© e.v. blackburn, 2011 aromaticity. © e.v. blackburn, 2011 aromatic hydrocarbons originally called...

© E.V. Blackburn, 2011

Aromaticity

N

N N

N

N

N

Hpurine pyrimidine


Aromatic hydrocarbons

Originally called aromatic due to fragrant odors, today this seems strange as many possess distinctly non-fragrant smells!

Their properties differ markedly from those of aliphatic hydrocarbons.

Aromatic hydrocarbons undergo ionic substitution whereas aliphatic hydrocarbons undergo free radical substitution coupled with ionic addition to double and triple bonds.


Nomenclature

Cl NO2

chlorobenzene nitrobenzene


Nomenclature

toluene

benzoic acid

aniline phenol

benzenesulfonic acid

anisole

CH3 NH2 OH

CO2H SO3H OCH3


Nomenclature

BrBr

o-dibromobenzene1,2-dibromobenzene

Cl

NO2

m-chloronitrobenzene1-chloro-3-nitrobenzene

NO2

O2N

p-dinitrobenzene1,4-dinitrobenzene


Nomenclature

NH2

I

p-iodoaniline4-iodoaniline

Cl

ClCl

1,3,5-trichlorobenzene

NO2

NO2O2NCH3

2,4,6-trinitrotoluene


CO2CH3OH

oil of wintergreen

CO2CH3NH2

methyl anthanylate - grape taste and odor

OO

CH2CH=CH2

safrole - root beer smell

CH2CH2NH2

OCH3

OCH3

H3CO

mescaline - euphoric

A few aromatic compounds


Benzene

The molecular formula of benzene is C6H6. How are the atoms arranged?

In 1865 Kekulé proposed that benzene has a “cyclohexatriene” structure:-


Benzene

However there are other structures having this molecular formula:-

Evidence points to the “cyclohexatriene” structure.


Benzene1. There is only one monosubstituted benzene of formula C6H5Y - all benzene hydrogens must therefore be equivalent.

BrBr

Br

Br

Br

Br

2. There are three disubstituted isomers:-


BenzeneHowever......

BrBr

BrBr

single bond

double bond

What is the structure of benzene?

What do we learn in the lab?


Reactions of benzene

Therefore benzene cannot be a simple triene as it does not react with bromine in carbon tetrachloride.

The benzene ring is very stable - it undergoes substitution reactions rather than addition reactions.

Br2/CCl4X

However:

Br2/CCl4 BrBr


Heats of hydrogenation

The heats of hydrogenation and combustion are lower than predicted for a cyclohexatriene structure.



E + H2

Ho = -120 kJ/mol



E + H2

Ho = -120 kJ/mol

+ 2H2

Ho = -232 kJ/mol



E + H2

Ho = -120 kJ/mol

+ 2H2

Ho = -232 kJ/mol

+ 3H2

Ho = -360 kJ/mol



E + H2

Ho = -120 kJ/mol

+ 2H2

Ho = -232 kJ/mol

+ 3H2

Ho = -360 kJ/mol

benzene + 3H2

Ho = -208 kJ/mol



The heats of hydrogenation and combustion are lower than predicted for a cyclohexatriene structure.

The heat of hydrogenation of one mole of benzene is 152 kJ less than that of three moles of cyclohexene.

Benzene is therefore 152 kJ more stable than expected for “cyclohexatriene.”


Benzene is a planar, cyclic molecule containing six atoms of carbon.

All carbon - carbon distances are 1.397Å and all angles are 120o.

The Kekulé structure cannot explain the physical and chemical properties of benzene.

Remember CHEM 261 and the concept of resonance......

“Whenever a molecule can be represented by 2 or more structures which differ only in the arrangement of their electrons, there is resonance.”

The structure of benzene


Resonance

The structure of benzene is a resonance hybrid of the two Kekulé structures:

The resonance hybrid is more stable than any one contributing canonical form (resonance-contributing structure). This energy, 150 kJ, is called the resonance energy.


Orbital description of benzene

HH

HH

H

H

120o

bond

sp2

A planar structure


Orbital description of benzene


Aromatic character

• Compounds do not readily undergo addition reactions.

• Compounds undergo electrophilic substitution reactions.

• Compounds whose molecules are cyclic and planar.

• Compounds whose molecular formulae indicate a high degree of unsaturation.


Hückel’s Rule

Hückel proposed the hypothesis that aromatic compounds possess molecules containing cyclic clouds of electrons delocalised above and below the plane of the molecule and that the electron clouds must contain a total of (4n+2) electrons.

Therefore, in order to possess aromatic character, the number of electrons must be 2 or 6 or 10 etc.


Cyclopentadiene

cyclopentadienyl cation cyclopentadienyl anion

cyclopentadienyl radical

5

aromatic

electrons: 4

antiaromatic

antiaromaticity: R. Breslow, D.R. Murayama, S. Murahashi, and R. Grubbs, J. Amer. Chem. Soc., 95, 6688 (1973).

+

.

-

6


DicyclopentadienylironFerrocene

Fe


The tropylium cation

• Tropylium bromide, C7H7Br, mp > 200C.

• It is soluble in water but insoluble in non-polar solvents.

• It forms a precipitate of silver bromide on addition of AgNO3.


Aromatic character?+ + +

+

HH

NH


Heme

Heme is the prosthetic group (non-peptide portion) of hemoglobin.

Fe

N

N

N

N

H3C

H2C=HC CH3

CH=CH2

CH3

CH2CH2CO2HHO2CH2CH2C

H3C


Aromatic compounds in biochemistry

Three amino acids necessary for protein synthesis contain a benzene ring:

CO2-

NH3+H

phenylalanine

CO2-

NH3+H

HO

tyrosine

CO2-

NH3+H

NH

tryptophan



Humans do not have the biochemical ability to synthesize the benzene ring. Thus phenylalanine and tryptophan derivatives are essential in our diet.

CO2-

NH3+H

CO2-

NH3+H

NH

Tyrosine can be synthesized from phenylalanine in a reaction catalyzed by phenylalanine hydroxylase.



Heterocyclic aromatics are present in many biochemical systems. Thus derivatives of purine and pyrimidine are essential parts of DNA and RNA.

N

N N

N

Hpurine

N

N

pyrimidine

© e.v. blackburn, 2011 aromaticity. © e.v. blackburn, 2011 aromatic hydrocarbons originally called...

Documents

structure of benzene

heats of hydrogenation

dinitrobenzene slide

trinitrotoluene slide

aromaticity slide

reactions of benzene

benzene hydrogens

mole of benzene