1. Aromaticity
Dr. A.K.M. Shafiqul Islam

2. Aromaticity
hydrocarbons
aliphatic aromatic
alkanes alkenes alkynes

3. Aliphatic compounds: open-chain compounds and ring compounds that are chemically similar to open-chain compounds. Alkanes, alkenes, alkynes, dienes, alicyclics, etc.
Aromatic compounds: unsaturated ring compounds that are far more stable than they should be and resist the addition reactions typical of unsaturated aliphatic compounds. Benzene and related compounds.

4. Aromatic Compounds
Aromatic was originally used to described some fragrant compounds in early 19th century.
Later, aromatic was used for specific chemical behavior – cyclic unsaturated compounds that undergo substitution rather than addition.
Current: aromatic compounds distinguished from aliphatic compounds by a special electronic configuration.

5. Benzene
This aromatic hydrocarbon was first discovered in 1825 but its structure was not generally agreed upon until 1946.
Facts about benzene:
Formula = C6H6
Isomer number:
one monosubstituted isomer C6H5Y known
two disubstituted isomers C6H4Y2 known
Benzene resists addition reaction, undergoes substitution reactions.
Heats of hydrogenation and combustion are far lower than they should be.
From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.

6. Reagent Cyclohexene Benzene
KMnO4
oxidation
no reaction
Br2/CCl4
addition HI
H2/Ni
reduction

7. Benzene + 3 H2, Ni, room temp.  NR
Benzene H2, Ni, 200oC, 1500 psi  cyclohexane
Although highly unsaturated, benzene does not react like alkenes, dienes, cycloalkenes, or alkynes (addition reactions) rather it undergoes substitution reactions instead.

8. Kekule Structure of Benzene
Open chain structure not possible
Cyclic structure of benzene proposed by Kekule (1872)

9. Benzene
The Lewis representation of benzene suggests that we deal with a six-membered ring of carbon atoms that are held together by alternating single and double bonds.
C: 6 • 4 = 24 valence electrons
H: 6 • 1 = 6 valence electrons
Total = 30 electrons
= 15 bonds

10. Benzene This implies that we should observe alternating
short (1.33 A) and long (1.54 A) bond lengths.
Measurements indicate that all bond lengths are the same (1.39 A). Again, we need to draw resonance structures.

11. Aromatic Compounds
Aromatic compound: a hydrocarbon that contains one or more benzene-like rings
arene: a term used to describe aromatic compounds
Ar-: a symbol for an aromatic group derived by removing an -H from an arene
Kekulé structure for benzene (1872)

12. But experiments show that the Kekulé structure is not correct
But experiments show that the Kekulé structure is not correct. All C-C bonds are identical and benzene does not undergo addition reactions typical of double bonds.
A correct description is given by resonance theory or by orbital models – valence bond or molecular orbital.

13. Benzene Resonance structure for benzene (1930s)
the theory of resonance, developed by Linus Pauling, provided the first adequate description of the structure of benzene
according to the theory of resonance, certain molecules and ions are best described by writing two or more Lewis structures; the real molecule or ion is a resonance hybrid of these structures
each individual Lewis structure is called a contributing structure
we show that the real molecule is a resonance hybrid of the two or more Lewis structures by using a double-headed arrow between them

14. Benzene here are two contributing structures for benzene
the resonance hybrid has some of the characteristics of each Lewis contributing structure
the length of a carbon-carbon bond in benzene, for example, is midway between that of a carbon-carbon single bond and a double bond

15. Delocalized electrons are not confined between two adjacent bonding atoms, but actually extend over three or more atoms.

16. Hybrid Resonance Structure

17. Hydrogenation of cyclohexene and benzene
The DHº for the hydrogenation of benzene is predicted to be kcal/mole

18. Hydrogenation of cyclohexene and benzene
The DHº for the hydrogenation of benzene was found experimentally to be kcal/mole
This number is much less than calculated

19. Hydrogenation of cyclohexene and benzene

20. The Two Criteria for Aromaticity
It must have an uninterrupted cyclic cloud of p electrons (called a p cloud) above and below the plane of the molecule.
The p cloud must contain an odd number of pairs of p electrons.

21. The Criteria for Aromacity: Hückel's Rule
A molecule must be cyclic, planar, completely conjugated and contain a particular number of p electorn. That is, they are the kind of hydrocarbons treated by Hückel's rule. 6

22. Hückel's rule A molecule must be cyclic a molecule must be planer
a molecule must be conjugated
a molecule must satisfy Hückel's rule and contain a particular number of p electron

23. Aromaticity and the 4n + 2 Rule
Huckel’s rule (based on calculations) – a planar cyclic molecule with conjugated double bonds has aromatic stability if it has 4n+ 2  electrons, where n is 0,1,2,3,4 (any integer)
For n=1: 4n+2 = 6; benzene is stable and the electrons are delocalized

24. Hückel's Rule
among planar, monocyclic, completely conjugated polyenes, only those with 4n p electrons possess special stability (are aromatic)
n 4n+2
0 2
1 6 benzene!
2 10
3 14
4 18 7

25. Applying the Criteria of Aromaticity
Monocyclic compounds with alternating single and double bonds are called annulenes.
Cyclobutadiene, benzene, and cyclooctatetraene are examples of annulenes.

26. Applying the Criteria of Aromaticity
Cyclopentadiene, cyclopentadienyl cation, and cyclopentadienyl anion.
Cyclopentadiene and the cyclopentadienyl cation are not aromatic. The cyclopentadienyl anion is aromatic.

27. Resonance contributors and resonance hybrid for cyclopentadienyl anion.
All carbons are equivalent in the cyclopentadienyl anion.

28. Naphthalene, phenanthrene and chrysene are aromatic

29. Arometic Heterocyclic Compounds
Pyridine, pyrrole, furan, and thiophene are heterocyclic aromatic compounds

30. Orbital structure of pyridine
A pyridine molecule showing p atomic orbitals on the nitrogen and carbon atoms

31. Resonance structures of pyrrole
The lone pair electrons are in a p orbital that overlaps the p orbital of the adjacent carbons, forming a pi bond

32. Orbital structures of pyrrole and furan
Schematic of pyrrole and furan molecules showing p atomic orbitals on the ring

33. Resonance contributors of furan
The oxygen is sp2 hybridized. One lone pair is in an sp2 orbital. One lone pair is in a p orbital that overlaps the p orbitals of adjacent carbons forming a pi bond