Why is Benzene so Unreactive to Addition Reactions?

BENZENE AND

AROMATIC COMPOUNDS

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

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.

C₆H₅CO₂H + CaO heat C₆H₆ + CaCO₃

Benzoic acid Benzene

Friedrich August Kekulé

( 1829 –

1896 )

German organic chemist

Structure of

Benzene

Hybridization of

Benzene

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

H

H

H H

H

Example:

H

If each double bond was independent

!

HCl

H

H

H H

H H

Cl 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?

HCl Cl₂ ⁺H₂O, H 1. BH₃

2. H₂O₂, HO^- H₂

Pt

KMnO₄ ReactionNO

ReactionNO

ReactionNO

ReactionNO ReactionNO ReactionNO

Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!

Loss of Conjugation

!

H Cl

Aromaticity Aromaticity Why is Benzene so Unreactive to Addition Reactions

?

A Substitution Reaction of Benzene

Undergoes substitution reactions that retain its planer conjugation – No Electrophilic Addition Reactions!

Cl

₂

FeCl

₃

Cl

+ Retains

Conjugation

!

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

=

Huckel’s Rule: The 4n+2  Electron Rule

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+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

12

Benzenoid Aromatic Compounds

Polycyclic benzenoid are aromatic compounds have two or more benzene rings fused together

Aromatic Compound Nomenclature

Common Names

1 Name the substituent and then the parent, benzene .

:

Aromatic Compound Nomenclature

IUPAC Names

Cl _NO

2

2. If the alkyl chain has more carbons, then the benzene ring becomes a substituent phenyl (Ph- , C

₆

H

₆

- , φ-):

Chlorobenzene Propylbenzene Nitrobenzene

F

flourobenzene

X

para (1,4-)

3. When two substituent are present, use these isomeric designations:

Br

Br

Br

Br

Br

Br OH

I

Aromatic Compound Nomenclature

IUPAC Names

1,2 dibromobenzene-

o-dibromobenzene

1,3 dibromobenzene-

m-dibromobenzene

1,4 dibromobenzene-

p-dibromobenzene 3-iodophenol m-iodophenol

ortho (1,2-) ortho (1,2-)

meta(1,3-)

meta(1,3-)

4. If more than two substituents, number the ring using the lowest possible numbers.

5. When more than two substituents are present and the substituents are different, list them in alphabetical order .

Aromatic Compound Nomenclature

IUPAC Names

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

REACTIONS OF AROMATIC

COMPOUNDS

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