Polynuclear Aromatic Hydrocarbons

Ref. books

1.Organic Chemistry, Vol.1 - I.L. Finar

2.Organic Chemistry - Morrison and Boyd

3.Advanced Organic Chemistry – Bahl and Bahl

4.Organic Chemistry - Herbert Meislich

AzuleneIsolated

Biphenyl

Linear

Naphthalene Phenanthrene

Polynuclear Hydrocarbons

Benzenoid Non- Benzenoid

Fused rings

Angular

Polynuclear aromatic hydrocarbons are composed by two or more benzene rings

Benzenoid: Similar to benzene in structure or linkage; having an aromatic ring system.

Fused or condensed ring system: When two rings share a pair of carbon atoms, the rings are said to be fused rings.

2

1

3

4

665

4

21

5

3

Isolated ring

Biphenyl or diphenyl

o m

mo

p

o

m o

m

p

Naphthalene (C10H8)

18

2

3

45

6

7

Shows aromatic propertiesSatisfy Huckel’s rule (4n+2) =(4*2+2)=10

All C=C are not same (X-ray diffraction study)

C1=C2=1.36 Å

C2=C3=1.40 Å

Resonance energy of naphthalene is 61 Kcal/mol Benzene, 36 Kcal/mol

2nd aromatic ring is less stable (61-36)=25 Kcal/mol

Naphthalene is less aromatic (more reactive) than benzene

Structure elucidation of naphthalene

1. Molecular Formula: C10H8

NaphthaleneO

COOH

COOH

2.

So naphthalene contains the skeletonC

C

So nitro group is present in benzene ring

3. O

Nitrophthalic acid

Naphthalenenitration

Nitronaphthalene

COOH

COOH

NO2

O

Phthalic acid

COOH

COOH

4. Naphthalene nitration Nitronaphthalene aminonaphthaleneredn.

The benzene ring in phthalic acid produced by oxidation of aminonaphthalene is not the same ring is that obtained by oxidation of nitronaphthalene.

i.e. Naphthalene contains two benzene rings and we can explain this by this equation

B

NO2

HOOC

HOOC

O

NH2

A Bredn.

COOH

COOH

A

O

A B

HNO3

NO2

A B

The structure of naphthalene is confirmed by method of its synthesis

Howarth method

R

+ O

O

OSuccinic anhydride

AlCl3

R

O

OHO

Zn-Hg/HCl

RO

HO

RO

conc.H2SO4

-H2O

Zn-Hg/HCl

R

Se

R

Other way of cyclization

R

+ O

O

O

AlCl3

R

O

O

HO

Succinic anhydride

R

O

Cl

SOCl2

R

O

intramoluclar

AlCl3

Friedel Craf t

Zn-Hg/HCl

R

Se

R

The reaction occurs if R is o- or p- directing group such as NH2, NHR, OH, OR, R, halogen.

If R is m- directing group (e.g. NO2, CN, COOH, COCH3, SO3H) no reaction occur.

The above reaction gives -substituted naphthalene.

R

Synthesis of 1-alkyl naphthalene

+ O

O

O

AlCl3

O

Zn-Hg/HCl

Succinic anhydride

COOH COOHbenzene 4-oxo-4-phenylbutanic acid 4-phenylbutanoic acid

O RHOR

conc. H2SO4

-H2O

1) RMgX

2) H2O

1-tetralone1- Alkyl naphthalene

Se

From -benzylidene – propenoic acid

Zn-ZnO

naphthalene

OOH

O

conc. H2SO4

-H2O

Benzylidene-3-propenoic acid OH

Reduction

Naphthalene

1,4- dihydronaphthalene

NaEtOH

Naisoamyl alc.

