The origin of August Kekulé’s view of benzene structure:

“There I sat and wrote mytextbook, but things did notgo well; my mind was occupiedwith other matters, I turned thechair towards the fireplace andbegan to doze.

“Once again the atoms dancedbefore my eyes. This time smaller groups modestly remained in the background.My mental eye, sharpened byrepeated apparitions of similarkind, now distinguished largerunits of various shapes.”

“Long rows, frequently joined more densely; everything in motion, twisting and turninglike snakes. And behold, whatwas that? One of the snakescaught hold of its own tail andmockingly whirled around myeyes.

“1 awoke, as if by lightening; this time, too, I spent the rest of the night working out theconsequences of this hypothesis”

“Let us learn to dream, gentlemen, then perhaps we shall find the truth” AUGUST KEKULÉ

Edison, Einstein and many others have used the subconscious mind to give them the insight and the “know-how” to bring about their great achievements.

August Kekulé

“In variably, my device works as I imagined it should. In twenty years there has not been single exception.” NIKOLA TESLA

“Imagination is more important than knowledge.” ALBERT EINSTEIN

AROMATIC COMPOUNDS

Introduction to aromatic Compounds14.1

Arenes = aromatic compounds:the word aromatic has nothing to do with odour.

Kekulé structure

resonance structure

aromatic

ortho meta para

Friedel-Craft acylationelectrophilic substitution

activating and deactivating group

ortho-para and meta director

oxidation ofalkylbenzene

substitution oftoluene

KEYWORDS

1825 Michael Faraday (British)

Isolated a pure compound of boiling point 80oC

Empirical formula = CH

Named “bicarburet of hydrogen”

DISCOVERY OF BENZENE

1834 Eilhard Mitscherlich (German)

Prepared benzene from benzoic acid

C6H5CO2H + CaO C6H6 + CaCO3

heat

Molar mass = 78, molecular formula = C6H6

named “benzin”

benzene benzin benzoin

benjoin(French)

gum benzoin Luban Jawi(Arabic)

1866 Friedrich August Kekulé (German)

Proposed a cyclic structure for benzene.

AROMATIC COMPOUNDS

In earlier time, compounds are called aromatic because of their pleasant odours.

Today, we use the word aromatic to refer to benzene and its structural relatives.

Benzene has strong pleasant odour.

AROMATIC COMPOUNDS

Aromatic compound is a cyclic conjugated molecule or ion that is stabilized by ∏ electron delocalisation.

it is characterised by substitution reactions.

C

CC

CC

C

H

H

H

H

H

H

or

KEKULÉ’S STRUCTURE

Kekule was the first to formulate a reasonable representation of benzene

The Kekule structure suggests alternating double and single carbon-carbon bonds

Benzene is actually a resonance hybrid of the two Kekulé structures.

All C–C bond length equal = 139 pm

Shorter than typical C–C (148 pm)

Longer than typical C=C (134 pm)

resonance hybrid

equivalent to

RESONANCE STRUCTURE

The six electrons completely delocalized around the ring

All six C atoms and six p orbitals are equivalent

The circle represents the six electrons, distributed over the six atoms of the ring!

THE CRITERIA FOR AROMATICITY

4 structural criteria must be satisfied for compound to be aromatic

Completely

conjugated

CyclicPlanar

Contain particular number of ∏ electron obeys HÜckel’s Rule

THE CRITERIA FOR AROMATICITY

HÜckel’s Rule

* cyclic, planar and completely conjugated compounds that contain [4n+2] ∏ electron (n=0,1,2…..) are said to be aromatic

planar monocyclic rings with 2,6,10,14 and so forth ∏ electrons are aromatic

Erich HÜckel (1896-1980)

THE CRITERIA FOR AROMATICITY

EXAMPLE OF AROMATIC COMPOUNDS

Benzene

1. Aromatic compounds with a single ring

[4n+2] ∏ = [4(1) + 2]∏ = 6 ∏ electrons

aromatic

Benzene is aromatic because:

contains 6∏ electrons (obeys HÜckels Rule)

cyclic, planar and has double bond in the ring

THE CRITERIA FOR AROMATICITY

2. Aromatic compounds with more than one ring

naphthalene

4n+2= 4(2)+ 2 10 ∏ electrons

Aromatic

* Two benzene rings joined together forms naphthalene

EXAMPLE

AROMATIC COMPOUNDS

Nomenclature of benzene and its derivatives14.2

Arenes = aromatic compounds:the word aromatic has nothing to do with odour.

