© e.v. blackburn, 2011 aromaticity. © e.v. blackburn, 2011 aromatic hydrocarbons originally called...
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© E.V. Blackburn, 2011
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.
© E.V. Blackburn, 2011
Nomenclature
toluene
benzoic acid
aniline phenol
benzenesulfonic acid
anisole
CH3 NH2 OH
CO2H SO3H OCH3
© E.V. Blackburn, 2011
Nomenclature
BrBr
o-dibromobenzene1,2-dibromobenzene
Cl
NO2
m-chloronitrobenzene1-chloro-3-nitrobenzene
NO2
O2N
p-dinitrobenzene1,4-dinitrobenzene
© E.V. Blackburn, 2011
Nomenclature
NH2
I
p-iodoaniline4-iodoaniline
Cl
ClCl
1,3,5-trichlorobenzene
NO2
NO2O2NCH3
2,4,6-trinitrotoluene
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
Benzene
The molecular formula of benzene is C6H6. How are the atoms arranged?
In 1865 Kekulé proposed that benzene has a “cyclohexatriene” structure:-
© E.V. Blackburn, 2011
Benzene
However there are other structures having this molecular formula:-
Evidence points to the “cyclohexatriene” structure.
© E.V. Blackburn, 2011
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:-
© E.V. Blackburn, 2011
BenzeneHowever......
BrBr
BrBr
single bond
double bond
What is the structure of benzene?
What do we learn in the lab?
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
Heats of hydrogenation
The heats of hydrogenation and combustion are lower than predicted for a cyclohexatriene structure.
© E.V. Blackburn, 2011
Heats of hydrogenation
E + H2
Ho = -120 kJ/mol
+ 2H2
Ho = -232 kJ/mol
+ 3H2
Ho = -360 kJ/mol
© E.V. Blackburn, 2011
Heats of hydrogenation
E + H2
Ho = -120 kJ/mol
+ 2H2
Ho = -232 kJ/mol
+ 3H2
Ho = -360 kJ/mol
benzene + 3H2
Ho = -208 kJ/mol
© E.V. Blackburn, 2011
Heats of hydrogenation
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.”
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
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.
© E.V. Blackburn, 2011
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.
© E.V. Blackburn, 2011
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.
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
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.
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
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
© E.V. Blackburn, 2011
Aromatic compounds in biochemistry
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.
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