Matriculation Chemistry ( Aromatic Compound )

The origin of August of August Kekulé’s Kekulé’s view of benzene structure: “There I sat and wrote my textbook, but things did not go well; my mind was occupied with other matters, matters, I turned the chair towards the fireplace and began to doze. doze.

“Once again the atoms danced before my eyes. eyes. This time smaller groups modestly remained in the background. b ackground. My mental eye, sharpened by repeated apparitions of similar  kind, now distinguished larger  units of various shapes shapes.” .” “Long rows, frequently joined more densely; everything in motion, twisting and turning like snakes. snakes. And behold, what was that? One of the snakes caught hold of its own tail and mockingly whirled around my eyes.

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

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

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

August Kekulé

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

“Imagination is more important than knowledge.” ALBERT EINSTEIN

AROMATIC COMPOUNDS 14.1

Introduction to aromatic Compounds

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

KEYWORDS aromatic resonance structure electrophilic substitution activating and deactivating group ortho-para and meta director 

Kekulé structure ortho meta para Friedel-Craft acylation oxidation of  alkylbenzene substitution of  toluene

DISCOVERY OF BENZENE 1825

Michael Faraday (British)

Isolated a pure compound of  boiling point 80oC Empirical formula = CH Named “bicarburet “bicarburet of hydrogen” hydrogen”

1834

Eilhard Mitscherlich (German)

Prepared benzene from benzoic acid C6H5CO2H + CaO

heat

C6H6 + CaCO3

Molar mass = 78 78,, molecular formula = C6H6 named “benzin “benzin”” benzene gum benzoin

benzin benjoin (French)

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 their pleasant odours. odours.

Benzene has strong pleasant odour .

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

AROMATIC COMPOUNDS Aromatic compound is a cyclic conjugated molecule or ion that is stabilized by ∏ electron delocalisation. delocalisation. it is characterised by substitution reactions.

KEKULÉ’S STRUCTURE 

Kekule was the first to formulate a reasonable representation of benzene H

H

C

C

H C C

C C

H

or  H

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

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

equivalent to

resonance hybrid All C–C bond length equal = 139 pm Shorter than typical C–C (148 pm) Longer than typical C=C (134 pm)

The six

electrons completely delocalized around the ring

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

All six C atoms and six  p orbitals are equivalent

THE CRITERIA FOR AROMATICITY 4 structural criteria must be satisfied for  compound to be aromatic C y  yc    l l   i i c  c c 

 y  l y  e   e  t  e   e  l  p  C o m  e  e d  t  a  g  u   u  j  c o n

P l la   nar 

C o  on     t  n ta    i  a i  n  n p a  ar  r  t  t   i  ic   cu    l  u n u  l a   ar  um   mb  r     be   er   r o f  f ∏  e l  le   ec    t  c   r  t r  o    n  o    n  o  b  be    y  e   s  y    H Ü  s Üc    k  c  e   l  R u  l  ’  ’ s  ul   le  s   e  ke 

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 Erich HÜckel (1896-1980) 

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

THE CRITERIA FOR AROMATICITY EXAMPLE OF AROMATIC COMPOUNDS

1. Aromatic compounds with a single ring Benzene

aromatic Benzene is aromatic because: 

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



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 EXAMPLE Aromatic

naphthalene 4n+2= 4(2)+ 2 10 ∏ electrons * Two benzene rings joined together forms naphthalene naphthalene

AROMATIC COMPOUNDS Nomenclature of benzene 14.2 and its derivatives

Arenes = aromatic compounds 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 substituents.. Many common name are recognized by the IUPAC system EXAMPLE: CH3 Common: toluene IUPAC: methylbenzene

MONO SUBSTITUTED BENZENE Benzene is the parent name and the substituent is indicated by a prefix prefix.. Cl

F

Br 

fluorobenzene chlorobenzene NO2

nitrobenzene

bromobenzene CH2CH3

ethylbenzene

IUPAC rules allow some common names to be retained. CH3

toluene COOH

benzoic acid

OH

NH2

phenol

aniline CHO

benzaldehyde

DISUBSTITUTED BENZENE Two Same Substituents

Relative position of subsituents are indicated by prefixes ortho, meta, and  para ( o –  –,, m –  –,, and  p –  –)) or by the use of number  of number . Br  Br  1 2 1 2 Br  Br  1

2 3

4

Br 

3

Br 

1,2–dibromobenzene 1,4–dibromobenzene or  or  1,3–dibromobenzene o –dibromobenzene  p –dibromobenzene or  m –dibromobenzene

NO2

1

2

NO2

NO2

1,2–dinitrobenzene or  o –dinitrobenzene

1

NO2

1

NO2

1,3–dinitrobenzene or  m –dinitrobenzene

2 4

2 3

3

NO2 1,4–dinitrobenzene or   p –dinitrobenzene

DISUBSTITUTED BENZENE Two Different Substituents

Select one of the substituent that give new parent name and numbered as C1 C1.. COOH 1 2 NO 2