NAMING BENZENE DERIVATIVES

Many organic molecules contain a benzene ring with one or more substituents.

Many common name are recognized by the IUPAC system

CH3

Common: tolueneIUPAC: methylbenzene

EXAMPLE:

Benzene is the parent name and the substituent is indicated by a prefix.

F

fluorobenzene

Cl

chlorobenzene

Br

bromobenzene

NO2

nitrobenzene

CH2CH3

ethylbenzene

MONO SUBSTITUTED BENZENE

IUPAC rules allow some common names to be retained.

CH3

toluene

OH

phenol

NH2

aniline

COOH

benzoic acid

CHO

benzaldehyde

Relative position of subsituents are indicated by prefixes ortho, meta, and para ( o–, m–, and p–) or by the use of number.

Br

Br12

1,2–dibromobenzeneor

o–dibromobenzene

Br

Br

12

3

1,3–dibromobenzeneor

m–dibromobenzene

Br

Br

1,4–dibromobenzeneor

p–dibromobenzene

12

34

DISUBSTITUTED BENZENE

Two Same Substituents

NO2

NO21

2

1,2–dinitrobenzeneor

o–dinitrobenzene

12

3

1,3–dinitrobenzeneor

m–dinitrobenzene

NO2

NO2

NO2

NO2

1,4–dinitrobenzeneor

p–dinitrobenzene

12

34

COOH

NO21

2

2–nitrobenzoic acidor

o–nitrobenzoic acid

12

3

3–nitrobenzoic acidor

m–nitrobenzoic acid

COOH

NO2

4–nitrobenzoic acidor

p–nitrobenzoic acid

12

34

COOH

NO2

Select one of the substituent that give new parent name and numbered as C1.

DISUBSTITUTED BENZENE

Two Different Substituents

Position of substituents must be indicated by numbers.

Br

Br

Br

1 2

34

1,2,4–tribromobenzene

The substituents are listed alphabetically when writing the name.

Cl

Br

I2–bromo–1–chloro–3–iodobenzene

THREE OR MORE SUBSTITUENTS

C atom bearing the subtituent that define the new parent name is numbered as C1.

OH

NO2

NO2

2,4–dinitrophenol

HO OH

COOH1

2

34

12

3

4

5

6

3,5–dihydroxybenzoic acid

Br

CH3

CH3

1

23

4

4–bromo–1,2–dimethylbenzene

Br

CH3

CH3

1

23

4

4–bromo–1,2–dimethylbenzene

o–, m– and p– naming system is used for arenes with 2 substituents only!

4–bromo–o–dimethylbenzene

correct

If alkyl substituent is larger than the ring (more than 6 C), the compound is named as phenyl-substituted alkane.

CH–CH2–CH2–CH2–CH2–CH3

CH21

23 4 5 6 7

2–phenylheptane

14.2-11

PHENYL GROUP

Benzene ring as substituent.

Phenyl = C6H5– = Ph

If the chain is unsaturated (have C═C or C≡C) or contains important functional group, the benzene ring is considered as phenyl substituent.

CH2–C C–CH3

1 2 3 4

1–phenyl–2–butene

CH2–CH2–OH

2–phenylethanol

12

14.2-12

phenyl group

CH2—

benzyl group

CH2Br

benzyl bromide

CH2OH

benzyl alcohol

14.2-13

BENZYL GROUP

AROMATIC COMPOUNDS

Chemical properties of benzene and its derivatives14.3

Arenes = aromatic compounds:the word aromatic has nothing to do with odour.

