ARENIUM ION.pptx

THIS PRESENTATION HAS BASIC CONTENTS OF ARENIUM ION.

1. GUIDED BY – SARASWATI SHARMA MA’AM DEPARTMENT OF CHEMISTRY PRESENTED BY- ADITI SAHU M.Sc. 2 nd 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  ( C 6 H + 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 ∆G 2 # The ring regains its aromatic stabilization, which is a highly favorable process.

8. 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

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