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Electrophilic Aromatic Substitution

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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Electrophilic Aromatic Substitution

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  1. 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.

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

  3. 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. 2 3 4 5

  4. Concept: 1 2 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. 3 4 5

  5. Concept: 1 2 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. 3 4 5

  6. Credits What will you learn Play/pause Restart 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

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

  8. REACTION MECHANISM

  9. REACTION MECHANISM

  10. REACTION MECHANISM

  11. REACTION MECHANISM

  12. REACTION MECHANISM

  13. REACTION MECHANISM

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  16. REACTION MECHANISM

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

  18. Step 1 1 2 3 Molecular orbital structure of non- resonating benzene 4 5

  19. Master layout 2 1 2 3 П orbital Figure (a) E 4 A EA orbital Figure (b) Only for reference 5

  20. Step 2: The attack 1 Refer to master layout 2 2 3 4 5

  21. Step 3: Increase and decrease in lobe sizes 1 Figure (c) Figure (d) 2 3 4 5

  22. Step 4: Transference of electrons 1 Figure (e) Figure (f) 2 3 4 5

  23. Step 5: Formation and breaking of bonds 1 2 3 Figure (g) Figure (h) 4 5

  24. Step 6: 1 2 3 Figure (i) 4 5

  25. Step 7: 1 2 3 Figure (j) 4 5

  26. Step 8: Transference of electrons 1 5 2 6 3 1 4 2 3 Figure (k) 4 5

  27. Step 9: Formation of П bond 1 2 6 3 4 3 Figure (l) 4 5

  28. 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

  29. Links for further reading Books: a) Fundamentals of Organic Chemistry by Solomon and Graham b) Organic Chemistry by Clayden, Greevs, Warren and W others

  30. Questionnaire To be given by Prof Anindya Datta

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