Electrophilic Aromatic Substitution Part 1

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Barbra Shaw
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1 Electrophilic Aromatic Substitution Part 1 Generic reaction: Optional reading: OCATSA instructor for access Introduction Fact: Alkenes undergo electrophilic addition Addition reaction: Increases the number of groups attached to the substrate at the expense of a pi bond Nucleophile (pi bond) Electrophile (induced dipole) How is the Br-Br dipole induced? δ - δ + δ - δ + ΔEN = 0 No bond dipole Weak C=C/Br-Br repulsion Weak Br-Br polarization Strong Br-Br polarization

2 Addition to Benzene Pi Bonds? Question: Benzene has pi bonds...also adds Br 2? No reaction Why NR? What is special about benzene? Is benzene a nucleophile? Benzene has pi electrons Benzene is a nucleophile Is ΔG too large? Benzene + Br 2 gives carbocation with resonance ΔG probably ok So What It Up With Benzene? How else is benzene different from an alkene? Aromaticity - worth 36 kcal mol -1 of stabilization Loss of aromaticity = large increase in ΔG = mechanism step too expensive How to solve this problem? Make Br 2 more electrophilic FeBr 3 Electrophile = e - deficient Therefore take e - away from Br Lewis acid More electrophilic than Br 2 alone

3 Reaction and Mechanism Mechanism? Arenium ion Arenium ion resonance hybrid: Arenium Ion Fate? Arenium ion = a carbocation, so carbocation fates apply Capture a nucleophile: Be deprotonated; form pi bond: Weak base adequate Rearrangement: Does not change overall reaction product Which carbocation fate is favored?

4 Overall reaction Reaction Name and Kinetics Electrophilic aromatic substitution (EAS) Mechanism Kinetics: Which step is rds? Carbocation formed Aromaticity lost Reaction rate depends on: Nucleophilicity of benzene ring Arenium ion stability Strength of electrophile Carbocation quenched Aromaticity restored What If Benzene Ring Has Substituent(s)? Benzene Only one product possible + + Toluene 2-Bromotoluene Ortho-bromotoluene 3-Bromotoluene Meta-bromotoluene 4-Bromotoluene Para-bromotoluene

5 Which Product is Major? Number of positions (probability) Steric hindrance to electrophilic attack o o Ortho attack (between CH 3 and H) More hindered m p m Para attack (between H and H) Less hindered Meta attack (between H and H) Less hindered Product ratio conclusion: Product ratio conclusion: Arenium ion stability Which Product is Major? Ortho attack: Meta attack: Para attack: Product ratio conclusion:

6 Summary Which factor dominates? Ask Mother Nature... Which Product is Major? Number of positions: Ortho, meta > para Steric effects: Meta, para > ortho Arenium ion stability: Ortho, para > meta Conclusion: Arenium ion stability number of positions steric effects CH 3 is an ortho/para director Directing Effects Why? Arenium ion stability CH 3 stabilizes adjacent carbocation by electron-donating inductive effect Extension: Any carbocation stabilizing group = ortho/para director Ortho/para directors Alkyl groups (-CH 3, -CH 2 CH 3, etc.) Pi bonds: Alkene, alkyne, aromatic rings Lone pairs (-X:) -OH, -OR Electron donation (resonance) -NH 2, -NHR, -NR 2 } outweighs -F, -Cl, -Br, -I electron withdrawing (induction) Others... In general... If it stabilizes a carbocation it is an ortho/para director.

7 Substituent Effects: Kinetics Select the faster reaction: or Rate-determining step: Electrophile + nucleophile arenium ion Electrophile: Same in both cases Nucleophilicity: Nucleophile: CH 3 is electron-donating group (EDG) H is neither EDG or EWG Arenium ion: CH 3 is electron-donating group H is neither EDG or EWG Arenium ion stability: Conclusion: Br 2 /FeBr 3 reacts with toluene than with benzene. CH 3 is an EAS activator Activating Effects Why? Relative to H... CH 3 enhances benzene ring nucleophilicity CH 3 increases arenium ion stability CH 3 is electron-donating group; stabilizes adjacent carbocation Extension: Any carbocation-stabilizing group is an activator EAS Activating Groups Alkyl groups (-CH 3, -CH 2 CH 3, etc.) Pi bonds: Alkene, alkyne, aromatic rings Lone pairs (-X:) -OH, -OR -NH 2, -NHR, -NR 2 Better electron donors = more powerful activators } Electron donation (resonance) outweighs electron withdrawing (induction)

8 Activating Effects: An Exception Are all ortho/para directors also activators? Yes, except F, Cl, Br, and I Why? Balance of electron-donation versus electron-withdrawal Does transition state look more like benzene ring or arenium ion? Net effect: F, Cl, Br, I are ortho/para directors, but deactivators OCH 3 is ortho/para director OCH 3 is activator Ph-OCH 3 EAS faster than Ph-H EAS F is ortho/para director F is deactivator Ph-F is slower than Ph-H EAS Substituent Effects: the Nitro Group (NO 2 ) Predict major product: Nitro group Nitro group structure?

