THIS PRESENTATION HAS BASIC CONTENTS OF ARENIUM ION.

1. GUIDED BY –
SARASWATI SHARMA MA’AM
DEPARTMENT OF CHEMISTRY
PRESENTED BY-
ADITI SAHU
M.Sc. 2nd SEMESTER
SOS IN CHEMISTRY
PRESENTATION ON
ARENIUM ION
PT.RAVISHANKAR SHUKLA UNIVERSITY,RAIPUR
SOS IN CHEMISTRY

2. CONTENTS
• Introduction
• Structure
• Mechanism
• Orientation and reactivity

3. INTRODUCTION
An arenium ion is a cyclohexadienyl cation that appears as a reactive
intermediate in electrophillic aromatic substitution.
• For historic reasons this complex is also called a Wheland
intermediate, after American chemist George Willard Wheland (1907-
1976).They are also called sigma complexes.
• The smallest arenium ion is the benzenium ion (C6H+
7), which is
protonated benzene.

4. STRUCTURE
• Two hydrogen atoms bonded to one carbon lie in a plane
perpendicular to the benzene ring.
• The arenium ion is no longer an aromatic species; however it is
relatively stable due to delocalization: the positive charge is
delocalized over 3 carbon atoms by the pi system, as depicted on the
following resonance structure:

5. ARENIUM ION MECHANISM
• The mechanism aromatic electrophilic substitution is known as the
arenium ion mechanism and has two main steps-
• Step 1:
The initial step is the attack of an electrophile creating a resonance
stabilized carbocation /intermediate called arenium ion, which is also
known as the Wheland intermediate. Although the Wheland
intermediate or σ-complex is stabilized by resonance, this step is
accompanied by loss of aromaticity so the energy of activation is
high.
This is also the rate-determining step of the reaction because of the
disruption of aromaticity.

6. Step 2:
• In the second step the leaving group departs. This leads to
regeneration of aromatic stabilization.
• The second step is nearly always faster than the first, making the first
rate determining, and the reaction is second order.

7. Energy Profile Diagram of the Arenium Ion
Mechanism of Electrophilic Aromatic Substitution
The energy diagram of this reaction shows that step 1 is highly
endothermic and has a large ∆G #
1.
• The first step requires the loss of aromaticity of the very stable
benzene ring, which is highly unfavourable .
The first step being a slow step, is rate-determining slow.
• Step 2 is highly exothermic and has a small ∆G2
#
• The ring regains its aromatic stabilization, which is a highly favorable
process.

9. ORIENTATION AND REACTIVITY
When an electrophilic substitution reaction is performed on a
monosubstituted benzene, the new group may be directed
primarily to the ortho, meta, or para position.
Also, sometimes, a fourth type of substitution may be encountered
viz., ipso substitution, a special case of electrophilic aromatic
substitution where the leaving group is not hydrogen but the
original substituent itself.
ipso