Transcript Why is Benzene so Unreactive to Addition Reactions? = Aromatic

The Discovery of Benzene
 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.
Micheal Faraday
(1791 – 1867)
English organic
chemist
 In 1834 the German chemist Eilhardt Mitscherlich
(University of Berlin) synthesised benzene by
heating benzoic acid with calcium oxide.
C6H5CO2H
Benzoic acid
+
CaO
heat
C6H6
Benzene
+
CaCO3
Eilhardt Mitscherlich
(1794 – 1863)
German organic
chemist
Structure of Benzene
Friedrich August Kekulé
(1829 – 1896)
German organic chemist
Hybridization of Benzene
Why is Benzene so Unreactive to Addition Reactions?
H
H
H
H
H
H
H
H
H
H
H
H
• 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!
H
H
H
H
H
H
H
H
H
H
HCl
Cl
H
H
H
Why is Benzene so Unreactive to Addition Reactions?
H
H
H
H
H
H
Why is Benzene so Unreactive to Addition Reactions?
H
H
H
H
H
H
H
H
H
H
H
=
H
H
H
H
H
H
H
Aromatic Properties:
• Planar
• Cyclic Conjugated
• Undergoes substitution reactions that retain its planer
conjugation – No Electrophilic Addition Reactions!
Why is Benzene so Unreactive to Addition Reactions?
Aromaticity
NO
Reaction
Cl
H
Loss of NO
Reaction
Conjugation!
HCl
Cl2
H2
NO
Reaction
H2O, H+
1. BH3
Pt
2. H2O2, HO-
NO
Reaction
KMnO4
NO
Reaction
NO
Reaction
Undergoes substitution reactions that retain its planer
conjugation – No Electrophilic Addition Reactions!
A Substitution Reaction of Benzene
+
Cl2
FeCl3
Cl
Retains
Conjugation!
Undergoes substitution reactions that retain its
planer conjugation – No Electrophilic Addition
Reactions!
Huckel’s Rule: The 4n+2 Electron Rule
Aromatic Properties:
• Planar
• Cyclic Conjugated
• Undergoes Substitution Reactions that retain its
Planer Conjugation – No Electrophilic Addition Rxs!
• Hückel 4n +2  electrons
H
H
H
H
H
H
H
H
H
H
H
=
H
H
H
H
H
H
H
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+2 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
11
Benzenoid Aromatic Compounds
• Polycyclic benzenoid are aromatic compounds have two or
more benzene rings fused together
12
Aromatic Compound Nomenclature
Common Names
Aromatic Compound Nomenclature
IUPAC Names
1. Name the substituent and then the parent, benzene:
Cl
Chlorobenzene
NO2
Propylbenzene
Nitrobenzene
F
flourobenzene
2. If the alkyl chain has more carbons, then the benzene ring becomes a
substituent phenyl (Ph- , C6H6- , φ-):
Aromatic Compound Nomenclature
IUPAC Names
3. When two substituent are present, use these isomeric designations:
X
ortho (1,2-)
ortho (1,2-)
meta(1,3-)
meta(1,3-)
para (1,4-)
Br
Br
Br
1,2-dibromobenzene
o-dibromobenzene
Br
OH
Br
Br
1,3-dibromobenzene
m-dibromobenzene
I
1,4-dibromobenzene
p-dibromobenzene
3-iodophenol
m-iodophenol
Aromatic Compound Nomenclature
IUPAC Names
4.
5.
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
2,6-dibromophenol
2,4,6-trinitrotoluene
3-chlorobenzoic acid
m-chlorobenzoic acid
• 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
• A General Mechanism for Electrophilic Aromatic
Substitution: Arenium Ion Intermediates
• Benzene reacts with an electrophile using two of its  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 
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
• 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.
• 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
• 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
• 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.
• 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
• 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
– 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:
Classification of Substitutents
• Oxidation of the Side Chain
• Alkyl and unsaturated side chains of aromatic rings can be oxidized to the
carboxylic acid using hot KMnO4
• 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