1,2,3,4-tetrahydronaphthaleneTetralene

decahydronaphthaleneDecalene

H2

Ni

Oxidation

Naphthalene

CHO

CHO

Phthaldehyde

1) O3

2) H2O/Zn

1,4- naphthoquinone

CrO3

AcOH

O

O

Phthalic anhydride

O2

V2O5 O

O

O

COOH

COOHPhthalic acid

KMnO4

H

Addition of Cl2

Naphthalene

1,4- dichloro- 1,4- dihydronaphthalene

Cl2

Cl

Cl

excessCl2

ClCl

ClCl

1,2,3,4- tetrachloro- 1,2,3,4-tetrahydronaphthalene

Naphthalene undergoes ES mostly at alpha-position

Resonance forms determine higher reactivity at C-1

C-1 attack has 2 resonance structures with benzene rings

C-2 attack has only 1 resonance structure with a benzene ring

The most stable intermediate (C-1 attack) gives faster reaction

Attack at C-2

Attack at C-1

Electrophilic substitution reaction

Naphthalene

EE

one resonance structure

At position 1; carbocation intermediate stabilize by two resonance

So carbocation is more stable position 1 than 2

Naphthalene

E

E E

Naphthalene

naphthalene-1- sulfonic acid

Cl2

SO3H

Cl

1- chloronaphthalene

NO2

conc. HNO3conc. H2SO4

1- nitronaphthalene

conc. H2SO4

40°C

conc. H2SO4

180°C

SO3H

naphthalene-2- sulfonic acid

FeCl3

CH3COCl

AlCl3CS2

COCH3

1- Acetylnaphthalene

CH3COCl

AlCl3PhNO2

COCH3

2- Acetylnaphthalene

The lower stability of 1-S is attributed to the steric interaction between the sulfonic group and the hydrogen atom in the 8-position.

Sulfonation

Substituted naphthalene

Activating groups direct the electrophile to the same ring; i.e. Elctrodonating group (EDG): NH2, OH, OR, alkyl

Deactivating groups direct it to the other ring; i.e. Electrowithdrawing group (EWG): NO2, CO, COOH, CN, SO3H

Homonuclear attack

EDG E

E

EDG

E

Major

Minor

Heteronuclear attack

EWGE

E

EWG

E

Major E Major

Examples

conc. HNO3

conc. H2SO4

OH OH

NO2

+

OHNO2

Major

conc. HNO3

conc. H2SO4

NO2

OHOH

Examples

conc. HNO3

conc. H2SO4

NO2 NO2NO2

NO2

+

NO2

MajorMinor

conc. HNO3

conc. H2SO4

NO2

+

NO2 NO2

O2N

NO2

Summary of naphthalene reactionsSummary of naphthalene reactions

Anthracene (C14H10)

2

1

34

7

56

8 9

10

Anthracene (C14H10)

2

1

34

7

5

6

89

10

monosubstitution (C14H9X) = 3 isomers

Disubstitution (C14H8X2) = 15 isomers

Anthracene (C14H10)

C1-C2 bond to have more double bond character (shorter bond length)

C2-C3 bond to have more single bond character (longer bond length)

From X-ray diffraction study: C1-C2 bond = 1.37 Å C2-C3 bond = 1.42 Å Resonance energy 84 kcal mol-1, average 28, less

aromatic than benzene

Synthesis of anthracene

(i) By Friedel Crafts reaction

(a)

CH2Cl

+ClH2C

AlCl3

Benzyl chloride

-2HCl

-2H

Anthracene

Synthesis of anthracene

(b)

+ AlCl3

Acetylene tetrabromide

-4HBr

C

C

H

Br Br

BrBr

H

+

(c)

+ AlCl3

Methylenedibromide

-4HBr+

CBrBr

HH

CHH

BrBr

-2H

Anthracene

(ii) By Haworth synthesis

Synthesis of anthracene

HOOC

AlCl3+ O

O

O

O

Phthalic anhydride o-benzoylbenzoic acid

O

O Anthraquinone

H2SO4

-H2OZn

Anthracene

distil.

(iii) By Diels-Alder reaction

Synthesis of anthracene

O

O

+

1,3- Butadiene

1,4- Naphthaquinone

O

O

O

O

CrO3

AcOH

9,10- anthraquinone

Zn

Anthracene

Chemical reactions

Anthracene

+ E+

EH

EH

Leaves naphthalene intactLoss of RE=84-61=23 kcal

Anthracene

+ E+

H

E

Attack at C-1

Attack at C-2

EH

H Nu

Nu-

Chemical reactions

Attack at C-9

Anthracene

+ E+

EH

E

-H+

EH

HLeaves two benzene intactLoss of RE=84-72 =12 kcal

Substitution productAddition product

Reactions preferentially occur at C-9 & C-10

Diels Alder reaction

+ O

O

OAnthracene

Maleic anhydride

O

O

OEndo- anthracene- maleic anhydride adduct

Chemical reactions

Addition of one molecule of O2

Anthracene

+ O2

Anthracene epoxide

OO

SO3H

Major at LT

Major at HT

SO3H+

H2SO4

HH

H H

COH3C

CH

3CO

Cl

AlC

l 3 i n

ben

z en e

Cl2

in CCl4

ClH

H Cl

-HC

l

Cl

O

O

NO2 NO2

NO2

+

[HNO3+H2SO4 is not used, leads formation of 9,10 anthraqunone by oxidation]

