Electro Phil i c Aromatic Substitution

8/12/2019 Electro Phil i c Aromatic Substitution

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+

HO-NO2 + H2SO4 H2O-NO2 + HSO4-

+ +H2O-NO2 H2O + NO2

H2SO4 + H2O HSO4- + H3O

+

HNO3 + 2 H2SO4 H3O+ + 2 HSO4

- + NO2+

nitration


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nitration:

1) HONO2 + 2 H2SO4 H3O+ + 2 HSO4

- + NO2+

2) + NO2+ RDS

electrophile


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H

NO2

H

NO2

H

NO2

H

NO2

resonance


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Mechanism for nitration:

1) HONO2 + 2 H2SO4 H3O+ + 2 HSO4

- + NO2+

2)

+ NO2+

H

NO23)

RDS

NO2 + H+

H

NO2


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Mechanism for sulfonation:

1) 2 H2SO4 H3O

+

+ HSO4-

+ SO3

2) + SO3

RDS

H

SO3-

3)H

SO3-

SO3- + H+

4) SO3- SO3H+ H3O

+ + H2O


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Mechanism for halogenation:

1) Cl2 + AlCl3 Cl-Cl-AlCl3

2) + Cl-Cl-AlCl3 RDS

H

Cl + AlCl4-

3)H

Cl+ AlCl4

-Cl + HCl + AlCl3


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Mechanism for Friedel-Crafts alkylation:

1) R-X + FeX3 R + FeX4-

2) + R

RDS

3)

H

R

H

R+ FeX4

-R + HX + FeX3


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1) R-OH + H+ ROH2+

3)+ R

RDS

4)

H

R

H

RR

2) ROH2+

R + H2O

+ H+

Mechanism for Friedel-Crafts with an alcohol & acid


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2) + R

RDS

3)

H

R

H

RR

1) C C + H+ R

+ H+

Mechanism for Friedel-Crafts with alkene &

acid:

electrophile in Friedel-Crafts alkylation = carbocation


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Generic Electrophilic Aromatic Substitution mechanism:

1) + Y+Z-RDS

H

Y

+ Z-

2)H

Y+ Z- Y + HZ


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Common substituent groups and their effect on EAS:

-NH2, -NHR, -NR2

-OH-OR

-NHCOCH3

-CH3

-R-H

-X

-CHO, -COR

-SO3H

-COOH, -COOR

-CN

-NR3+

-NO2

in

creasingr

eac

tivity ortho/paradirectors

metadirectors


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Why do substituent groups on a benzene ring affect the

reactivity and orientation in the way they do?

electronic effects, pushing or pulling electrons by

the substituent.

Electrons can be donated (pushed) or withdrawn

(pulled) by atoms or groups of atoms via:

Inductiondue to differences in electronegativities

Resonancedelocalization via resonance


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N

H

H

unshared pair of electrons on the nitrogenresonance donatinggroups

(weaker inductive withdrawal)

N

R

R

N

R

H

NR

R

Rstrong inductivewithdrawal(no unshared pair of electrons on thenitrogen & no resonance possible


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H3C C

O

N

H

resonance donation


HO

resonancedonation


R

O resonancedonation(weaker inductive withdrawal)


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resonance donation

H3Cinductive donation

sp3 sp2 ring carbon

inductive withdrawal X


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H

C

O

HO

C

O

RO

C

O

R

C

O

resoance withdrawalandinductive withdrawal


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N

O

O

resonance and

inductive withdrawal

resonance and

inductive withdrawal

CN


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Common substituent groups and their effect on reactivityin EAS:

-NH2, -NHR, -NR2

-OH-OR

-NHCOCH3 electron donating

-C6H5

-R

-H

-X

-CHO, -COR

-SO3H

-COOH, -COOR electron withdrawing-CN

-NR3+

-NO2

in

creasingr

eac

tivity


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Electron donating groups activate the benzene ring to

electrophilic aromatic substitution.

1. electron donating groups increase the electron density

in the ring and make it more reactive with electrophiles.

2. electron donation stabilizes the intermediatecarbocation, lowers the Eact and increases the rate.

H Y

CH3


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H Y

NO2

Electron withdrawing groups deactivate the benzene ring to

electrophilic aromatic substitution.

1. electron withdrawing groups decrease the electron density

in the ring and make it less reactive with electrophiles.

2. electron withdrawal destabilizes the intermediate

carbocation, raising the Eact and slowing the rate.


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CF3

PH2

PO3H

electron withdrawing = deactivating & meta-director

electron withdrawing = deactivating & meta-director

electron donating = activating & ortho-/para-director


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Br2, Fe

Br + ortho-

NO2

Br2, Fe

NO2Br + ortho-

O

OBr2, Fe

O

O

Br

+ ortho-


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How to draw resonance structures for EAS

Y

H

Y

HY

HY


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G G G

G G G

G G

H H H

Y Y Y

H

Y

H

Y

H

Y

H Y H Y

G

H Y

ortho-attack

meta-attack

para-attack


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G

H

YG

If G is an electron donating group, these structures are

especially stable.


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G G G

G G G

G G

H H H

Y Y Y

H

Y

H

Y

H

Y

H Y H Y

G

H Y

ortho-attack

meta-attack

para-attack


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Electron donating groups stabilize the intermediatecarbocations for ortho-and para-in EAS more than for

meta-. The Eacts for ortho-/para-are lower and the

rates are faster.

Electron donating groups direct ortho-/para-in EAS


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G

H

YG

H Y

If G is an electron withdrawinggroup, these structures

are especially unstable.


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G G G

G G G

G G

H H H

Y Y Y

H

Y

H

Y

H

Y

H Y H Y

G

H Y

ortho-attack

meta-attack

para-attack


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Electron withdrawing groups destabilize the intermediatecarbocations for ortho-and para-in EAS more than for

meta-. The Eacts for ortho-/para-are higher and the

rates are slower.

Electron withdrawing groups direct meta-in EAS


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Halogens are electron withdrawing but are ortho/para

directing in EAS.

The halogen atom is unusual in that it is highly

electronegative but also has unshared pairs of electronsthat can be resonance donated to the carbocation.


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X X X

X X X

X X

H H H

Y Y Y

HY H

Y

H

Y

H Y H Y

X

H Y

X

H Y

X

H

Y

ortho-

meta-

para-

halogens are deactivating in EAS but direct ortho and para

Electro Phil i c Aromatic Substitution

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