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Chapter 4Aromatic compounds

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BenzeneBenzene: planar, ring, all atoms have p orbitals, 3 pairs of p electrons.

• Has six identical carbon–carbon bonds

• Each π electron is shared by all six carbons

• The π electrons are delocalized

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Resonance Contributors and the Resonance Hybrid of Benzene

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Nomenclature of Monosubstituted Benzenes

Some are named by attaching “benzene” after the name of the substituent

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When a benzene ring is a substituent, it is called a phenyl group

A benzene ring with a methylene group is called a benzylgroup

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With the exception of toluene, benzene rings with an alkylsubstituent are named as alkyl-substituted benzenes or as phenyl-substituted alkanes

Aryl group (Ar) is the general term for either a phenylgroup or a substituted phenyl group

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In disubstituted benzenes, the relative positions of the two substituents are indicated by numbers or by prefixes

Nomenclature of Substituted Benzenes

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The two substituents are listed in alphabetical order

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If one of the substituents can be incorporated into a name, that name is used and the incorporated substituent is given the 1-position

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Naming Polysubstituted Benzenes

The substituents are numbered in the direction that results in the lowest possible number

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The incorporated substituent is given the 1-position; the ring is numbered in the direction that yields the lowestpossible number

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The Resonance Energy of Benzene

-36 kcal/mole

+H2

+3H2

+3H2 -49.8 kcal/mole-28.6 kcal/mole

-85.8 kcal/mole

(28.6 × 2)

calculated

RESONANCE ENERGY

benzene

1,3,5-cyclohexatrienehypothetical

(-208 kJ)(-120 kJ)

(-360 kJ)

(- 152 kJ)

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Electrophilic Aromatic Substitution Reactions

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Electrophilic Aromatic Substitution

Aromatic ring systems react with electrophiles differently than alkenes.

H+ E+

E+ H+

+ E+Nu-E

Nu

We obtain a substitution rather than an addition product.

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Mechanism

H+ E+

First, we get a carbocation intermediate.At the carbon that adds the electrophile, rehybridization (sp2 sp3) takes place. Molecule loses aromatic character.

H

EEH

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Mechanism

Carbocation is resonance stabilized.

H

EEH

H

EEH

H

EEH

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Mechanism

H

EEH

+

Nu- E

NuHH

E

+ HB

B-

The electron-rich part of the reagent can act either as nucleophile in an addition reaction or as a base to remove a proton.

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Mechanism

The two reactions would result in different products.

E

NuHH E

The exceptional stabilization for the aromatic compounds leads only to the substitution (addition elimination) product.

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Electrophilic Aromatic Substitution

Halogenation:1st step is generating an electrophile:

Br Br + FeBr3 FeBr3 Br Brδ+δ−

FeBr3 is a Lewis acid that coordinates with Br2 andpolarizes it, generating an electrophilic Br+

Η

Br

Η

+ FeBr3

Br Br + HBr

+ FeBr3

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Electrophilic Aromatic Substitution

Nitration:

1st step is generating an electrophile, the nitronium ion:

HO NO2 + H OSO3H HSO4-

+ HO NO2

H

NO2+ + H2O

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Electrophilic Aromatic Substitution

NO2+HNO2

H+HSO4

-

NO2

The highly electrophilic nitronium ion attacks the aromatic ring.

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Aromatic Sulfonation

Substitution of H by SO3 (sulfonation)Reaction with a mixture of sulfuric acid and SO3 (“Fuming H2SO4)Reactive species is sulfur trioxide or its conjugate acid

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Preparation of Phenols

From aromatic sulfonic acids with NaOH at high temperature

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Alkylation of Aromatic Rings: The Friedel–Crafts Reaction

Alkylation among most useful electrophilic aromatic substitution reactions

Aromatic substitution of R+ for H+Aluminum chloride promotes the formation of the carbocation

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Friedel-Crafts Acylationacyl group, ⎯COR

Similar to alkylation Reactive electrophile: resonance-stabilized acyl cation

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Substituent Effects in Aromatic RingsSubstituents can cause a compound to be (much) more or (much) less reactive than benzene

Substituents affect the orientation of the reaction – the positional relationship is controlled

ortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators.

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Ring-Activating and Ring Deactivating Substituents

Monosubstituted benzenes undergo electrophilic aromatic substitution.Substituents influence reactivity.Substituents determine position of second electrophilic substitution.Substituents that increase electron density increase reactivity.Substituents that decrease electron density decrease reactivity.