Phenanthrene C14H10

2

1

3

4

75

6

8

9

10

Phenanthrene C14H10

2

1

3

4

7

5

6

8 9

10

2

1

34

7

56

10

8

9

monosubstitution (C14H9X) = 5 isomers

Disubstitution (C14H8X2) = 25 isomers

Position of double bond

2

1

3

4

7

5

6

89

10

C9-C10 bond to have more double bond character RE 92 kcal/mole, 92-72=20 Kcal/mole to remove the

aromaticity of the middle ring

Preparation of phenanthrene

1) Howrth method

+ O

O

O

AlCl3

COOH

O

NaphthaleneSuccinic anhydride

Zn-Hg/HCl

COOH

conc.H2SO4 O(i) Zn-Hg/HCl

(ii) Pd,

2) Posher synthesis

NO2

CHO

+

CH2COONa

Ac2O

NO2

COOH

Zn-Hg/HCl

NH2

COOH

NaNO2/H2SO4

N2HSO4

COOH

Cu

Phenanthrene

Preparation of 2- alkyl phenanthrene:

Preparation of 1- alkyl phenanthrene:

O 1) RMgX

2) H2OOH

RSe

R

+ O

O

O

AlCl3COOH

O

Zn-Hg/HCl

COOHconc.H2SO4 O Zn-Hg/HCl

Naphthalene

Se

R

R

R R R

R

Oxidation:

K2Cr2O7

H2SO4

O

O

PhenanthraquinonePhenanthrene

Na, heatC2H5OH

Reduction:

Na/EtOH

9,10-dihydro-phenanthrene

1) O2 2) H2O

CHO

CHO

Biphenyl-2,2'-dicarbaldehyde

Br2 Br

Br

9,10-dibromo-9,10-dihydro-phenanthrene

Br2

FeBr3

Br

9-bromo-9,10-dihydro-phenanthrene

H2O2 AcOH

COOH

COOH

Diphenic acid

EAS in anthracene or phenanthrene yields mixtures and is not generally useful. For example, in sulfonation:

13%

8%

18%

18%0%

Bromination is an exception:

Br2

FeBr3

Br2, CCl4

Br

BrBr

CH2

2

1

3

4

665

4

2

1

5

3

Biphenyl methane or diphenyl methane

o m

mo

p

o

m o

m

p

Diphenyl methane (C13H12)

7

1. Friedel- CrafteCH2Cl

+AlCl3

CH2

Diphenyl methaneBenzyl chloride

AlCl3CH2

Diphenyl methane

2 + CH2Cl2

Methods of preparation

2. From benzophenone

CH2

Diphenyl methane

O

HI/ P

or Zn-Hg/ HClor NH2NH2/ NaOEt

Benzophenone

Nitration

CH2

Diphenyl methane

conc. HNO2

conc. H2SO4CH2 NO2

1-benzyl-4-nitrobenzene

conc. HNO2

conc. H2SO4

CH2 NO2O2N

bis(4- nitrophenyl)methane

Halogenation

CH2

Diphenyl methane

hv CH

Diphenylmethylbromide

Br2Br

Oxidation

CH2

Diphenyl methane

K2Cr2O7H2SO4

C

benzophenone

[O]O

ret-hot

H2C

+ H2

Stilbene

Trans-stilbebestable

Cis-stilbebeunstable

(C6H5-CH=CH-C6H5)

C

C

HC6H5

H C6H5

Trans-stilbene Stable

C

C

HC6H5

C6H5 H

Cis-stilbene Stable

C6H5CHO + C6H5CH2MgBr C6H5CH

OMgBr

CH2C6H5

H+

C6H5CHOHCH2C6H5 heatH2O

CCH

C6H5H

C6H5

Syntheis of trans-stilbene

(I)

Syntheis of trans-stilbene

C6H5CHOHCOC6H5 EtOHZn/Hg

CCH

C6H5H

C6H5HCl

(II)

C6H5CHO + C6H5CH2COOKacetone

CCCOOH

C6H5H

C6H5

(III)

heatCC

H

C6H5H

C6H5

-Phenylcinnamic acid

Reactions of trans-stilbene

C6H5CH2CH2C6H5EtOH

NaCCH

C6H5H

C6H5

C6H5CHBrCHBrC6H5CCCOOH

C6H5H

C6H5

Br2

KOH

EtOH C6H5C CC6H5

Stilbebe dibromide

bibenzyl

Dphenyl acetylene

Synthesis of cis-stilbene

Zn

EtOHC6H5C CC6H5 CC

C6H5

HH

C6H5

Cis-stilbebe is readily converted into trans-stilbebe under the catalytic influence of traces of hydrogen bromide and peroxides