1. Benzene and
Aromatic Compounds

2. The Discovery of Benzene
Micheal Faraday
(1791 – 1867)
English organic chemist
Benzene was discovered in 1825 by the English chemist Michael Faraday (Royal Institution)
Faraday called this new hydrocarbon “bicarburet of hydrogen”.
Faraday isolated benzene from a compressed illuminating gas that had been made by pyrolyzing whale oil.
Eilhardt Mitscherlich
(1794 – 1863)
German organic chemist
In 1834 the German chemist Eilhardt Mitscherlich (University of Berlin) synthesised benzene by heating benzoic acid with calcium oxide.

3. Friedrich August Kekulé German organic chemist
Structure of Benzene
Friedrich August Kekulé
(1829 – 1896)
German organic chemist

4. Hybridization of Benzene

5. Why is Benzene so Unreactive to Addition Reactions?
C-C bond is 1.54 Å
C=C bond is 1.34 Å
But, All Benzene C to C bonds are 1.39 Å
Example:
If each double bond was independent!
HCl

6. Why is Benzene so Unreactive to Addition Reactions?

7. = Aromatic Properties: Planar Cyclic Conjugated
Why is Benzene so Unreactive to Addition Reactions? =
Aromatic Properties:
Planar
Cyclic Conjugated
Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!

8. Aromaticity Why is Benzene so Unreactive to Addition Reactions?
Loss of
Conjugation! NO
Reaction NO
Reaction
Cl2
H2O, H+
HCl H2 Pt NO
Reaction
BH3
H2O2, HO- NO
Reaction
KMnO4 NO
Reaction NO
Reaction
Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!

9. A Substitution Reaction of Benzene
Retains
Conjugation! +
Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!

10. = Huckel’s Rule: The 4n+2p Electron Rule Aromatic Properties: Planar
Cyclic Conjugated
Undergoes Substitution Reactions that retain its
Planer Conjugation – No Electrophilic Addition Rxs!
Hückel 4n +2  electrons =

11. The Annulenes
Annulenes are monocyclic compounds with alternating double and single bonds
Annulenes are named using a number in brackets that indicates the ring size
Benzene is [6]annulene and cyclooctatetraene is [8]annulene
An annulene is aromatic if it has 4n+2p electrons and a planar carbon skeleton
The [14]and [18]annulenes are aromatic (4n+2, where n= 3,4)
The [16] annulene is not aromatic

12. Benzenoid Aromatic Compounds
Polycyclic benzenoid are aromatic compounds have two or more benzene rings fused together

13. Aromatic Compound Nomenclature
Common Names

14. Aromatic Compound Nomenclature
IUPAC Names
1. Name the substituent and then the parent, benzene:
Chlorobenzene
Propylbenzene
Nitrobenzene
flourobenzene
If the alkyl chain has more carbons, then the benzene ring becomes a substituent phenyl (Ph- , C6H6- , φ-):

15. Aromatic Compound Nomenclature
IUPAC Names
3. When two substituent are present, use these isomeric designations:
ortho (1,2-)
meta(1,3-)
para (1,4-)
1,2-dibromobenzene
o-dibromobenzene
1,3-dibromobenzene
m-dibromobenzene
1,4-dibromobenzene
p-dibromobenzene
3-iodophenol
m-iodophenol

16. Aromatic Compound Nomenclature
IUPAC Names
If more than two substituents, number the ring using the lowest possible numbers.
When more than two substituents are present and the substituents are different, list them in alphabetical order .
4-bromo-1,2-dimethyl
benzene
2-chloro-1,4-dinitro
benzene
2,4,6-trinitrotoluene
2,6-dibromophenol
3-chlorobenzoic acid
m-chlorobenzoic acid

17. Reactions of Aromatic Compounds

18. Electrophilic Aromatic Substitution
Arene (Ar-H) is the generic term for an aromatic hydrocarbon
The aryl group (Ar) is derived by removal of a hydrogen atom from an arene
Aromatic compounds undergo electrophilic aromatic substitution (EAS)
The electrophile has a full or partial positive charge

19. A General Mechanism for Electrophilic Aromatic Substitution: Arenium Ion Intermediates
Benzene reacts with an electrophile using two of its p electrons
This first step is like an addition to an ordinary double bond
Unlike an addition reaction, the benzene ring reacts further so that it may regenerate the very stable aromatic system
In step 1 of the mechanism, the electrophile reacts with two p electrons from the aromatic ring to form an arenium ion
The arenium ion is stabilized by resonance which delocalizes the charge
In step 2, a proton is removed and the aromatic system is regenerated

20. Halogenation of Benzene
Halogenation of benzene requires the presence of a Lewis acid.
Fluorination occurs so rapidly it is hard to stop at monofluorination of the ring.
A special apparatus is used to perform this reaction.
Iodine is so unreactive that an alternative method must be used.

21. Sulfonation of Benzene
Nitration of Benzene
Nitration of benzene occurs with a mixture of concentrated nitric and sulfuric acids
The electrophile for the reaction is the nitronium ion (NO2+)
Sulfonation of Benzene
Sulfonation occurs most rapidly using fuming sulfuric acid (concentrated sulfuric acid that contains SO3)
The reaction also occurs in conc. sulfuric acid, which generates small quantities of SO3, as shown in step 1 below

22. Friedel-Crafts Alkylation
An aromatic ring can be alkylated by an alkyl halide in the presence of a Lewis acid
The Lewis acid serves to generate a carbocation electrophile

23. Synthetic Applications of Friedel-Crafts Acylations:
The Clemmensen Reduction
Primary alkyl halides often yield rearranged products in Friedel-Crafts alkylation which is a major limitation of this reaction.
Unbranched alkylbenzenes can be obtained in good yield by acylation followed by Clemmensen reduction.
Clemmensen Reduction reduces phenyl ketones to the methylene (CH2) group.

24. Effects of Substituents on Reactivity and Orientation
The nature of groups already on an aromatic ring affect both the reactivity and orientation of future substitution
Activating groups cause the aromatic ring to be more reactive than benzene
Deactivating groups cause the aromatic ring to be less reactive than benzene
Ortho-para directors direct future substitution to the ortho and para positions
Meta directors direct future substitution to the meta position
Activating Groups: Ortho-Para Directors
All activating groups are also ortho-para directors
The halides are also ortho-para directors but are mildly deactivating
The methyl group of toluene is an ortho-para director
Toluene reacts more readily than benzene, e.g. at a lower temperatures than benzene

25. The methyl group of toluene is an ortho-para director
Amino and hydroxyl groups are also activating and ortho-para directors
These groups are so activating that catalysts are often not necessary
Alkyl groups and heteroatoms with one or more unshared electron pairs directly bonded to the aromatic ring will be ortho-para directors

26. Deactivating Groups: Meta Directors
Strong electron-withdrawing groups such as nitro, carboxyl, and sulfonate are deactivators and meta directors
Halo Substitutents: Deactivating Ortho-Para Directors
Chloro and bromo groups are weakly deactivating but are also ortho, para directors
In electrophilic substitution of chlorobenzene, the ortho and para products are major:

27. Classification of Substitutents

28. Oxidation of the Side Chain
Alkyl and unsaturated side chains of aromatic rings can be oxidized to the carboxylic acid using hot KMnO4

29. Synthetic Applications
When designing a synthesis of substituted benzenes, the order in which the substituents are introduced is crucial
Example: Synthesize ortho-, meta-, and para-nitrobenzoic acid from toluene