1. Benzene and Aromatic Compounds
Chem-160
4/6/2019
D rSeemal Jelani

2. The Structure of Benzene
In benzene, the actual bond length (1.39 Å) is intermediate between the carbon—carbon single bond (1.53 Å) and the carbon—carbon double bond (1.34 Å).
4/6/2019
D rSeemal Jelani

3. Interesting Aromatic Compounds
Benzene and toluene, the simplest aromatic hydrocarbons obtained from petroleum refining, are useful starting materials for synthetic polymers. They are also two of the components of the BTX mixture added to gasoline to boost octane ratings.
Compounds containing two or more benzene rings that share carbon—carbon bonds are called polycyclic aromatic hydrocarbons (PAHs). Naphthalene, the simplest PAH, is the active ingredient in mothballs.
4/6/2019
D rSeemal Jelani

4. KMnO4 oxidation no reaction Br2/CCl4 addition HI H2/Ni reduction
Reagent Cyclohexene Benzene
KMnO4
oxidation
no reaction
Br2/CCl4
addition HI
H2/Ni
reduction
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D rSeemal Jelani

5. 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.
4/6/2019
D rSeemal Jelani

6. Freidel-Crafts alkylation C6H6 + RX, AlCl3  C6H5R + HX substitutions
Reactions of benzene:
Nitration
C6H HNO3, H2SO4  C6H5NO H2O
Sulfonation
C6H H2SO4, SO3  C6H5SO3H H2O
Halogenation
C6H X2, Fe  C6H5X HX
Freidel-Crafts alkylation
C6H RX, AlCl3  C6H5R HX
substitutions
4/6/2019
D rSeemal Jelani

7. The bonds in benzene are all equal and 1.39 Å
e) From X-ray, all of the C—C bonds in benzene are the same length and intermediate in length between single and double bonds.
C—C single bonds  1.50 Å
C = C double bonds  1.34 Å
The bonds in benzene are all equal and 1.39 Å
but 1,3,5-cyclohexatriene has three double bonds and three single bonds!
4/6/2019
D rSeemal Jelani

8. Key Ideas on Benzene Unusually stable
heat of hydrogenation 150 kJ/mol lower than a cyclic triene
Planar hexagon:
bond angles are 120°
carbon–carbon bond lengths 139 pm
Undergoes substitution not addition
Resonance hybrid
One more important factor is the number of electrons in the cyclic orbital
4/6/2019
D rSeemal Jelani

9. Aromaticity E Huckel (1931)
Aromaticity is a property of certain molecules
Chemistry would be similar to benzene
Meet the following criteria
Planar
Mono cyclic system
Conjugated pi system
Contains 4n + 2  electrons
Can apply rules to variety of compounds and determine aromatic nature.
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D rSeemal Jelani

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12. Aromaticity and the 4n + 2 Rule
Huckel’s rule, based on calculations – a planar cyclic molecule with alternating double and single bonds has aromatic stability if it has 4n+ 2  electrons (n is 0,1,2,3,4)
For n=1: 4n+2 = 6
benzene is stable and the electrons are delocalized
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D rSeemal Jelani

13. Number of π electrons = 4 n + 2 6 = 4 n + 2 n = 1
For example, the benzene molecule, which has 3 π bonds or 6 π electrons, is aromatic.
Number of π electrons = 4 n + 2
6 = 4 n + 2
n = 1
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D rSeemal Jelani

14. For n = 0, we get (4 × 0 + 2) = 2 For n = 1, we get (4 × 1 + 2) = 6
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D rSeemal Jelani

15. The letter “n” is not a characteristic of the molecule!
Molecules that have the 3 characteristics listed above (cyclic, conjugated, flat) and have this number of π electrons [4n +2] will be aromatic.
The letter “n” is not a characteristic of the molecule!
4/6/2019
D rSeemal Jelani