2. Benzene
• 1825, Michael Faraday found a compound
with C:H ratio of 1:1.
• 1834, Eilhard Mitscherlich produced the same
compound and found the molecular formula
of C6H6.
• It was then named benzene.
• Other compounds with low C:H ratios were
then classified as aromatic compounds.
2
3. Benzene
• Benzene occurs in two Kekulé structures.
Kekulé structures: resonance structure of the
benzene ring with alternate double and single bonds
3
4. Properties of Benzene
• Benzene ring is planar.
• All C-C bond lengths are the same (1.397 Å)
and all bond angles are 120o.
• Delocalisation of the electrons gives benzene
great stability.
4
5. Reactions of Benzene
• Reacts with Br2 in the presence of FeCl3
catalyst to form bromobenzene + HBr
5
6. Annulenes
• Annulenes: cyclic hydrocarbons with
alternating single and double bonds.
• Eg.: Benzene = 6-annulene.
• Criteria for annulenes: MUST have
– Cycle with conjugated double bonds
– Planar to allow overlapping of π-orbitals
6
7. Exceptions of Aromatic Compounds
• Cyclobutadiene and Cyclooctatetraene are NOT
aromatic compounds because they don’t form
Kekulé structures.
• Cyclobutadiene (C4H4) is very reactive, it
dimerizes (forms C8H8) very quickly.
• Cyclooctatetraene adds Br2 readily.
7
8. Hückel’s Rule
• For a cyclic compound with alternating double
and single bonds, if:
– (4N+2) = aromatic
– (4N) = antiaromatic
– N = 0, 1, 2, …
– Example:
• Benzene (C6H6):
• 6 = 4N+2, N = 1
• ∴ aromatic
• Cyclobutadiene (C4H4):
• 4 = 4N, N =
• ∴ anti-aromatic
8
9. Aromatic, anti-aromatic, non-
anti- non-
aromatic
Cyclic Planar Every C-atom Fulfill
with π orbital Hückel’s Rule
Aromatic Yes Yes Yes 4N+2
Anti-aromatic Yes Yes Yes 4N
Non-aromatic No Maybe No No
9
10. Reactions of Aromatic Compounds
• Halogenation of Benzene
• I) Bromination
– Br2 donates a pair of electron to a strong Lewis
acid catalyst, FeBr3 to form a stronger electrophile.
10
11. Bromination of Benzene
– Benzene attacts to form sigma complex.
– Bromine ion from FeBr4- acts as a weak base to
remove a proton to form bromobenzene and HBr.
11
12. Chlorination of Benzene
• Chlorination is similar to bromination.
• AlCl3 is used as a Lewis acid catalyst together
with Cl2 to form chlorobenzene.
12
14. Nitration of Benzene
• Use sulfuric acid (H2SO4) with nitric acid
(HNO3) to form nitronium ion electrophile.
• Sulfuric acid acts as a catalyst.
14
18. Nitration of Nitrobenzene
• Nitrobenzene is 100,000 less reactive than
benzene.
• Nitration is performed in concentrated HNO3
and H2SO4 >100oC.
• Forms majority meta- isomers.
18
19. Friedel-
Friedel-Crafts Alkylation
• Synthesis of alkylbenzenes from alkyl halides
and a Lewis acid (usually AlCl3 or FeCl3).
19
20. Friedel-
Friedel-Crafts Acylation
• Acyl chloride (RCO-Cl) reacts with benzene to
form phenyl ketone (an acylbenzene).
• Reaction is analogous to alkylation, but the
final product is a phenyl ketone.
20
22. Other Reactions… (I)
• Clemmensen Reduction
acylbenzene alkylbenzene upon treatment with
HCl and amalgamated zinc
Amalgamated: to mix a metal with mercury. 22
23. Other Reactions… (II)
• Chlorination of Benzene
– Occurs with high heat and pressure (or light)
– Results in benzene hexachloride, used as an
insecticide.
Benzene Benzene hexachloride
23
24. Other Reactions… (III)
• Catalytic Hydrogenation of benzene
– With catalysts: Pt, Pd, Ni, Ru or Rh
24
25. Side Chain Oxidation (I)
• Alkylbenzenes can be oxidized to benzoic acid
by hot KMnO4 or Na2Cr2O7 in H2SO4.
• Produces carboxylate salt of benzoic acid.
25