COOH 1

2

COOH 1

2 3

4

NO2

3

NO2

2–nitrobenzoic acid 4–nitrobenzoic acid or  or  3–nitrobenzoic acid o –nitrobenzoic acid  p –nitrobenzoic acid or  m –nitrobenzoic acid

THREE OR MORE SUBSTITUENTS Position of substituents must be indicated by numbers.. numbers The substituents are listed alphabetically when writing the name. Cl

Br  1

2 4

Br 

3

Br  1,2,4–tribromobenzene

Br  I 2–bromo–1–chloro–3–iodobenzene

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

1 2

COOH

NO2

3 4

5

3

OH HO 4 3,5–dihydroxybenzoic 3,5–dihydroxyben zoic acid

NO2 2,4–dinitrophenol Br  4

6

1 2

3

2

CH3

CH3 4–bromo–1,2–dimethylbenzene 1

Br  4

3

2

CH3

1

CH3

4–bromo–1,2–dimethylbenzene

correct

4–bromo–o –dimethylbenzene o –  –,, m –

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

14.2-11

PHENYL GROUP Benzene ring as substituent substituent.. If alkyl substituent is larger than the ring (more than 6 C), the compound is named as phenyl-substituted alkane. alkane. 1 CH2

6 3 4 5 7 2 CH–CH  –CH  –CH  –CH  –CH 2

2

2

2–phenylheptane

Phenyl = C6H5  – = Ph

2

3

14.2-12

If the chain is unsaturated (have C═C or C≡C or C≡C)) or contains important functional group, group, the benzene ring is considered as phenyl substituent. substituent. 1

2

CH2  –C

3 4

C–CH 3

1–phenyl–2–butene

2

1

CH2 –CH2 –OH

2–phenylethanol

14.2-13

BENZYL GROUP CH2—

phenyl group

benzyl group

CH2Br 

CH2OH

benzyl bromide

benzyl alcohol

AROMATIC COMPOUNDS Chemical properties of benzene 14.3 and its derivatives

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

14.3-01

UNUSUAL REACTIONS OF BENZENE Br 2 / CCl4 BENZENE

KMnO4 / H2O H2 / Ni

no addition of Br 2 (no decolorization) decolorization) no oxidation (no decolorization) decolorization) slow addition at high temperature and pressure

Alkenes readily undergo addition reaction,, benzene does not! reaction not!

14.3-03

REACTION OF ARENES Involves the benzene ring itself  Electrophilic aromatic substitution + Br 2

Br 

FeBr 3

benzene

bromobenzene

Involves substituents attached to the ring CH3CH2CH2CH3 KMnO 4 H2O

COOH

ELECTROPHILIC SUBSTITUTION

14.3-04

Most characteristic reaction of benzene. A H atom is replaced by an electrophile electrophile.. H

E +

E+ electrophile

+

H+

14.3-05

X2, FeX3 (X = Cl Br)

X + HX

halogenatio n

HONO2

NO2 + H2O

nitratio n

R + HCl

FriedelCrafts Alkylation

H2SO4 RCl , AlCl3 (R can rearrange rearrange)) O RCCl , AlCl3

O C–R + HCl

FriedelCrafts Acylation

14.3-07

GENERAL MECHANISM Formation of arenium of arenium ion

STEP 1

H H

H

H

H

E+

H H H H

+

H

E H H

H H

H

E H

+

H

H H arenium ion

E H

+

H

14.3-08

ARENIUM ION

+ E+

E H arenium ion

benzene ring

Electrophile takes two electrons of six–electrons system to form bond. This interrupts of cyclic of cyclic system of  Benzene ring (aromatic aromatic))

electrons.. electrons

arenium ion (nonaromatic nonaromatic))

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

Loss of H of H

STEP 2 H H

H

E

+

H

H

14.3-09

+

H

H

E

H

H H

H

Substitution reaction allow aromatic six electrons to be regenerated regenerated.. E H

E+ 6

electrons

4

electrons

+

 – H+

E 6

electrons

14.3-10

Kekulé structures are more appropriate for writing mechanisms.. mechanisms For simplicity For simplicity,, however, we can show the mechanism in the following way: E

STEP 1 + E+

H

+

arenium ion E

STEP 2

+

E H

+ H+

HALOGENATION

14.3-11

Reactants: benzene and halogens (Cl2 or Br 2). Reactants: Conditions: Lewis acid such as FeCl3 and FeBr 3 no reaction (decolorization not observed)

+ Br 2

+ Br 2

FeBr 3

Br  bromobenzene

+ HBr 

14.2-13

MECHANISM STEP 1

Formation of Br  of Br + +

Br —Br–FeBr  —Br–FeBr 3 complex

Br  –Br + FeB3r   STEP 2

Br +

Loss of H of H+ Br  FeBr 4 –

+

H

+ FeBr 4 – Br 

Formation of arenium of arenium ion + Br +

STEP 3

 –

+

H

Br  + HBr  + FeBr 3

FUNCTION OF LEWIS ACIDS

14.3-14

Increase polarity of halogen of halogen molecules. molecules. Produce positive halogen ions (Br + or Cl or Cl+).

