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Generic reaction:. Electrophilic Aromatic Substitution Part 1. Optional reading: OCATSA email for access: [email protected] Nucleophile (pi bond). Electrophile (induced dipole). d -. d +. d -. d +. Introduction. Fact : Alkenes undergo electrophilic addition

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introduction

Nucleophile (pi bond)

Electrophile (induced dipole)

d-

d+

d-

d+

Introduction
  • Fact: Alkenes undergo electrophilic addition
  • Addition reaction: Increases the number of groups attached to the substrate at the expense of a pi bond

How is the Br-Br dipole induced?

DEN = 0

No bond dipole

Weak C=C/Br-Br repulsion

Weak Br-Br polarization

Strong Br-Br polarization

addition to benzene pi bonds
Addition to Benzene Pi Bonds?

Question: Benzene has pi bonds...also adds Br2?

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 Eact too large? Benzene + Br2 gives a stable intermediate  Eact probably ok

Resonance-stabilized carbocation

so what it up with benzene

Not aromatic

e- acceptor

Permanent d+

Electrophile = e- deficient

Therefore take e- away from Br

So What It Up With Benzene?

How else is benzene different from an alkene?

Aromatic

  • Aromaticity - worth 36 kcal mol-1 of stabilization
  • Loss of aromaticity = large increase in Eact = mechanism step too expensive
  • How to solve this problem?
  • Make Br2 more electrophilic

FeBr3

More electrophilic than Br2 alone

reaction and mechanism

Useful reminder

Arenium ion

Reaction and Mechanism

Mechanism?

Arenium ion resonance hybrid:

arenium ion fate
Arenium Ion Fate?

Arenium ion = a carbocation, so carbocation fates apply

Capture a nucleophile:

Aromaticity not restored

  • Be deprotonated; form pi bond:
  • Weak base adequate

Aromaticity restored

reaction name and kinetics

Electrophilic aromatic substitution (EAS)

Reaction Name and Kinetics

Overall reaction

Mechanism

Kinetics: Which is rds?

  • 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

what if benzene ring has substituent s

Benzene

+

+

What If Benzene Ring Has Substituent(s)?

Only one product possible

2-Bromotoluene

Ortho-bromotoluene

3-Bromotoluene

Meta-bromotoluene

4-Bromotoluene

Para-bromotoluene

Toluene

which product is major
Which Product is Major?

Number of positions (probability)

Steric hindrance to electrophilic attack

Ortho attack

(between CH3 and H)

More hindered

Two ortho positions

Two meta positions

One para position

Meta attack

(between H and H)

Less hindered

Product ratio conclusion:

40% ortho, 40% meta, 20% para

Para attack

(between H and H)

Less hindered

Product ratio conclusion:

meta, para > ortho

which product is major10
Which Product is Major?

Arenium ion stability

More stable

Ortho attack:

3o

2o

2o

Less stable

Meta attack:

2o

2o

2o

More stable

Para attack:

2o

2o

3o

Product ratio conclusion: ortho, para > meta

which product is major11

Reversed when steric effects are large enough

Which Product is Major?
  • Summary
  • Number of positions: ortho, meta > para
  • Steric effects: meta, para > ortho
  • Arenium ion stability: ortho, para > meta

Which factor dominates?

Ask Mother Nature...

60%

<1%

40%

Conclusion: arenium ion stability >>> number of positions > steric effects

directing effects

}

Electron donation (resonance)

outweighs

electron withdrawing (induction)

Directing Effects
  • CH3 is an ortho/para director
  • Why? Arenium ion stability
  • CH3 stabilizes adjacent carbocation by electron-donating inductive effect

Extension: Any carbocation stabilizing group = ortho/para director

Ortho/para directors

Alkyl groups (-CH3, -CH2CH3, etc.)

Pi bonds: alkene, alkyne, aromatic rings

Lone pairs (-X:) -OH, -OR

-NH2, -NHR, -NR2

-F, -Cl, -Br, -I

Others...

In general... If it stabilizes a carbocation it is an ortho/para director.

substituent effects kinetics

Nucleophilicity:

Arenium ion stability:

Substituent Effects: Kinetics

?

or

Which reacts faster with Br2/FeBr3:

  • Rate-determining step: electrophile + nucleophile  arenium ion
  • Electrophile: same in both cases
  • Nucleophile: CH3 is electron-donating group (EDG)
  • H is neither EDG or EWG

>

  • Arenium ion: CH3 is electron-donating group
  • H is neither EDG or EWG

>

Conclusion: Br2/FeBr3 reacts faster with toluene than with benzene

activating effects

Electron donation (resonance)

outweighs

electron withdrawing (induction)

}

Activating Effects

CH3 is an EAS activator

  • Why? Relative to H...
  • CH3 enhances benzene ring nucleophilicity
  • CH3 increases arenium ion stability
  • CH3 is electron-donating group; stabilizes adjacent carbocation
  • Extension: Any carbocation-stabilizing group is an activator
  • EAS Activating Groups
  • Alkyl groups (-CH3, -CH2CH3, etc.)
  • Pi bonds: alkene, alkyne, aromatic rings
  • Lone pairs (-X:) -OH, -OR
  • -NH2, -NHR, -NR2

