14D_EAS - Electrophilic Aromatic Substitution Part 1 E H...

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1 Electrophilic Aromatic Substitution Part 1 Generic reaction: H E (an electrophile) E Optional reading: OCATSA email for access: harding@chem.ucla.edu
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2 Br Br Br Br Br Br Introduction Fact : Alkenes undergo electrophilic addition •Addition reaction: Increases the number of groups attached to the substrate at the expense of a pi bond Electrophile (induced dipole) Nucleophile (pi bond) How is the Br-Br dipole induced ? δ + δ - δ - δ + EN = 0 No bond dipole Weak C=C/Br-Br repulsion Weak Br-Br polarization Br Br Strong Br-Br polarization Br Br
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3 Br 2 Br Br Addition to Benzene Pi Bonds? Question : Benzene has pi bonds. ..also adds Br 2 ? X No reaction occurs (NR) Why NR? What is special about benzene? •Is benzene a nucleophile? Benzene has pi electrons Benzene is a nucleophile •Is E act too large? Benzene + Br 2 gives a stable intermediate E act probably ok Br Br δ + δ - Η Βρ Η Βρ Η Βρ Resonance-stabilized carbocation
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4 So What It Up With Benzene? Br Br δ + δ - Η Βρ Η Βρ Η Βρ Aromaticity - worth 36 kcal mol -1 of stabilization •Loss of aromaticity = large increase in E act = mechanism step too expensive How to solve this problem? •Make Br 2 more electrophilic Aromatic Not aromatic Br Br FeBr 3 Br Br FeBr 3 δ + e - acceptor Permanent δ + Electrophile = e - deficient Therefore take e - away from Br More electrophilic than Br 2 alone How else is benzene different from an alkene?
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5 Reaction and Mechanism Mechanism ? Br Br FeBr 3 Br H Br H Br H Arenium ion Br H δ + δ + δ + Arenium ion resonance hybrid: Useful reminder Br 2 FeBr 3 product Br Br FeBr 3 Br Br FeBr 3 δ +
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6 Arenium Ion Fate? Arenium ion = a carbocation, so carbocation fates apply Be deprotonated; form pi bond : •Weak base adequate Capture a nucleophile : Aromaticity not restored Aromaticity restored Br H Br FeBr 3 Br + HBr + FeBr 3 Br H Br FeBr 3 Br H Br + FeBr 3
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7 Reaction Name and Kinetics Overall reaction Electrophilic aromatic substitution (EAS) Br 2 FeBr 3 Br + HBr + FeBr 3 Mechanism Br Br FeBr 3 Br H Br H Br FeBr 3 Br + HBr + FeBr 3 Carbocation formed Aromaticity lost Rate-determining step •Carbocation quenched •Aromaticity restored Reaction rate depends on: Nucleophilicity of benzene ring Arenium ion stability Strength of electrophile Kinetics : Which is rds? Br Br FeBr 3 Br Br FeBr 3 δ +
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8 What If Benzene Ring Has Substituent(s)? Toluene 2-Bromotoluene Ortho -bromotoluene 3-Bromotoluene Meta -bromotoluene 4-Bromotoluene Para -bromotoluene CH 3 Br + CH 3 Br + CH 3 Br Br 2 FeBr 3 Br 2 FeBr 3 Br Benzene Only one product possible CH 3 H H H H H
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9 CH 3 Which Product is Major?
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This note was uploaded on 03/12/2012 for the course CHEM 14D taught by Professor Hardinger during the Winter '08 term at UCLA.

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14D_EAS - Electrophilic Aromatic Substitution Part 1 E H...

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