BENZENE

Br2 / CCl4 no addition of Br2

(no decolorization)

KMnO4 / H2O no oxidation(no decolorization)

H2 / Nislow addition at hightemperature and pressure

UNUSUAL REACTIONS OF BENZENE14.3-01

Alkenes readily undergo addition reaction, benzene does not!

Involves substituents attached to the ring

Electrophilic aromatic substitution

+ Br2

FeBr3

Br

benzene bromobenzene

Involves the benzene ring itself

CH3CH2CH2CH3 KMnO4

H2O

COOH

butylbenzene benzoic acid

REACTION OF ARENES14.3-03

Most characteristic reaction of benzene.

ELECTROPHILIC SUBSTITUTION14.3-04

A H atom is replaced by an electrophile.

H+ E+

E+ H+

electrophile

XX2, FeX3

(X = Cl Br) + HX halogenation

HONO2

H2SO4

NO2

+ H2O nitration

RCl , AlCl3

(R can rearrange)

R+ HCl

Friedel-CraftsAlkylation

RCCl , AlCl3

O

C–RO

+ HClFriedel-CraftsAcylation

14.3-05

STEP 1 Formation of arenium ion

H

H

H

H

H

H E+

H

H

H

H

+

E

H

H

H

H

H

H

E

H

+H

H

H

H

H

H

E

H+

GENERAL MECHANISM 14.3-07

arenium ion

E+

Electrophile takes two electrons of six–electrons system to form bond.

benzene ring

This interrupts of cyclic system of electrons.

Benzene ring arenium ion (aromatic) (nonaromatic)

The four electrons delocalized through these the five C atom (p orbitals)

E

H

+

arenium ion

ARENIUM ION 14.3-08

STEP 2 Loss of H+

H

H

H

H

H

EH

H

H

H

H

E

H+

EH

+E

6 electrons 4 electrons

E+ – H+

6 electrons

Substitution reaction allowaromatic six electrons tobe regenerated.

14.3-09

Kekulé structures are more appropriate for writing mechanisms.

For simplicity, however, we can show the mechanism in the following way:

STEP 1

+ E+ +

E

H

arenium ion

STEP 2 E

+ H+

14.3-10

E

H+

Br

+ Br2no reaction(decolorization not observed)

+ Br2

FeBr3

bromobenzene

+ HBr

HALOGENATION 14.3-11

Reactants: benzene and halogens (Cl2 or Br2).

Conditions:

Lewis acid such as FeCl3 and FeBr3

STEP 1 Formation of Br+

Br–Br + FeBr3 Br—Br–FeBr3

+ –Br+ + FeBr4

–

complex

+ Br+

STEP 2 Formation of arenium ion

STEP 3 Loss of H+

Br

+ HBr

MECHANISM 14.2-13

FeBr4–

+ FeBr3

+

Br

H

+

Br

H

Increase polarity of halogen molecules.

Produce positive halogen ions (Br+ or Cl+).

FUNCTION OF LEWIS ACIDS 14.3-14

NO2HNO3

H2SO4nitrobenzene

+ H+ + HSO4–

NITRATION 14.3-15

Reactants: benzene and concd. HNO3.

Conditions:

Concd. H2SO4

STEP 1 Formation of nitronium ion (NO2+)

H2O + NO2+

MECHANISM 12.5-49

H

..

..

H-O-NO2 + H-OSO3H H-O+-NO2 + HSO4–

..

nitronium ion

+ NO2+

STEP 2 Formation of arenium ion

STEP 3 Loss of H+

NO2

+ H2SO4

HSO4–

+

NO2

H

+

NO2

H

R+ R–X

AlCl3

alkylbenzene

+ HCl

FRIEDEL–CRAFT ALKYLATION 12.5-51

Reactants: benzene and haloalkane.

Conditions:

Catalyst: Lewis acid such as AlCl3.

