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H. E.  +.  –. +. +. E. Y. H. Y. Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution. part 1. Reactions of Arenes; Electrophilic Aromatic Substitution. Electrophilic Aromatic Substitution

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Chapter 12 Reactions of Arenes: Electrophilic Aromatic Substitution

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H

E

+

–

+

+

E

Y

H

Y

Chapter 12Reactions of Arenes:Electrophilic Aromatic Substitution

part 1


Reactions of Arenes; Electrophilic Aromatic Substitution

  • Electrophilic Aromatic Substitution

    • Halogenation, nitration, sulfonation, Friedel Crafts alkylation, Fridel Crafts Acylation

  • Second Substitution

    • Placement of 2nd substituent (o,p or m)

    • Rate of substitution

  • Third Substitution

  • Electrophilic Aromatic Substitution of Napthalene and Heterocycles


Aromatic Compounds React Differently Than Alkenes

Addition

Substitution


H

E

+

–

+

+

E

Y

H

Y

O

-C-R

Electrophilic aromatic substitutions include:

Nitration (-NO2)

Sulfonation (-SO3H)

Halogenation (-Br or –Cl)

Friedel-Crafts Alkylation (-R)

Friedel-Crafts Acylation


H

H

H

SO2OH

NO2

Br

FeBr3

H2SO4

heat

+

+

+

HONO2

Br2

HOSO2OH

+

+

+

H2O

HBr

H2O


O

O

H

H

C(CH3)3

CCH2CH3

AlCl3

AlCl3

+

+

(CH3)3CCl

CH3CH2CCl

+

+

HCl

HCl


Basic Electrophilic Aromatic Substitution Mechanism

  • Form Strong Electrophile, E+

  • E+ Attacks P electrons of Aromatic Ring

  • Restoration of Aromatic Ring / Catalyst


E+

H

H

E

H

H

H

H

H

+

H

H

H

H

H

Electrophilic Aromatic Substitution Mechanism

Step 1: attack of electrophileon -electron system of aromatic ring

highly endothermic

carbocation is allylic, but not aromatic


H

H

E

H

H

H

H

E

+

H

H

H

H

H

H+

Electrophilic Aromatic Substitution Mechanism

Step 2: loss of a proton from the carbocationintermediate

highly exothermic

this step restores aromaticity of ring


H

NO2

+

Electrophile isnitronium ion

O

N

O

••

••

Nitration of Benzene

NET REACTION

H2SO4

+

HONO2

+

H2O


Step 1a; Formation of Strong Electrophile


NO2+

H

H

H

H

NO2

NO2

H

H

H

H

H

H

+

+

H

H

H

H

H

H

H

H

Step 1b: attack of nitronium cationon -electron system of aromatic ring

Step 2: Water molecule will abstract a proton from the carbocation intermediate

H

H

H

NO2

H

H

H+


H

SO2OH

••

••

O

O

+

••

Several electrophiles present:

a major one is sulfur trioxide

S

O

••

Sulfonation of Benzene

heat

+

HOSO2OH

+

H2O


+

••

••

••

••

+

Br

Br

Br

Br

FeBr3

FeBr3

H

Br

FeBr3

••

••

••

••

+

Br2

+

HBr

Lewis base

Lewis acid

Complex

Halogenation of Benzene

NET REACTION

Electrophile is a Lewis acid-Lewis basecomplex between FeBr3 and Br2.


H

C(CH3)3

H3C

+

Electrophile is tert-butyl cation

CH3

C

H3C

Friedel-Crafts Alkylation of Benzene

AlCl3

+

(CH3)3CCl

+

HCl


+

••

AlCl3

(CH3)3C

Cl

••

+

••

+

AlCl3

(CH3)3C

Cl

••

Role of AlCl3

acts as a Lewis acid to promote ionizationof the alkyl halide

••

+

(CH3)3C

Cl

AlCl3

••


Limitations to Friedel Crafts Alkylation

  • Linear alkyl groups can not be put on ring this way due to carbocation rearrangement.


H

C(CH3)3

AlCl3

+

(CH3)2CHCH2Cl

Isobutyl chloride

tert-Butylbenzene(66%)

Rearrangements in Friedel-Crafts Alkylation

Carbocations are intermediates.

