Reactions of Benzene & Its Derivatives

[Pages:29]Organic Lecture Series

Reactions of Benzene & Its Derivatives

Chapter 22 1

Organic Lecture Series

Reactions of Benzene

The most characteristic reaction of aromatic compounds is substitution at a ring carbon:

Halogenation: H + Cl2

Fe Cl3

Cl + HCl

Chlorobenzene

Nitration: H + HN O3 H2 SO4

N O2 + H2 O

Nitrobenzene 2

Organic Lecture Series

Reactions of Benzene

Sulfonation: H + SO3 H2 SO4

SO3 H

Benzenesulfonic acid

Alkylation: H + RX

A lX3

R + HX

An alkylbenzene

Acylation: O

H + RCX

A lX3

O CR + HX

An acylbenzene

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Organic Lecture Series

Carbon-Carbon Bond Formations:

R R Cl

AlCl3

Arenes

Alkylbenzenes

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Organic Lecture Series

Electrophilic Aromatic Substitution

? Electrophilic aromatic substitution: a reaction in which a hydrogen atom of an aromatic ring is replaced by an electrophile

H + E+

E + H+

? In this section:

? several common types of electrophiles ? how each is generated ? the mechanism by which each replaces

hydrogen

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Organic Lecture Series

EAS: General Mechanism

? A general mechanism

Step 1: Step 2:

s low , rate

+

H + E+ determin ing

H

El e ctro phile

E Res on ance-s tabilized

cation intermed iate

+

H

fast

E

E + H+

? Key question: What is the electrophile and how is it generated?

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Organic Lecture Series

+ +

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Chlorination

Organic Lecture Series

Step 1: formation of a chloronium ion

Cl Cl Cl + Fe Cl

Cl

Chlorin e Ferric chloride

(a Lew is (a Lew is

bas e)

acid )

+

Cl

-

Cl Cl Fe Cl

Cl

A molecular comp lex w ith a positive ch arge

on ch lorine

+

Cl

FeCl4 -

An ion p air contain ing a ch loronium ion

Step 2: attack of the chloronium ion on the ring

slow , rate de te rm i ni ng + Cl

+H

H

H

+

Cl

Cl

+ Cl

Resonan ce-stab ilized cation in termediate; th e positive charge is delocalized onto three atoms of the ring 8

Chlorination

Organic Lecture Series

Step 3: proton transfer regenerates the aromatic character of the ring

+H +

Cl Cation intermediate

Cl-FeCl3

f ast

Cl + HCl + FeCl3

Chlorob enzene

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Bromination

Organic Lecture Series

H + Br 2

F eBr3

Br + HBr

Bromobenzene

This is the general method for Substitution of halogen onto a benzene ring

(CANNOT be halogenated by Free Radical Mechanism)

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Organic Lecture Series

Bromination-Why not addn of Br2?

Regains Aromatic Energy

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Nitration

Organic Lecture Series

? Generation of the nitronium ion, NO2+

? Step 1: proton transfer to nitric acid

O

HSO3 O H + H O N O

Sul f uri c

N itric

acid pKa= -3 acid pKa= -1.4

HSO4

H

O

+

ON

H

O

Conju gate acid of nitric acid

? Step 2: loss of H2O gives the nitronium ion, a very strong electrophile

H

O

ON

H

O

H O + O NO

H The nitronium

ion

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Nitration

Organic Lecture Series

Step 1: attack of the nitronium ion (an electrophile) on the aromatic ring (a nucleophile)

+ + ONO

H NO2 +

H NO2

H NO2 +

+ Resonance-stabilized cation intermediate

Step 2: proton transfer regenerates the aromatic ring

H O

H

H NO2

+

+

NO2 H

+ OH H

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Nitration

Organic Lecture Series

? A particular value of nitration is that the nitro group can be reduced to a 1? amino group

COOH

COOH

+ 3 H2

NO2 4-N itroben zoic acid

Ni (3 atm)

+ 2H2O

NH2 4-Aminoben zoic acid

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Sulfonation

Organic Lecture Series

? Carried out using concentrated sulfuric acid containing dissolved sulfur trioxide

Benzene

+ SO3 H2 SO4

SO3 H

B enzenesulfonic acid

(SO3 in H2SO4 is sometimes called "fuming" sulfuric acid.)

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Organic Lecture Series

Friedel-Crafts Alkylation

? Friedel-Crafts alkylation forms a new C-C bond between an aromatic ring and an alkyl group

+ Cl

AlCl3

+ HCl

Benzene 2-Chloropropane

Cumene

(Isoprop yl chlorid e) (Isopropylbenzen e)

The electrophilic partner is a carbocation; it will arrange to the most stable ion: allylic>3o>2o>1o

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