Alkylation = transfer an alkyl group to benzene

12.5-52

EXAMPLE:

+ CH3CHCH3

ClCH(CH3)2AlCl3

2–chloropropane+ HCl

isopropylbenzene

+ (CH3)3C–ClC(CH3)3AlCl3

2–chloro–2–methylpropane+ HCl

tert–butylbenzene

12.5-53

STEP 1 Formation of carbocation

(CH3)2CHCl + AlCl3 (CH3)2CH–Cl–AlCl3

carbocation

(CH3)2CH+ + AlCl4–

MECHANISM 12.5-54

:

:

:

:

:

+ -

+ +CH(CH3)2

STEP 2 Formation of arenium ion

STEP 3 Loss of H+

CH(CH3)2

+ HCl

AlCl4–

+

CH(CH3)2

H

+

CH(CH3)2

H

Rearrangement can occur, especially when 1o haloalkanes are used.

EXAMPLE:

CH3CH2CH2CH2Cl

CH2CH2CH2CH3

+

butylbenzene

CHCH2CH3

CH3

sec–butylbenzene

65%

35%

Rearrangement:

CH3—CH2—CH—CH2

H+

1o carbocationCH3—CH2—CH—CH3

+

2o carbocation

AlCl3

OTHER FACTS ABOUTFRIEDEL–CRAFT ALKYLATION

12.5-55

EXAMPLE:

+ CH3C—Cl

O

acetyl chloride

AlCl3CCH3

O

acetophenone

+ HCl

FRIEDEL–CRAFT ACYLATION 12.5-56

Reactants: benzene and acid chloride.

Conditions:

Catalyst: Lewis acid such as AlCl3.

Product: ketone.

EXAMPLE:

CH3C—

O

acyl group

benzoyl group

–C—

O

RC—

O

acetyl group

ACYL GROUP 12.5-57

Acylation = transfer an acyl group to benzene

STEP 1 Formation of acylium ion

acylium ion

R–C–Cl + AlCl3 R–C–Cl–AlCl3

:O: :O:

R–C+═O R–C≡O+ + AlCl4–

MECHANISM 12.5-59

: :

+ R–C+═O

STEP 2 Formation of arenium ion

STEP 3 Loss of H+

+ HCl

C═O

R

+

C=O

H

R

+

C=O

H

R AlCl4–

But, what would happen if we were to carry out a reactionon aromatic ring that already has a substituent?

SUBSTITUENT EFFECT 12.5-61

Activating groups:

Substituents that activate the ring, making it more reactive than benzene.

NO2 Cl H OH

relative rateof nitration

EXAMPLE:

6 x 10–8 0.033 1 1000 reactivity

EFFECT ON REACTIVITY 12.5-62

Deactivating groups:

Substituents that deactivate the ring, making it less reactive than benzene.

EXAMPLE:

CH3

HNO3

H2SO4

CH3

NO2

+

CH3

NO2

CH3

NO2(59%) (37%)

+

(4%)

ortho-para director

Ortho–para directors

EFFECT ON ORIENTATION 12.5-63

Meta directors

Tend to direct the incoming group into ortho and para positions.

Tend to direct the incoming group into meta position.

CLASSIFICATION OF SUBSTITUENTS

ortho– , para– directingactivators

ortho– , para– directingdeactivators

meta– directing deactivators

12.5-64

ORTHO– , PARA– DIRECTING ACTIVATORS

12.5-65

SMH, 1 ed, p.625*WAD, 4 ed, p.217CRY: 5 ed., p.651MCM: 6 ed., p.300SOL: 8 ed., p. 300

—NH2

●●

—R

—NHCOR●●

—OR●●

●●

—OH●●

●●

—NHR●●

—NR2

●●

Increasing activation

General structure:—R or —Z●

●

Alkyl groups or havenonbonded electron pairon the atom bonded to

benzene ring

EXAMPLE:

CH2CH3

Br2

FeBr3

CH2CH3

Br

+

CH2CH3

Br(38%)ethylbenzene (62%)

12.5-66

ORTHO– , PARA– DIRECTING DEACTIVATORS

12.5-67

SMH, 1 ed, p.625*WAD, 4 ed, p.217CRY: 5 ed., p.651MCM: 6 ed., p.300SOL: 8 ed., p. 300

