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: 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 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 Concept:

Want to know more… (Further Reading) ‏ Definitions Animation Area Test 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) ‏ (if any) ‏ Play/pauseRestart Output result of interactivity (if any) ‏ What will you learn Credits Reaction at Orbital level

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

REACTION MECHANISM

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

Step Molecular orbital structure of non- resonating benzene Description Animator’s FeedbackAudio Narration Display 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 Figure (b) ‏ a E A EA orbital П orbital Only for reference Figure (a) ‏ A Label : Right Left Labelling for figures should be separate – For Benzene keep figures only on left part of each slide to show change taking place and label as one set; For EA keep figures only on right part of each slide to show change taking place and label as one set [see above]

Description Animator’s FeedbackAudio Narration Display the master layout 2 as following: Figure (a) approaches from left [Diagonally?? Or straight?? Angle??] Figure (b) approaches from right Show the captions EA Orbital and ∏ Orbital before they start moving towards each other. Refer to master layout 2 Step 2: The attack

DESCRIPTION Animator’s FeedbackTEXT & AUDIO From 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 : RightLeft

DESCRIPTION Animator’s FeedbackTEXT & 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 Indexing for figure?? Midway transfer of black material in EA [ see above] ?? Figure (e) C Figure (f) c Step 4: Transference of electrons LeftRight Label :

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 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) ‏ 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) DFigure (h) E Label : Left Right

DESCRIPTION Animator’s Feedback TEXT & AUDIO Display image (i) as shown above with all its labels 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) I Label :

DESCRIPTIONTEXT & 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 Step 7: Figure (j) ‏ Label :II

DESCRIPTIONTEXT & AUDIO Show formation of conical bondElectron 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. Lobe 4 starts increasing in size Lobe 5 and 6 become equal in size Step 8: Transference of electrons Figure (k) III Label :

DESCRIPTIONTEXT & AUDIO The conical bond now becomes a rodThus ∏ 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 Label :

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

Questionnaire To be given by Prof Anindya Datta