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AROMATIC SUBSTITUTION REACTIONS . 36 kcal/mol resonance. “Double” bonds in a benzene ring do not react like those in an alkene. easier . E +. Instead of addition reactions, benzene rings give substitution reactions (resonance preserved). More powerful electrophiles

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AROMATIC SUBSTITUTION

REACTIONS

36 kcal/mol

resonance

“Double” bonds in a benzene

ring do not react like those

in an alkene.

easier

E+

Instead of addition reactions,

benzene rings give substitution

reactions (resonance preserved).

More powerful electrophiles

are required. A few of the

common aromatic substitution

reactions are on the next slide.

E+

more

difficult


Slide2 l.jpg

Some Substitution Reactions of Benzene

Halogenation

+

Friedel-Crafts

Alkylation

+

Friedel-Crafts

Acylation

+

-

+

+

Nitration

+

-

Sulfonation

+


Slide3 l.jpg

MECHANISMS

All of the reactions follow the same pattern of mechanism.

The reagents combine to form a strong electrophile E+ ,and

its partner (:X ), which react as follows:

ELECTROPHILIC AROMATIC SUBSTITUTION

(+)

+

+ HX

(+)

+

slow

:X

intermediate

restores

ring resonance

benzenium ion*

resonance structures are shown

by the (+) symbols

* also called a benzenonium ion


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ENERGY PROFILE FOR AROMATIC SUBSTITUTION

(+)

benzenium

intermediate

Transition

state 1

+

(+)

Transition

state 2

intermediate

Ea

activation

energy

+

+

H

slow

fast

STEP 1

STEP 2


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Cl

Al

Cl

Cl

HALOGENATION - 1

Formation of the Chloronium Ion Complex

..

..

:

:

:

:

d+

..

..

..

..

..

..

d-

:

:

:

:

:

..

..

..

..

..

..

:

:

:

:

..

..

sp2

..

:

:

..

..

..

+

-

:

:

:

..

..

..

:

:

..

chloronium

ion complex


Slide6 l.jpg

HALOGENATION - 2

Chlorination of Benzene

-

Cl2 + AlCl3

-

+

+

[

]

benzenium

ion

chloronium

ion complex

+ HAlCl4

HCl + AlCl3


Slide7 l.jpg

FRIEDEL-CRAFTS ALKYLATION - 1

Formation of a Carbocation Complex

..

..

:

:

:

:

d+

..

..

..

..

d-

:

:

:

..

..

..

..

:

:

:

:

..

..

Other aliphatic

R-Cl may be used

..

:

:

..

..

-

+

:

:

..

..

:

:

..

carbocation


Slide8 l.jpg

FRIEDEL-CRAFTS ALKYLATION - 2

Friedel-Crafts Alkylation

-

CH3Cl + AlCl3

-

+

+

[

]

+ HAlCl4

HCl + AlCl3


Slide9 l.jpg

FRIEDEL-CRAFTS ALKYLATION - 3

REARRANGEMENTS ARE COMMON IN

FRIEDEL-CRAFTS ALKYLATION

AlCl3

-

+

-

+

carbocation rearrangement

AlCl3

+


Slide10 l.jpg

FRIEDEL-CRAFTS ACYLATION - 1

Formation of an Acylonium Complex

..

..

:

:

:

:

..

..

..

..

d+

d-

:

:

:

..

..

..

..

:

:

:

:

..

..

..

Other acid chlorides

(RCOCl) may be used

:

:

..

..

-

:

:

..

..

+

:

:

..

acylium ion

(acylonium ion)

Rearrangements DO NOT occur


Slide11 l.jpg

FRIEDEL-CRAFTS ACYLATION - 2

Friedel-Crafts Acylation

-

+ AlCl3

-

+

+

[

]

+ HAlCl4

HCl + AlCl3


Slide12 l.jpg

..

..

..

:

:

:

..

..

..

+

+

+

H2SO4

:

+

..

:

:

:

:

:

..

..

-

-

..

NITRATION - 1

Formation of Nitronium Ion

nitronium ion

..

:

Powerful

Electrophile

+

Reacts with benzene.

:

..


Slide13 l.jpg

NITRATION - 2

Nitration of Benzene

:

..

..

..

HNO3

+

:

:

+

:

:

+

H2SO4

..

-

:

..

..

:

..

:

+

..

-


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SULFONATION - 1

Fuming Sulfuric Acid

.

H2SO4 SO3

..

..

:

:

..

..

:

..

..

:

:

..

..

sulfur

trioxide


Slide15 l.jpg

SULFONATION - 2

Sulfonation of Benzene

-

.

