Electrophilic Aromatic Substitution (EAS)Reactions

Overall reactionOverall reaction

General Mechanism for Electrophilic Aromatic Substitution

Summary of ERS Reactions

Halogenation of Benzene

+ X2

F eX 3

c a ta lys t

X

+ HX

General Equation:General Equation:

Electrophile = X+Electrophile = X+

X X + FeX3X FeX3 + X+ electrophile

EX:EX:

Nitration of Benzene

Electrophile = +NO2 (nitronium ion)Electrophile = +NO2 (nitronium ion)

+ HNO3NO2H 2SO 4

c a ta lys t

General Equation:General Equation:

N+

O

O-

OH + H2SO4N

+

O

O-

OH2

+O N

+O

-H 2O

Sulfonation of Benzene

+ SO3

SO3HH 2SO 4

c a ta lys t

General Equation:General Equation:

S

O

O

O + H2SO4 S+

O

O

OH

Electrophile = +SO3H(protonated sulfer trioxide)Electrophile = +SO3H(protonated sulfer trioxide)

Friedel–Crafts Alkylation

Electrophile = R(carbocation) Electrophile = R(carbocation)

General Equation:General Equation:

+ ClA lC l3

c a ta lys tEX:EX:

Friedel–Crafts Acylation

Acyl group:

Electrophile is R–C≡O⊕ (acylium ion)

General Equation:General Equation:

Substituents effect on reactivity and orientation

EDG (electron-donating group): (R, OH, OR, NR2,)

orthoortho

para

EDG

Reactivity: Ring is electron rich; Ring reacts faster than benzene with E+ (Activation)

Reactivity: Ring is electron rich; Ring reacts faster than benzene with E+ (Activation)

Orientation: Ortho (o), Para (p) position

EWG (electron-withdrawing group):

(CF3 , NO2, SO3H,CN, )

Substituents effect on reactivity and orientation

EWG

metameta

Reactivity: Ring is electron poor; Ring reacts slower than benzene with E+ (Deactivation)

Orientation: Meta (m) position

C

O

R

Xorthoortho

para

Reactivity: Ring reacts slower than benzene with E+ (Deactivation)

Orientation: Ortho (o), Para (p) position

Substituents effect on reactivity and orientation

Halogens (X): (F, Cl, Br, I)

Reactivity towards electrophilic aromatic substitution

then reaction is faster than with benzene

EffectEffect of of Electron-Donating Electron-Donating (releasing) and (releasing) and Electron-Withdrawing GroupsElectron-Withdrawing Groups

arenium ionstabilized

If G is electron-donating group

If G is an electron-withdrawingthen reaction is slower than with benzene

arenium iondestabilized

(1) Mesomeric (+M) Effects:Mesomeric (+M) Effects:

Activate the ring by resonance

(2) ) Inductive (+I): Inductive (+I):

Activate the inductive (through σ bond)

EDG activate the ring by:EDG activate the ring by:

Note: Mesomeric effect stronger than Inductive effect

Deactivate the ring by resonance effect

Deactivate the ring by negative inductive effect

EWG deactivate the ring by:EWG deactivate the ring by:

(1) Mesomeric (-M) Effects:Mesomeric (-M) Effects:

(2) ) Inductive (-I): Inductive (-I):

Note: Mesomeric effect stronger than Inductive effect

Ex1: Write the formulas for benzyl chloride, toluene, and benzoic acid, ortho-xylene, 4-ethyl-2,6-difluorotoluene,

Ex2: Draw the important resonance contributors for the benzenoniumintermediate in the bromination of aniline

Ex3: Devise a synthesis for each of the following, starting with benzene:a. m-bromobenzenesulfonic acidb. p-nitrotoluene

Ex4: Which compound is more reactive toward electrophilic substitution

Exercises

OCH3

OHOCl

CH3

Ex5: using benzene or toluene as the only aromatic starting material, devise a synthesis of each compound.a.2-bromo-4-nitrotoluene b.p-nitroethylbenzene