Better electron donors = more powerful activators

activating effects an exception
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

Example:

  • OCH3 is ortho/para director
  • OCH3 is activator
  • Reaction faster than benzene + Br2/FeBr3
  • F is ortho/para director
  • F is deactivator
  • Reaction is slower than benzene + Br2/FeBr3
substituent effects the nitro group no 2

Nitro group

Substituent Effects: the Nitro Group (NO2)

Predict major product:

Nitro group structure?

part 1 summary

Example:

arenium ion

Part 1 Summary

Electrophilic aromatic substitution (EAS): electrophilic attack on aromatic ring leads to hydrogen atom replacement

ortho

meta

para

Mechanism:

rds

Substituent effects: CH3 is an ortho/para director and activator

substituent effects the nitro group no 219

Nitro group

Substituent Effects: the Nitro Group (NO2)

Predict major product:

Nitro group structure?

substituent effects the nitro group no 220

Bad !

Substituent Effects: the Nitro Group (NO2)

Arenium ion stability

Ortho attack:

2o

2o

2o

Meta attack:

2o

2o

2o

Para attack:

2o

2o

2o

Product ratio conclusion: meta > ortho > para

directing effects21
Directing Effects
  • NO2 is an meta director
  • Why? Arenium ion stability
  • NO2 destabilizes adjacent carbocation by electron-withdrawing inductive effect
  • Extension: Any carbocation destabilizing group = meta director
  • Meta directors
  • -NO2
  • -+NH3, -+NH2R, -+NHR2, -+NR3
  • -C=O (ketone, aldehyde, ester, carboxylic acid, etc.)
  • -CN
  • -CF3
  • -SO3H sulfonic acid
  • Others...

In general: If it destabilizes a carbocation it is a meta director.

substituent effects kinetics22

Nucleophilicity:

Arenium ion stability:

Substituent Effects: Kinetics

?

or

Which reacts faster with Br2/FeBr3:

  • Rate-determining step: electrophile + nucleophile  arenium ion
  • Electrophile: same in both cases
  • Nucleophile: NO2 is electron-withdrawing group (EWG)
  • H is neither EDG or EWG

<

  • Arenium ion: NO2 is electron-withdrawing group
  • H is neither EDG or EWG

<

Conclusion: Br2/FeBr3 reacts more slowly with nitrobenzene than with benzene

other electrophiles bromination ar h ar br

o/p director

Other Electrophiles: BrominationAr-H  Ar-Br

Origin of electrophile:

--or--

:

Example:

Write out the complete mechanism for yourself

other electrophiles chlorination ar h ar cl

more hindered

*

*

o/p director

*

*

*

*

Other Electrophiles: ChlorinationAr-H  Ar-Cl

Origin of electrophile:

--or--

EDG

Example:

major minor

Write out the complete mechanism for yourself

what is the electrophile

+

electrophile

+

+

What is the Electrophile?

A useful pattern for remembering EAS reactions...

comes from

Bromination:

comes from

Chlorination:

General EAS:

comes from

other electrophiles nitration ar h ar no 2

o/p director

Other Electrophiles: NitrationAr-H  Ar-NO2

Electrophile: +NO2

Origin of electrophile:

--or--

:

Example:

Write out the complete mechanism for yourself

just who is in charge here

ortho/para director

meta director

director

Just Who Is In Charge Here?

Nitration of bromobenzene:

Bromination of nitrobenzene:

Generic EAS reaction:

other electrophiles sulfonation ar h ar so 3 h sulfonic acid

meta director

Other Electrophiles: SulfonationAr-H  Ar-SO3H (sulfonic acid)

Electrophile: +SO3H

Origin of electrophile:

--or--

EWG

Example:

Write out the complete mechanism for yourself

other electrophiles friedel crafts alkylation ar h ar r r alkyl group

How?

Other Electrophiles: Friedel-Crafts AlkylationAr-H  Ar-R (R = alkyl group)

Electrophile: R+ (a carbocation)

Origin of electrophile:

Less stable carbocations

More stable carbocations

  • Carbocation rearrangements possible

Example:

Electrophile = +C(CH3)3

Write out the complete mechanism for yourself

other electrophiles friedel crafts acylation

(a ketone)

How?

Electrophile =

Other Electrophiles: Friedel-Crafts Acylation

Electrophile:

Acylium ion

Origin of electrophile:

Example:

Write out the complete mechanism for yourself

other electrophiles diazo coupling
Other Electrophiles: Diazo Coupling

Electrophile:

Diazonium cation

Origin of electrophile:

  • Mechanism? An OWLS problem

Example:

Write out the complete mechanism for yourself

eas application example synthesis of allura red c

Ar-N=N-Ar suggests synthesis via diazo coupling

Strawberry soda colored with Allura Red AC

EAS Application Example:Synthesis of Allura Red C
  • Allura Red AC (an azo dye)
  • Color due to extensive conjugation
  • Many dyes are diazo compounds
synthesis of allura red c

EWG

Nucleophile

Electrophile

Deactivated

Activated

*

EDG

*

Synthesis of Allura Red C

o/p director

Allura Red AC

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