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Author: Sukumar Honkote Chemistry Department, IIT Bombay. Electrophilic Aromatic Substitution. Electrophilic Aromatic Substitution involves the attack on the electrophile by the ∏ electrons of the aromatic ring and replacing one of the hydrogen molecules on the ring.

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author sukumar honkote chemistry department iit bombay
Author: Sukumar Honkote

Chemistry Department, IIT Bombay

Electrophilic Aromatic Substitution

Electrophilic Aromatic Substitution involves the attack on the electrophile by the ∏ electrons of the aromatic ring and replacing one of the hydrogen molecules on the ring.

In this Learning object, this reaction has been analysed by understanding the changes that occur in

the molecular orbitals of the reactants.

learning objectives
Learning objectives:
  • After interacting with this learning object, the user will be able to:
  • Explain the process of electrophilic aromatic substitution
definitions
Definitions:

1

  • Electrophile: is a reagent that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile.
  • 2. Nucleophile: is a reagent that forms a chemical bond to its reaction partner (the electrolyte) by donating both bonding electrons.
  • 3. Molecular Orbital: is a mathematical function that
  • describes the wave-like behavior of an electron in a
  • molecule.
  • 4. ∏ Orbital: The molecular orbital of the ∏ bond. It is
  • the shape of the maximum probability function of the ∏
  • electrons.

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concept
Concept:

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1) Aromatic compounds like benzene undergo electrophilic

aromatic substitution reactions.

2) Electrons in a molecule do not remain stationary but move

about the molecule in defined volumes. The shape of these

volumes are given by molecular orbitals. The probability of

finding electrons in these volumes is maximum.

3) The ∏ bond (double bond) is electron rich while an electrophile is electron poor.

4) In the reaction the electron rich ∏ bond is attracted towards

the electrophile E and vice versa. Due to this attraction the

shape of the orbitals (volumes) change and bend towards each other.

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concept1
Concept:

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5) Thus the ∏ bond breaks and a new bond between C & E is

formed. The E-A bond also gets broken.

6) Now a positive charge is created on the adjacent carbon also

the benzene became unstable. Thus the electrons from the

C-H bond reform the ∏ bond and the C-H bond breaks to

form a proton (electronless hydrogen) which leaves. Now the

stability of the benzene is regained.

7) In benzene, the hydrogen is replaced and electrophile E

comes there. Hence its a substitution reaction.

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Credits

What will you learn

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Reaction Mechanism

Reaction at Orbital level

Lets Learn!

Interactivity options

Sliders(IO1)‏

/ Input Boxes(IO2)‏

/Drop down(IO3)‏

(if any)‏

Definitions

Concepts

Test your understanding

(questionnaire)‏

Animation Area

Lets Sum up (summary)‏

Want to know more…

(Further Reading)‏

Output result of interactivity (if any)‏

Instructions/ Working area

reaction mechanism

REACTION MECHANISM

Note: The animation will begin with Reaction Mechanism (default)

reaction mechanism10

REACTION MECHANISM

The reaction mechanism animation ends here. The next slide onwards are animation details for reaction at orbital level.

step 1
Step 1

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Molecular orbital structure of non- resonating benzene

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master layout 2
Master layout 2

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П orbital

Figure (a)

E

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A

EA orbital

Figure (b)

Only for reference

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Step 2: The attack

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Refer to master layout 2

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slide22
Step 4: Transference of electrons

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Figure (e) Figure (f)

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slide24
Step 6:

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Figure (i)

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slide25
Step 7:

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Figure (j)

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summary
Summary:
  • Electrophilic aromatic substitution or EAS is an organic reaction in

which an atom, usually hydrogen, appended to an aromatic system is

replaced by an electrophile.

  • It involves the attack on the electrophile by the ∏ electrons of the aromatic ring and replacing one of the hydrogen molecules on the ring.
  • There are three fundamental components to an electrophilic aromatic substitution mechanism:
    • formation of the new σ bond from a C=C in the arene nucleophile
    • removal of the proton by breaking the C-H σ bond
    • reforming the C=C to restore the aromaticity
links for further reading
Links for further reading

Books:

a) Fundamentals of Organic Chemistry by Solomon and Graham

b) Organic Chemistry by Clayden, Greevs, Warren and W others

questionnaire
Questionnaire

To be given by Prof Anindya Datta

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