H2SO4 SO3

+

H3O+

D

can be

reversed

in boiling water

or steam (acidic)

+ H2SO4


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DIRECTIVITY AND

RING ACTIVATION / DEACTIVATION


Slide17 l.jpg

ortho, meta and para Positions

m-nitrotoluene

3-nitrotoluene

ipso

1

o-

ortho

6

2

1-methyl-3-nitrobenzene

meta

m-

5

3

4

para

p-dichlorobenzene

p-

1,4-dichlorobenzene


Slide18 l.jpg

Reacts faster

than benzene

Nitration of Anisole

HNO3

+

H2SO4

anisole

ortho

para

“ACTIVATED”

The -OCH3 group when placed on the ring gives only

ortho and para products, and no meta:

Substituents that cause this result are called o,p directors

and they usually activate the ring.


Slide19 l.jpg

Nitration of Methyl Benzoate

HNO3

H2SO4

meta

methyl benzoate

Reacts slower

than benzene

“DEACTIVATED”

The -COOMe group when placed on the ring gives only meta,

and no ortho or para products:

Substituents that cause this result are called m directors

and they usually deactivate the ring.


Slide20 l.jpg

SUBSTITUENT CATEGORIES

Most ring substituents fall into one of these two categories:

O,P = ACTIVATE

M = DEACTIVATE

o,p - directors

m- directors

activate the ring

deactivate the ring

We will look at one of each kind in order to

understand the difference…..



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Nitration of Anisole

ortho

meta

para

Activated

ring

actual

products

para

ortho

+


Slide23 l.jpg

:

ortho

EXTRA!

energy

lowered

meta

:

para

EXTRA!


Slide24 l.jpg

RECALL:

HAMMOND

POSTULATE

Ea

Energy Profiles

NITRATION OF ANISOLE

benzenium

intermediate

meta

benzenium

intermediates

have more

resonance

ortho

para

ortho-para

director



Slide26 l.jpg

Nitration of Methyl Benzoate

ortho

meta

para

Deactivated

ring

actual

product

meta


Slide27 l.jpg

d-

d+

BAD!

ortho

energy

raised

meta

d-

d+

para

BAD!

energy

raised


Slide28 l.jpg

Energy Profiles

NITRATION OF METHYL BENZOATE

some

benzenium

resonance

structures

have a bad

situation

ortho

para

meta

meta

director



Slide30 l.jpg

ortho, para - Directing Groups

Groups that donate

electron density

to the ring.

PROFILE:

:

E+

increased

reactivity

+I Substituent

+R Substituent

..

These groups also

“activate” the ring, or

make it more reactive.

CH3-O-

..

CH3-

..

R-

CH3-N-

..

-NH2

..

The +R groups activate

the ring more strongly

than +I groups.

-O-H

..


Slide31 l.jpg

meta - Directing Groups

Groups that withdraw

electron density from

the ring.

PROFILE:

d+

d-

E+

decreased

reactivity

-I Substituent

-R Substituent

These groups also

“deactivate” the ring,

or make it less reactive.

+

+

-

-SO3H


Slide32 l.jpg

EXCEPTION

Halides - o,p Directors / Deactivating

Halides represent a special case:

..

:

:

They are o,p directing groups

that are deactivating

E+

They are o,p directors (+R effect )

They are deactivating ( -I effect )

Most other other substituents fall

into one of these four categories:

+R / -I / o,p / deactivating

1) +R / o,p / activating

-F

-Cl

-Br

-I

2) +I / o,p / activating

3) -R / m / deactivating

4) -I / m / deactivating


Slide33 l.jpg

GROUPS ACTING IN CONCERT

steric

crowding

o,p director

m-director

very

little

formed

HNO3

H2SO4

When groups direct to the

same positions it is easy to

predict the product.

major

product


Slide34 l.jpg

GROUPS COMPETING

o,p-directing groups win

over m-directing groups

too

crowded

X

HNO3

+

H2SO4


Slide35 l.jpg

RESONANCE VERSUS INDUCTIVE EFFECT

+R

HNO3

H2SO4

major

product

+I

resonance effects are more

important than inductive effects


Slide36 l.jpg

SOME GENERAL RULES

1) Activating (o,p) groups (+R, +I) win over

deactivating (m) groups (-R, -I).

2) Resonance groups (+R) win over inductive (+I) groups.

3) 1,2,3-Trisubstituted products rarely form due to

excessive steric crowding.

4) With bulky directing groups, there will usually be more

p-substitution than o-substitution.

5) The incoming group replaces a hydrogen, it will not

usually displace a substituent already in place.