Lokesh Jangid semester 1

1. ARENIUM ION MECHANISM
IN
AROMATIC ELECTROPHILIC SUBSTITUTION
2020-21
SUBMITTED BY : LOKESH JANGID
M.Sc. CHEMISTRY(SEM 1)
DEPARTMENT OF CHEMISTRY
S.P
.C. GOVERNMENT COLLEGE, AJMER

2. CONTENT :-
 Introduction
 Why Aromatic Compounds Undergoes Electrophilic
Substitution Reactions Rather Than Addition Reactions?
 Examples Of Aromatic Electrophilic Substitution Reactions
 Arenium Ion Mechanism
 Steps Involved In Arenium Ion Mechanism
 Evidence Of Arenium Ion Mechanism
 Energy Profile Diagram Of The Arenium Ion Mechanism.
 Summary

3. Introduction
 In aromatic compounds substitution may take place by
electrophilic, nucleophilic, or free radical mechanism.
 There is a pi-electron cloud above and below the aromatic
ring. These π-electrons are loosely held as compared to σ-
electrons and are available to electrophiles.
Thus, electrophilic substitutions are characteristic
reactions of aromatic compounds.
On the other hand, the electron cloud above and below the
plane of an aromatic ring shielded the ring carbon from the
attack of a nucleophile.

4. Why Aromatic Compounds Undergoes
Electrophilic Substitution Reactions
Rather Than Addition Reactions ?
 One of the characteristics of benzene derivatives is that
they tend to undergoed substitution at aromatic carbon
rather than addition reaction (to the double bonds). This
property aromatic compounds is mainly due to their
‘aromaticity’.

5. Examples Of Aromatic Electrophilic
Substitution Reactions

6. Arenium Ion Mechanism
 The most widely accepted mechanism of aromatic
electrophilic substitution reaction is biomolecular and
involves arenium ion Intermediate.
 Some time this mechanism is called as SE2 Mechanism, as
it is biomolecular.

7. Steps Involved In 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 or
intermediate called arenium ion, which is also known as
the Wheland Intermediate.Although the Wheland
Intermediate or sigma-complex or now popularly known as
arenium ion is stabilised by resonance (with charge
dispersal over the carbon ortho and para to the site of
attachment of the electrophile).

8.  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 and reaction follow second
order kinetics.
 The attacking electrophile may be a positive ion or a dipole.
Resonance stabilised arenium
Resonance stabilised arenium ion intermediate

9.  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.

10. Evidence For The Arenium Ion Mechanism
1. Isolation of arenium ion intermediates : A very
strong evidence for the arenium ion mechanism is the
isolation of arenium ion intermediates in many cases.
For example, the arenium ion 1 was heated in the
following reaction and when it was heated, the normal
substitution product 2 was obtained.
Arenium ions have also been detected spectrally

11. 2. Isotope effects : A difference in the rate of a reaction
due to a diffrence in the isotope present in the reaction
system is called isotope effect.
 If the proton is lost before the arrival of the electrophile
(SE1 mechanism) or if the loss of the proton and the
arrival of the electrophile are simultaneous then there
should be a substantial isotope effect(i.e. , deuterated
substrates should undergo substitution more slowly than
undeuterated substrates) because, in each case, the C-H
bond is not broken in rate- determining step, hence no
isotope effect should be found.
 As expected, isotope effect has not been observed in most
aromatic electrophilic substitutions.
 For example, the rate of nitration of deuterio and
tritiobenzene are same as the rate of benzene.

12.  This clearly shows that aromatic electrophilic
substitutions involve two-steps and that the loss of proton
is not the rate-determining step.
 The above facts are incompatible with either SE1 or the
following concerted(simultaneous) mechanism resembling
SN2 mechanism.

13. Energy Profile Diagram Of The Arenium Ion
Mechanism
A more detailed picture of the arenium ion mechanism may be presented by its energy
profile.

14.  The energy diagrame of this reaction shows that step 1 is highly
endothermic and has a large ΔG(1)
 The first step Requires the loss of Aromaticity of very stable benze
ring, which is highly unfavourable.
 The first step is a slow step, is rate-determining.
 Step 2 is highly exothermic and has a small ΔG(2)
 The ring regains its aromatic stabilization, which is highly
favourable process.
 The rate of reaction at ortho, para or meta positions depends on
the height of the energy barrier (ΔG) b/w the reactants and the
T.S.

15. Summary
 Arenium ion/ Wheland intermediate/ σ-complex is the
name of the intermediate involved in the electrophilic
substitution of aromatic compounds. Arenium ion
mechanism has two main steps.
 The first step is rate determining step, involves the attack
of electrophile and loss of aromaticity. The second step
involves departure of the leaving group leading to regain
of aromaticity.
 The isotope effect and the isolation of arenium ion using
various techniques and their spectral studies form the
main evidence for the arenium ion mechanism.
 The first step is highly endothermic and has a large ΔG
while the later step is highly exothermic and has a small
Δ G.

16. Refrences :
 Advanced Organic Chemistry-Reactions, Mechanism and
Structure, Jerry March, John Wiley.
 epgp.inflibnet.ac.in (e-PG Pathshala- A Gateway to all
Post Graduate Courses).

17. Thank You