electrophilic aromatic substitution author: sukumar honkote chemistry department, iit bombay...

Electrophilic Aromatic Substitution

Author: Sukumar Honkote Chemistry Department, IIT Bombay

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

Definitions:

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1 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.

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:

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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Concept:

Want to know more…(Further Reading)

Definitions

Animation AreaTest your understanding (questionnaire)

Lets Learn!

Concepts

Lets Sum up (summary)

Instructions/ Working area

Reaction Mechanism

Interactivity options

Sliders(IO1)/ Input Boxes(IO2)/Drop down(IO3)

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Output result of interactivity (if any)

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Reaction at Orbital level

REACTION MECHANISM

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

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

Step 11

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4Molecular orbital structure of non- resonating benzene

Description Animator’s Feedback Audio NarrationDisplay the above image.1.Indexing for figure?? [see above]2.Size of black lobes??3.Time length for slide to appear on screen [in seconds]??

This is the molecular orbital structure of non- resonating benzene.

Index –

Orbital ??

Bond ??

Atom ??

Benzene Ring ??

Master layout 2 1

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E A EA orbitalП orbital

Only for reference

Figure (a)A

Label :

Label :

RightLeft

Labelling for figures should be separate – For Benzenekeep figures only on left part of each slide to show change taking place and label as one set; For EA keepfigures only on right part of each slide to show changetaking place and label as one set [see above]

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Description Animator’s Feedback Audio NarrationDisplay the master layout 2 as following:•Figure (a) approaches from left [Diagonally?? Or straight?? Angle??]•Figure (b) approaches from rightShow the captions EA Orbital and ∏ Orbital before they start moving towards each other.

Refer to master layout 2Step 2: The attack

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DESCRIPTION Animator’s Feedback TEXT & AUDIOFrom master layout 2 , as the figures come closer show the following:a)Bending of lobes [from figure (a) in master layout 2]b)Transfer of black material [from figure (b) in master layout 2]1.Indexing for figure?? 2.Difference between black lobes and white lobes??

An electrostatic attraction is developed between the electron rich ∏ (pi) bond and the electron poor electrophile E.

Figure (c) B Figure (d) b

Step 3: Increase and decrease in lobe sizes

Label : Label :

RightLeft

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DESCRIPTION Animator’s Feedback TEXT & AUDIOFrom 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 conicalii) The black material will start moving from left

side lobes to right side as showniii) The lower, right side lobe becomes smaller• Indexing for figure?? • Midway transfer of black material in EA [see above]??

Figure (e) C Figure (f) c

Step 4: Transference of electronsLeft Right

Label : Label :

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DESCRIPTION Animator’s Feedback 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 formedThe 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)After some time, show the breaking of E-A bond as shown in figure (h)Figure E should be on next slide on

Left part of Slide

Step 5: Formation and breaking of bonds

Figure (g) D Figure (h) ELabel :

Left Right

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TEXT & AUDIO

Display image (i) as shown above with all its labels.1. 90 degree angle between C-H and C-E

bond ??2. This is new slide so Indexing and

labelling must be new from now on

Aromaticity of the ring is lost on attack of the ∏ bond onto the electrophile. Thus the compound is unstable in this state.

Step 6:

Figure (i) ILabel :

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4DESCRIPTION TEXT & AUDIOFrom previous slide, show bending ofwhite 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

Step 7:

Figure (j)Label : II

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4DESCRIPTION TEXT & AUDIOShow 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 rodbond.Lobe 4 starts increasing in size Lobe 5 and 6 become equal in size

Step 8: Transference of electrons

Figure (k) III

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Label :

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4DESCRIPTION TEXT & AUDIOThe 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.The white lobes of H (together) move away from the figure (l)

Step 9: Formation of П bond

Figure (l) IV

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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 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

Books: a) Fundamentals of Organic Chemistry by Solomon and

Grahamb) Organic Chemistry by Clayden, Greevs, Warren and W

others

QuestionnaireTo be given by Prof Anindya Datta