9 Electrophilic Aromatic Substitution Part 2 Part 1 Summary Electrophilic aromatic substitution (EAS): Electrophilic attack on aromatic ring leads to hydrogen atom replacement Mechanism: ortho meta para rds Substituent effects: CH 3 is an ortho/para director and activator

10 Substituent Effects: the Nitro Group (NO 2 ) Predict major product: Nitro group Nitro group structure? Substituent Effects: the Nitro Group (NO 2 ) Arenium ion stability Ortho attack: Meta attack: Para attack: Product ratio conclusion:

11 Directing Effects NO 2 is an meta director Why? Arenium ion stability NO 2 destabilizes adjacent carbocation by electron-withdrawing inductive effect Extension: Any carbocation destabilizing group = meta director Meta directors -NO NH 3, - + NH 2 R, - + NHR 2, - + NR 3 -C=O (ketone, aldehyde, ester, carboxylic acid, etc.) -C N -CF 3 -SO 3 H sulfonic acid Others... In general: If it destabilizes a carbocation it is a meta director. Substituent Effects: Kinetics Select the faster reaction: or Rate-determining step: electrophile + nucleophile arenium ion Electrophile: same in both cases Nucleophilicity: Nucleophile: NO 2 is electron-withdrawing group (EWG) H is neither EDG or EWG < Arenium ion: NO 2 is electron-withdrawing group H is neither EDG or EWG Arenium ion stability: < Conclusion: Br 2 /FeBr 3 reacts -NO 2 is a deactivator with nitrobenzene than with benzene.

12 Origin of electrophile: Other Electrophiles: Bromination Ar-H Ar-Br --or-- Write out the complete mechanism for yourself Origin of electrophile: Other Electrophiles: Chlorination Ar-H Ar-Cl --or-- Write out the complete mechanism for yourself

13 What is the Electrophile? A useful pattern for remembering EAS reactions... Bromination: comes from Chlorination: comes from General EAS: comes from Electrophile: + NO 2 Other Electrophiles: Nitration Ar-H Ar-NO 2 Origin of electrophile: Nitric acid --or-- Write out the complete mechanism for yourself

14 More on Directing Effects Just Who Is In Charge Here? Nitration of bromobenzene: Bromination of nitrobenzene: Generic EAS reaction: Electrophile: Other Electrophiles: Sulfonation Ar-H Ar-SO 3 H (sulfonic acid) Origin of electrophile: Sulfur trioxide --or-- Write out the complete mechanism for yourself

15 Other Electrophiles: Friedel-Crafts Alkylation Ar-H Ar-R (R = alkyl group) Electrophile: R + (a carbocation) Origin of electrophile: Elec when R + less stable Elec when R + more stable Carbocation rearrangements possible Elec = + C(CH 3 ) 3 Write out the complete mechanism for yourself Other Electrophiles: Friedel-Crafts Acylation (a ketone) Electrophile: Acylium ion Origin of electrophile: Acid chloride Electrophile = Write out the complete mechanism for yourself

16 Other Electrophiles: Azo Coupling Electrophile: Diazonium cation Origin of electrophile: Mechanism? An OWLS problem Hint: NO H 3 O + + NO Write out the complete mechanism for yourself EAS Application Synthesis of Allura Red AC Ar-N=N-Ar suggests synthesis via azo coupling Allura Red AC (an azo dye) Color due to extensive conjugation Many dyes are azo compounds Strawberry soda colored with Allura Red AC

17 Synthesis of Allura Red AC + Electrophile Azo coupling (EAS)

Benzene Benzene is best represented as a resonance hybrid:

Benzene Benzene is best represented as a resonance hybrid: Electrophilic Aromatic Substitution (EAS) is a substitution reaction usually involving the benzene ring; more specifically it is a reaction in which the hydrogen atom of an aromatic ring is replaced as