—F●●

●●

●●—Cl

●●●●

●●

—Br●●

●●

●●— I

●●●●

●●

General structure

—X (halogens)

●●●●

●●

EXAMPLE:

Cl

HNO3

H2SO4

Cl

NO2

+

Cl

NO2(35%)chlorobenzene (64%)

12.5-68

META– DIRECTING ACTIVATORS 12.5-69

SMH, 1 ed, p.625*WAD, 4 ed, p.217CRY: 5 ed., p.651MCM: 6 ed., p.300SOL: 8 ed., p. 300

—CHO

Increasing deactivation

General structure:—Y (+ or –)

—COR

—COOR

—COOH

—CN

—SO3H

—NO2

—NR3

+

Have a full or partial positive charge on the

atom bonded to benzene ring

EXAMPLE:

NO2

HNO3

H2SO4

nitrobenzene

NO2

NO2

(93%)

12.5-69

Due to:

INDUCTIVE EFFECT 12.5-70

Electronegativity of the atoms in the substituent.

Polarisability of the substituent.

EXAMPLE:

Cl

Cl— electron–withdrawing

CH3

CH3— electron–donating

Activating Groups

Release electrons to the ring

Stabilise arenium ion

Form faster

12.5-71

CH3

Deactivating Groups

Withdraw electrons from the ring

Destabilise arenium ion

Form slower

+

NO2

Cl

+

NO2

EXAMPLE:

CF3

(trifluoromethyl)benzene benzene

CH3

toluene increasing rate of nitration

CF3 withdraws e-, arenium ion less stable ring less reactive

CH3 releases e-, arenium ion more stable ring more reactive

12.5-72

CF3

+

NO2

+

NO2CH3

+

NO2

OXIDATION OF SIDE CHAIN 12.5-75

Reactants: arene with benzylic H.

Conditions:

Strong oxidizing agent such as KMnO4

and Na2Cr2O7.

Heat.

CH3

CH(CH3)2CH3

benzylic H

CH3

toluene

KMnO4

heat

COOH

CH3O2N

p–nitrotoluene

Na2Cr2O7

heatCOOHO2N

p–nitrobenzoic acid

CH(CH3)2CH3 COOHHOOC

KMnO4

heat

EXAMPLE:

benzoic acid

12.5-76

isopropyl toluene terepththalic acid

Alkyl group, regardless their chain length are converted to –COOH.

12.5-77

CH(CH3)2CH3 COOHHOOC

KMnO4

heat

Compounds without a benzylic H are inert to oxidation.

C(CH3)3CH3 C(CH3)3HOOC

KMnO4

heat

Take place at high temperature or in the presence of uv light.

Mechanism: free–radical substitution

Cl or Br replaces H atom of alkyl group

CH3

Cl2

heat or light

CH2Cl

EXAMPLE:

Cl2

heat or light

CHCl2

Cl2

heat or light

CCl3

toluene benzyl chloride

(dichloromethyl)benzene

(trichloromethyl)benzene

HALOGENATION OF TOLUENE 12.5-78

Free radical substitution reaction

HALOGENATION OF TOLUENE

Electrophilic aromatic substitution reaction

CH3

toluene

Br2

FeBr3

CH3

Br2 +

CH3

Br2

Benzylic radicals are even more stable than 3o radicals!

CH2

benzylicradical

●12.5-79

Many aromatic compounds are carcinorgenic and toxic.

CH3

toluene benzene

CARCINOGENIC EFFECT12.5-34

Example: benzene, benzo[a]pyrene.

At one time, benzene was widely used as solvent.

Studies revealed benzene is carcinorgenic (can cause cancer).

Replaced by toluene.

12.5-35

Benzo[a]pyrene is found in cigarette smoke, automobile exhaust, and the fumes from charcoal grills.

benzo[a]pyrene

When ingested or inhaled, it oxidised to carcinogenic products.

12.5-36

12.5-21

Benzoic acid, the simplest organic acid, prevent the growth of many organism.

12.5-20

widely used as a food preservative.

12.5-22

Fresh wild berries

12.5-19

END OF SLIDE SHOW