-like shape


7B. The Annulenes Hückel’s rule predicts that annulenes

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


(4n + 2) planar annulenes:

[14]Annulene(aromatic)

Benzene[6]Annulene

[18]Annulene(aromatic)

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


Non-planar (4n + 2) annulenes are non-aromatic


10

© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 by John Wiley & Sons, Inc. All rights reserved.


11


non anti

aromatic


12

7D. Aromatic Ions

H HH H

pka = 16pka = 36


Bu Li(a strong base)

H H H

H

HH

strong

base

sp3 sp2

6 electrons aromatic


H H

- H

6 electrons(aromatic)


Based on sound calculations or experiments● If the ring has lower -electron

energy, then the ring is aromatic● If the ring and the chain have the

same -electron energy, then the ring is nonaromatic

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


13

Cyclobutadiene-electron

energy increases+ H2

1,3-Butadiene4 electrons

Cyclobutadiene4 electrons(antiaromatic)

-electronenergy decreases

+ H2

1,3,5-Hexatriene6 electrons

Benzene6 electrons(aromatic)

Benzene

© 2014 by John Wiley & Sons, Inc. All rights reserved. © 2014 Pearson Education, Inc.

© 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc.

1. must have a ring2. every atom must have a p orbital. otherwise --> nonaromatic 3. ring must be flat. otherwise --> nonaromatic4. 4n+2. if 4n=antiaromatic

14

8. Other Aromatic Compounds

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

8A. Benzenoid Aromatic Compounds


8B. Nonbenzenoid AromaticCompounds

(Azulene)



8C. Fullerenes


15

9. Heterocyclic Aromatic Compounds

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


Examples of useful heterocyclic aromatic compounds

NH

HO

NH2

Serotonin(neurotransmitter)

S

N

N

S

HOOC

COOHO

H

O

Penicillin(antibiotic)

OO2NN N N

O

H

ONitrofurantoin(urinary antibacterial)

O

N

NN

N

S N

OH

OO "Viagra"

N



Aromaticity

X

X

NH

X = O, S

N H

6 e: aromatic


16


Aromaticity● Evidence: 1H NMR shift


Basicity of nitrogen-containing heterocycles

N

N

N NNH 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)


17


Basicity of nitrogen-containing heterocycles

(still aromatic)Imidazole

(a very common basein organic synthesis)

N

NH

+ H+N

N

N

NH

H

H

HN

N

H

H


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 electronsbasic

nitrogen

Non-basic nitrogen


10. Aromatic Compounds in Biochemistry

Two amino acids necessary for protein synthesis contain the benzene ring

Phenylalanine

O

O

NH3

Tyrosine

O

O

NH3HO


18

N

N1

2 3 4

5N

NH

N

N

6 7

8

9

5

6 1 2

34

PyrimidinePurine

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



•The bases with the carbonyl groups (uracil, thymine, cytosine and guanine) are not aromatic as drawn.•However, if we consider the important resonance contributor from the amide systems, then we can see their aromatic character as shown for cytosine below:

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


19

11. Spectroscopy of AromaticCompounds

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

11A. 1H NMR Spectra

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

11B. 13C NMR Spectra


7C. NMR Spectroscopy: Evidence forElectron Delocalization inAromatic Compounds

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

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


The circulation of 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 ( ~ 7) absorption


HH

HH

H

H

H H

H

H

H

H

H H

H

H

H

H

( -3.0)

( 9.3)


20


N

H O

(c)

(d)

N

H O

N

H O

N

H O

A B C D

(c)

(d)



11C. Infrared Spectra of SubstitutedBenzenes


21

11D. Ultraviolet–Visible Spectra ofAromatic Compounds



11E. Mass Spectra of AromaticCompounds


22



23


Documents

In the 19 century, organic compounds Chapter 14home.konkuk.ac.kr/~parkyong/Classes/ch14.pdf · 2 2. Nomenclature of Benzene Derivatives Naming monosubstituted benzenes In many simple