Electrophilic Aromatic Substitution

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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.

• Author Sukumar Honkote
• Chemistry Department, IIT Bombay

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Learning objectives

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

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Definitions
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• 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
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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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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
Play/pause
Restart
What will you learn
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
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REACTION MECHANISM
Note The animation will begin with Reaction
Mechanism (default)
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
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REACTION MECHANISM
The reaction mechanism animation ends here. The
next slide onwards are animation details for
reaction at orbital level.
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Step 1
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Molecular orbital structure of non- resonating
benzene
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Description Audio Narration
Display the above image. This is the molecular orbital structure of non- resonating benzene.
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Master layout 2
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? orbital
Figure (a)
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Only for reference
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Step 2 The attack
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Refer to master layout 2
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Description Audio Narration
Display the master layout 2 as following Figure (a) approaches from left Figure (b) approaches from right
Show the captions EA Orbital and ? Orbital before they start moving towards each other.
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Step 3 Increase and decrease in lobe sizes
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Figure (c)
Figure (d)
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DESCRIPTION TEXT AUDIO
From master layout 2 , as the figures come closer show the following Bending of lobes from figure (a) in master layout 2 Transfer of black material from figure (b) in master layout 2 An electrostatic attraction is developed between the electron rich ? (pi) bond and the electron poor electrophile E.
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Step 4 Transference of electrons
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Figure (e) Figure
(f)
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DESCRIPTION TEXT AUDIO
From step 3 , figure (d) remains as it is. Thus the electron cloud of the ? bond gets shifted to one of the carbons of the bond. Simultaneously A also starts to withdraw more electron from E
show the change in figure (c) and display the above image (e) i) The rods in figure (c) will become conical ii) The black material will start moving from left side lobes to right side as shown iii) The lower, right side lobe becomes smaller Thus the electron cloud of the ? bond gets shifted to one of the carbons of the bond. Simultaneously A also starts to withdraw more electron from E
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Step 5 Formation and breaking of bonds
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Figure (g)
Figure (h)
DESCRIPTION TEXT AUDIO
From step 4 once figure (f) gets attached to figure (e), the conical rods should disappear. Thus the electron cloud of the ? bond fills the E-A antibonding. Hence the E-A bond is broken and C-E bond gets formed The result of this bond breaking and making is also the formation of the empty p orbital and the nucleophile A-
The image should like figure (g) Thus the electron cloud of the ? bond fills the E-A antibonding. Hence the E-A bond is broken and C-E bond gets formed The result of this bond breaking and making is also the formation of the empty p orbital and the nucleophile A-
After some time, show the breaking of E-A bond as shown in figure (h) Thus the electron cloud of the ? bond fills the E-A antibonding. Hence the E-A bond is broken and C-E bond gets formed The result of this bond breaking and making is also the formation of the empty p orbital and the nucleophile A-
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Step 6
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Figure (i)
DESCRIPTION TEXT AUDIO
Display image (i) as shown above with all its labels. Aromaticity of the ring is lost on attack of the ? bond onto the electrophile. Thus the compound is unstable in this state.
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Step 7
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Figure (j)
DESCRIPTION TEXT AUDIO
From previous slide, show bending of white lobes and C- H bond as shown above. An electrostatic attraction is developed between the positively charged empty p orbital and the C-H bond
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Step 8 Transference of electrons
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Figure (k)
DESCRIPTION TEXT AUDIO
Show formation of conical bond Electron cloud moves from the C-H bond to the empty p orbital to form the ? bond and regain aromaticity
The black material is transferred from lobe number 6 and 3 to lobe number 1,2 and 4 via the rod bond. Electron cloud moves from the C-H bond to the empty p orbital to form the ? bond and regain aromaticity
Lobe 4 starts increasing in size Electron cloud moves from the C-H bond to the empty p orbital to form the ? bond and regain aromaticity
Lobe 5 and 6 become equal in size Electron cloud moves from the C-H bond to the empty p orbital to form the ? bond and regain aromaticity
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Step 9 Formation of ? bond
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Figure (l)
DESCRIPTION TEXT AUDIO
The conical bond now becomes a rod Thus ? bond is formed releasing an H (proton). Thus aromatic electrophilic substitution takes place with E replacing a proton.
From previous slide lobes 5 and 6 have become white. Thus ? bond is formed releasing an H (proton). Thus aromatic electrophilic substitution takes place with E replacing a proton.
The white lobes of H (together) move away from the figure (l) Thus ? bond is formed releasing an H (proton). Thus aromatic electrophilic substitution takes place with E replacing a proton.
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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 s bond from a CC in the
  arene nucleophile
• removal of the proton by breaking the C-H s bond
• reforming the CC to restore the aromaticity

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Links for further reading

• Books
• a) Fundamentals of Organic Chemistry by Solomon
  and Graham
• b) Organic Chemistry by Clayden, Greevs, Warren
  and W others

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Questionnaire

• To be given by Prof Anindya Datta