14.3-15

NITRATION Reactants: benzene and concd. HNO3. Reactants: Conditions: Concd. H2SO4 NO2

HNO3 H2SO4

nitrobenzene + H+

+ HSO4 –

12.5-49

MECHANISM STEP 1

 . . H-O-NO  . . 2

Formation of nitronium of nitronium ion (NO2+)

H

+ H-OSO3H

H-O ..+-NO2 + HSO4 –

H2O + NO2+ nitronium ion

STEP 2

Formation of arenium of arenium ion + NO

+ 2

STEP 3

Loss of H of H+  – HSO 4 NO2

+

H

NO2

+

H

NO2 + H2SO4

12.5-51

FRIEDEL–CRAFT ALKYLATION Reactants: benzene and haloalkane Reactants: haloalkane.. Conditions: Catalyst:: Lewis acid such as AlCl3. Catalyst

+ R –X

AlCl3

R

+ HCl

alkylbenzene Alkylation = transfer  transfer an an alkyl group to benzene

12.5-52

12.5-53

EXAMPLE: Cl

CH(CH3)2 + HCl

AlCl3

+ CH3CHCH3 2–chloropropane

isopropylbenzene

AlCl3

+ (CH3)3C –Cl 2–chloro–2–methylpropane

C(CH3)3 + HCl tert   –butylbenzene

12.5-54

MECHANISM STEP 1

Formation of carbocation of carbocation

(CH3)2CHCl: + AlCl3                                             :                        :

                                            :                        :

+ (CH3)2CH–Cl–AlCl 3                                             :                        :

                                            :                        :

(CH3)2CH+ + AlCl4 – carbocation

Formation of arenium of arenium ion

STEP 2

+

CH(CH3)2

+

CH(CH3)2

+

H

Loss of H of H+

STEP 3

CH(CH3)2

+

H

AlCl4 – CH(CH3)2 + HCl

OTHER FACTS ABOUT FRIEDEL–CRAFT ALKYLATION

12.5-55

Rearrangement can occur, especially when 1o haloalkanes are used. CH2CH2CH2CH3

EXAMPLE: CH3CH2CH2CH2Cl AlCl3

+

butylbenzene CH3 CHCH2CH3

65 %

sec   –butylbenzene

Rearrangement: H

35 %

+

CH3—CH2—CH—CH2

+

CH3—CH2—CH—CH3 2 carbocation

FRIEDEL–CRAFT ACYLATION

12.5-56

Reactants: benzene and acid chloride. Reactants: chloride. Product:: ketone Product ketone.. Conditions: Catalyst:: Lewis acid such as AlCl3. Catalyst EXAMPLE: O +

CH3C— C—Cl Cl

O AlCl3

CCH3

acetyl chloride acetophenone

+ HCl

ACYL GROUP O RC— acyl group EXAMPLE:

O

O CH3C— acetyl group

 –C— benzoyl group

Acylation = transfer  transfer an an acyl group to benzene

12.5-57

12.5-59

MECHANISM STEP 1

Formation of acylium of acylium ion

:O: R–C–Cl + AlCl3

:O: R–C–Cl–AlCl3

R–C  ═ O +

                      :

                      :

R–C≡O+ + AlCl4 –

acylium ion

STEP 2

STEP 3

Formation of arenium of arenium ion

R C=O

+ R–C+ ═O

H

Loss of H of H+ R C=O

+

H

AlCl4 –

+

R C═O + HCl

SUBSTITUENT EFFECT

12.5-61

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

EFFECT ON REACTIVITY

12.5-62

Activating groups: Substituents that activate the ring ring,, making it more reactive than benzene benzene.. Deactivating groups: Substituents that deactivate the ring ring,, making it less reactive than benzene benzene.. EXAMPLE: relative rate of nitration of nitration

