1. Aromatic hydrocarbons

2. Aromatic hydrocarbons
Originally called aromatic due to fragrant odors, today this seems strange as many posses distinctly non-fragrant smells!
Their properties differ markedly from those of aliphatic hydrocarbons.
Aromatic hydrocarbons undergo ionic substitution whereas aliphatic hydrocarbons undergo ionic addition to double and triple bonds and free radical substitution.

3. Nomenclature
chlorobenzene
nitrobenzene

4. Nomenclature toluene aniline phenol benzoic acid benzenesulfonic acid
anisole

5. Nomenclature o-dibromobenzene 1,2-dibromobenzene m-chloronitrobenzene
p-dinitrobenzene
1,4-dinitrobenzene

6. Nomenclature p-iodoaniline 4-iodoaniline 1,3,5-trichlorobenzene
2,4,6-trinitrotoluene

7. A few aromatic compounds
methyl anthanylate - grape taste and odor
oil of wintergreen
safrole - root beer smell
mescaline - euphoric

8. Benzene
The molecular formula of benzene is C6H6. How are the atoms arranged?
In 1865 Kekulé proposed that benzene has a “cyclohexatriene” structure:-

9. Benzene However there are other structures having this molecular
formula:-
Evidence points to the “cyclohexatriene” structure.

10. Benzene
1. There is only one monosubstituted benzene of formula C6H5Y - all benzene hydrogens must therefore be equivalent.
2. There are three disubstituted isomers:-

11. Benzene However...... double bond single bond
What is the structure of benzene?
What do we learn in the lab?

12. Reactions of benzene
The benzene ring is very stable - it undergoes substitution reactions rather than addition reactions.
However:
Therefore benzene cannot be a simple triene as it does not react with bromine in carbon tetrachloride.

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

14. Heats of hydrogenation

15. Heats of hydrogenation

16. Heats of hydrogenation

17. Heats of hydrogenation

18. 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.”

19. The structure of benzene
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.”

20. 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.

21. Orbital description of benzene
A planar structure

22. Orbital description of benzene

23. Aromatic character
Compounds whose molecular formulae indicate a high degree of unsaturation.
Compounds do not readily undergo addition reactions.
Compounds undergo electrophilic substitution reactions.
Compounds whose molecules are cyclic and planar.

24. 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, 6, 10 etc.

25. Cyclopentadiene cyclopentadienyl cation cyclopentadienyl anion
cyclopentadienyl radical
p electrons: 4 5 6
antiaromatic
aromatic
antiaromaticity: R. Breslow, D.R. Murayama, S. Murahashi, and R. Grubbs, J. Amer. Chem. Soc., 95, 6688 (1973).

26. Dicyclopentadienyliron Ferrocene

27. 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.

28. Aromatic character?

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

30. Aromatic compounds in biochemistry
Three amino acids necessary for protein synthesis contain a benzene ring:

31. 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.
Tyrosine can be synthesized from phenylalanine in a reaction catalyzed by phenylalanine hydroxylase.

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

33. IR spectroscopy Frequency Vibration (cm-1)
benzene ring C-H stretching
1500 and C=C stretching
, C-H bending, mono
C-H bending, ortho
, C-H bending, meta
C-H bending, para

34. Problems
Try problems – and – in Solomons.