Therefore, rearrangements can occur


O

O

H

CCH2CH3

AlCl3

+

CH3CH2CCl

+

HCl

Electrophile is an acyl cation

+

+

••

CH3CH2C

O

CH3CH2C

O

Friedel-Crafts Acylation of Benzene

Note; Since carbocation is stabalized by

resonance; rearrangement is unlikely

(this is different than alkylation)


O

O

H

CR

Zn(Hg), HCl

CH2R

Putting a Linear Alkyl Group on Ring;Acylation-Reduction

H2NNH2, KOH,triethylene glycol,heat

RCCl

OR

AlCl3


(CH3)2CHCH2Cl

CH2CH(CH3)3

C(CH3)3

AlCl3

Example: Prepare isobutylbenzene

isobutylbenzene

No! Friedel-Crafts alkylation of benzene using isobutyl chloride fails because of carbocation rearrangement.

Actual Product


O

(CH3)2CHCCl

CH2CH(CH3)3

Zn(Hg)HCl

O

CCH(CH3)2

Use Acylation-Reduction Instead

+

AlCl3


12.9Rate and Regioselectivity in Electrophilic Aromatic Substitution

A substituent already present on the ring can affect both the rate and regioselectivityof electrophilic aromatic substitution.


H

H

H

SO2OH

NO2

Br

FeBr3

H2SO4

heat

+

+

+

HONO2

Br2

HOSO2OH

+

+

+

H2O

HBr

H2O


O

O

H

H

C(CH3)3

CCH2CH3

AlCl3

AlCl3

+

+

(CH3)3CCl

CH3CH2CCl

+

+

HCl

HCl


Effect on Rate

Activating substituents increase the rate of EAS compared to that of benzene.

(-OH, -CH3)

Deactivating substituents decrease the rate of EAS compared to benzene.

(-Cl, -NO2)


Rate of Nitration Depends on What is Already on the Ring

Ring Deactivators

Ring Activators


Effect on Regioselectivity

Ortho-para directors direct an incoming electrophile to positions ortho and/or para to themselves.

Meta directors direct an incoming electrophile to positions meta to themselves.


Placement of 2nd Substituent Depends on What the 1st Substituent is

FeBr3

NO2 is a meta director;

while Br is an ortho /para director


CH3

CH3

CH3

CH3

NO2

HNO3

acid

NO2

NO2

Nitration of Toluene

o- and p-nitrotoluene together comprise 97% of the product

a methyl group is an ortho-para director

+

+

63%

3%

34%


E+

H

H

E

H

H

H

H

H

+

H

H

H

H

H

Mechanism

  • Form Strong Electrophile

  • Electrophile Adds to Benzene Ring to Form the Most Stable Carbocation

  • Restoration of Aromaticity


CH3

CH3

CH3

NO2

H

H

H

H

H

+

H

+

+

H

H

H

H

H

H

NO2

NO2

H

H

H

more stable

less stable

Three Possible Carbocations May Be Formed

Carbocation Stability Controls Regioselectivity

gives ortho

gives para

gives meta


CH3

CH3

NO2

NO2

H

H

+

H

H

+

+

H

H

H

H

H

H

ortho Nitration of Toluene

CH3

this resonance form is a tertiary carbocation

NO2

H

H

H

H

H


CH3

CH3

CH3

H

H

H

H

H

H

+

+

+

H

H

H

H

H

H

NO2

NO2

H

H

NO2

H

para Nitration of Toluene

this resonance form is a tertiary carbocation


CH3

CH3

CH3

H

H

H

H

H

H

+

+

+

H

H

H

H

H

H

NO2

NO2

NO2

H

H

H

meta Nitration of Toluene

all the resonance forms of the rate-determining intermediate in the meta nitration of toluene have their positive charge on a secondary carbon


Nitration of Toluene: Interpretation

  • The rate-determining intermediates for ortho and para nitration each have a resonance form that is a tertiary carbocation. All of the resonance forms for the rate-determining intermediate in meta nitration are secondary carbocations.

  • Tertiary carbocations, being more stable, are formed faster than secondary ones. Therefore, the intermediates for attack at the ortho and para positions are formed faster than the intermediate for attack at the meta position. This explains why the major products are o- and p-nitrotoluene.


Ortho/Para DirectorsA. Lone Pair on Atom Bond to Benzene Ring

-X ; -F, -Cl, -Br, -I

B. Alkyl Groups Bond to Benzene Ring

-CH3

-CH2CH3


NET REACTION


Ortho, Para Attack


+

+

ERG

ERG

H

H

H

H

X

H

H

H

H

X

H

H

Lone Pair Stabilizes Intermediates forortho and para Substitution

comparable stabilization not possible for intermediate leading to meta substitution


Meta Attack


Ortho/Para DirectorsA. Lone Pair on Atom Bond to Benzene Ring

-X ; -F, -Cl, -Br, -I

B. Alkyl Groups Bond to Benzene Ring

-CH3

-CH2CH3


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