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Classification of SubstituentsActivating Substituents

(ortho/para directing):• prim. Amines• sec. Amines • tertiary amines• phenolic OH• phenol ethers• amides• phenolic esters• alkyl substituents• halides

Strongly activating

weakly activating

deactivating

N H 2N H R

N R 2O H

O R

N H C R 'O

O C R 'O

RX

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Classification of SubstituentsDeactivating Substituents (meta directing):

• aldehydes

• esters

• carboxylic acid

• nitrile

• sulfonic acid

• nitro

• ammonium

weakly deactivating

strongly deactivating

CHO

δ+

CORO

δ+

COHO

δ+

C N

SO3H

NO

O

NR3

δ+

δ+

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Substituent Effects in Aromatic Rings

Substituents can cause a compound to be (much) more or (much) less reactive than benzene

Substituents affect the orientation of the reaction – the positional relationship is controlledortho- and para-directing activators, ortho- and para-directing deactivators, and meta-directing deactivators

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Origins of Substituent Effects

An interplay of inductive effects and resonance effectsInductive effect - withdrawal or donation of electrons through a σ bondResonance effect - withdrawal or donation of electrons through a π bond due to the overlap of a p orbital on the substituent with a p orbital on the aromatic ring

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Inductive Effects

Controlled by electronegativity and the polarity of bonds in functional groupsHalogens, C=O, CN, and NO2 withdraw electrons through σ bond connected to ring

Alkyl groups donate electrons

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Resonance Effects – Electron Withdrawal

C=O, CN, NO2 substituents withdraw electrons from the aromatic ring by resonanceπ electrons flow from the rings to the substituents

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Resonance Effects – Electron Donation

Halogen, OH, alkoxyl (OR), and amino substituents donate electronsπ electrons flow from the substituents to the ringEffect is greatest at ortho and para

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An Explanation of Substituent Effects

Activating groups donate electrons to the ring, stabilizing the intermediate (carbocation)Deactivating groups withdraw electrons from the ring, destabilizing the intermediate

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Halobenzenes

Cl EN= 3.5

EN= 2.5 +

Inductive effect lowers electrondensity in the ring system.

Effects act in opposite directions;inductive effect stronger.

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Ammoniumbenzenes

NRR

R +

Nitrogen lacks lone pair; no resonance effect.Compare to anilines.

Positively charged nitrogenhas strong polarizing effect,lowering electron density inring system.

anilinium cation

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Activating ortho/para

OH3C

o

m

p

OCH3HE

OCH3

EH

OCH3

HE

OCH3E

H

OCH3

EH

OCH3

HE

EOCH3H

OCH3

EH

H

OCH3

E

OCH3HE

OCH3

HE

There are additionalresonance contributorsfor cations inortho and parasubstitution.

These positions have adouble advantage:bigger electron densitybetter cation stabilization

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Deactivating meta

NO O

H

E

NO O

NH

E

O ON

H

E

O O

NO O

EH

NO O

EH

NO

EH

O

N

E H

O ON

E H

O ON

E H

O O

There are resonance contributors for cations in ortho and parasubstitution that are lessstable because of twoneighboring +ve. charges.

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Summary Table: Effect of Substituents in Aromatic Substitution

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Arrange the following groups?

A) -OCH3 B) CH3C C) CO

_ _ N D) NR3+_

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Trisubstituted Benzenes: Additivity of Effects

If the directing effects of the two groups are the same, the result is additive

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Substituents with Opposite Effects

If the directing effects of two groups oppose each other, the more powerful activating group decides the principal outcomeOften gives mixtures of products

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Meta-Disubstituted Positions Are UnreactiveThe reaction site is too hinderedTo make aromatic rings with three adjacent substituents, it is best to start with an ortho-disubstituted compound

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What is the major organic product?

CH3 CH3

CH3

CH3

COCH2

O

COCH2

O

COCH2

O

(D)(C)

(B)COCH2

O

(A)

CH3Cl, AlCl3COCH2

O

1 equivalent

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What is the suitable reactant to produce the following product?

AlCl3?

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What is the major organic product?

1)CH3COCl,AlCl3

2) Br2, FeBr3?

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Synthesis it from benzene?

CH3ClCl

NO2

OHO2N

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EX: Draw the four major resonance structures of the intermediate in the reaction of anisole with HNO3/H2SO4?

OCH3

HNO3/H2SO4

OCH3

NO2

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Provide the structure of the major monoitration product of the compounds below.

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CN

+ Br2FeBr3

CN CN CN CN

Br Br Br Br

Br

A B C D

Chapter 4�Aromatic compoundsThe Resonance Energy of Benzene Electrophilic Aromatic SubstitutionMechanismMechanismMechanismMechanismElectrophilic Aromatic SubstitutionElectrophilic Aromatic SubstitutionElectrophilic Aromatic SubstitutionAromatic SulfonationPreparation of PhenolsAlkylation of Aromatic Rings: The Friedel–Crafts Reaction Friedel-Crafts AcylationSubstituent Effects in Aromatic Rings Ring-Activating and Ring Deactivating SubstituentsClassification of SubstituentsClassification of SubstituentsSubstituent Effects in Aromatic Rings Origins of Substituent EffectsInductive EffectsResonance Effects – Electron WithdrawalResonance Effects – Electron DonationAn Explanation of Substituent Effects HalobenzenesAmmoniumbenzenesActivating ortho/paraDeactivating metaSummary Table: Effect of Substituents in Aromatic Substitution Arrange the following groups?Trisubstituted Benzenes: Additivity of Effects Substituents with Opposite EffectsMeta-Disubstituted Positions Are UnreactiveWhat is the major organic product?What is the suitable reactant to produce the following product? What is the major organic product? Synthesis it from benzene?