NO2 6 x 10 –8

Cl 0.033 reactivity

H 1

OH 1000

EFFECT ON ORIENTATION

12.5-63

Ortho–para directors Tend to direct the incoming group into ortho and para positions. positions. Meta directors Tend to direct the incoming group into meta position. position. EXAMPLE: CH3 HNO3 H2SO4

ortho-para director  CH3 CH3 NO2 + + (59

(37

NO

CH3

NO2 (4

12.5-64

CLASSIFICATION OF SUBSTITUENTS ortho – ,  para – directing

activators

ortho – ,  para –

directing

deactivators

meta –

directing deactivators

ORTHO– , PARA– DIRECTING ACTIVATORS

12.5-65

Increasing activation ●●

●●

●●

—NH2 —NHR —NR2 ●●

—OH ●● ●●

—OR ●●

General structure: —R or —Z

● ●

●●

—NHCOR —R

Alkyl groups or have nonbonded electron pair  on the atom bonded to benzene ring

12.5-66

EXAMPLE:

CH2CH3 Br 2

CH2CH3 Br 

CH2CH3 +

FeBr 3 ethylbenzene

(38 %)

(62 %)

Br 

ORTHO– , PARA– DIRECTING DEACTIVATORS ●●

●●

—F

—Cl

● ● ●●

●●

●●

●●

—Br  ●●

● ●

—I

● ●

● ● ●●

General structure —X (halogens) ●●

● ● ●●

12.5-67

12.5-68

EXAMPLE:

Cl

Cl

NO2

HNO3 H2SO4 chlorobenzene

Cl +

(35 %)

(64 %)

NO2

12.5-69

META– DIRECTING ACTIVATORS —CHO —COR —COOR —COOH —CN —SO3H —NO2 + —NR3 Increasing deactivation

General structure: —Y ( + or   –) Have a full or partial or partial positive charge on the atom bonded to benzene ring

12.5-69

EXAMPLE:

NO2 HNO3

NO2

H2SO4 nitrobenzene

(93 %)

NO2

12.5-70

INDUCTIVE EFFECT Due to: Electronegativity of the atoms in the substituent. Polarisability of the substituent. EXAMPLE: Cl Cl— electron–withdrawing

CH3 CH3— electron–donating

12.5-71

Activating Groups Release electrons electrons to the ring Stabilise arenium ion

CH3

NO2

+

Form faster  Deactivating Groups Withdraw electrons from the ring Destabilise arenium ion Form slower 

Cl

+

NO2

12.5-72

EXAMPLE: CF3

CH3

trifluoromethyl)benzene

benzene

toluene

increasing rate of nitration CF3

NO2

NO2

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

+

CH3

NO2

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

OXIDATION OF SIDE CHAIN

12.5-75

Reactants:: arene with benzylic H. Reactants H. Conditions: Strong oxidizing agent such as KMnO4 and Na2Cr 2O7. Heat.. Heat benzylic H CH3 CH3

CH(CH3)2

EXAMPLE:

12.5-76

CH3

COOH KMnO4 heat

toluene O2N

benzoic acid CH3

Na2Cr 2O7 heat

 p –nitrotoluene

CH3

CH(CH3)2

isopropyl toluene

O2N

COOH

 p –nitrobenzoic acid

KMnO4 heat

HOOC

COOH

terepththalic acid

12.5-77

CH3

CH(CH3)2

KMnO4 heat

HOOC

COOH

Alkyl group, regardless their chain their chain length are converted to –COOH to –COOH.. Compounds without a benzylic H are inert to oxidation oxidation..

CH3

C(CH3)3

KMnO4 heat

HOOC

C(CH3)3

HALOGENATION OF TOLUENE

12.5-78

Free radical substitution reaction

Take place at high temperature or in the presence of uv of uv light. light. Mechanism: free–radical substitution Cl or Br  or Br replaces replaces H atom of alkyl of alkyl group EXAMPLE: CH3

(dichloromethyl)benzene CH2Cl

Cl2

Cl2

heat or  light

toluene

heat or  light

benzyl chloride

CHCl CH Cl2

Cl2

CCl3

heat or  light

(trichloromethyl)benzene

HALOGENATION OF TOLUENE Electrophilic aromatic substitution reaction

CH3

CH3

CH3 Br 2

Br 2

+

FeBr 3

toluene

Br 2

●

CH2

12.5-79

benzylic radical

Benzylic radicals

more stable than 3o radicals radicals!!

CARCINOGENIC EFFECT

12.5-34

CH3

benzene

toluene

Many aromatic compounds are carcinorgenic and toxic toxic.. Example: benzene benzene,, benzo[a]pyrene benzo[a]pyrene..

12.5-35

At one time, benzene was widely used as solvent solvent.. Studies revealed benzene is carcinorgenic (can cause cancer ). ). Replaced by toluene

12.5-36

benzo[a]pyrene Benzo[a]pyrene is found in cigarette smoke, smoke, automobile exhaust,, and the fumes from charcoal grills. exhaust grills. When ingested or inhaled, it oxidised to carcinogenic products.. products

12.5-21

12.5-20

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

12.5-22

widely used as a food preservative

12.5-19

Fresh wild berries

END OF SLIDE SHOW