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Biology and chemistry, Study notes of Medical Biochemistry

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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
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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

Azulene

Isolated

Biphenyl

Linear

Naphthalene

Phenanthrene

Polynuclear Hydrocarbons

Benzenoid

Non- Benzenoid

Fused rings

Angular

Polynuclear aromatic hydrocarbons are

composed by two or more benzene rings

Naphthalene (C

H

8 1

2

3

4

5

6

7

Shows aromatic properties

Satisfy Huckel’s rule (4n+2)

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

C

1

=C

2

=1.36 Å

C

2

=C

3

=1.40 Å

Resonance energy of naphthalene is 61 Kcal/mol

Benzene, 36 Kcal/mol

2nd aromatic ring is less stable (61-36)=

Kcal/mol

Naphthalene is less aromatic (more reactive)

than benzene

So nitro group is present in benzene ring

O

Nitrophthalic acid

Naphthalene

nitration

Nitronaphthalene

COOH
COOH
NO

2

O

Phthalic acid

COOH

COOH

Naphthalene

nitration

Nitronaphthalene
aminonaphthalene

redn.

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
NO

2

HOOC
HOOC
O

NH

2

A B

redn.

COOH

COOH

A

O

A
B
HNO

3

NO

2

A
B

Other way of cyclization

R

O

O

O

AlCl

3

R

O

O

HO

Succinic anhydride

R

O

Cl

SOCl

2

R

O

intramoluclar

AlCl

3

Friedel Craf t

Zn-Hg/HCl

R

Se

R

The reaction occurs if R is o - or p - directing

group such as NH

2

, NHR, OH, OR, R,

halogen.

If R is m - directing group (e.g. NO

2

, CN,

COOH, COCH

3

, SO

3

H) no reaction occur.

The above reaction gives-substituted

naphthalene.

R

From-benzylidene – propenoic acid

Zn

  • ZnO

naphthalene

O

OH

O

conc. H

2

SO

4

  • H

2

O

Benzylidene- 3 - propenoic acid

OH

Reduction

Naphthalene

1 , 4 - dihydronaphthalene

Na

EtOH

Na

isoamyl alc.

1 , 2 , 3 , 4 - tetrahydronaphthalene

Tetralene

decahydronaphthalene

Decalene

H

2

Ni

Addition of Cl

Naphthalene

1 , 4 - dichloro- 1 , 4 - dihydronaphthalene

Cl

2

Cl

Cl

excess

Cl

2

Cl

Cl

Cl

Cl

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

Naphthalene undergoes ES mostly at alpha-position

Resonance forms determine higher reactivity at C-

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-

Attack at C-

Electrophilic substitution reaction

Naphthalene

naphthalene- 1 - sulfonic acid

Cl

2

SO

3

H

Cl

1 - chloronaphthalene

NO

2

conc. HNO 3

conc. H

2

SO

4

1 - nitronaphthalene

conc. H

2

SO

4

40 ° C

conc. H 2

SO 4

180 ° C

SO

3

H

naphthalene- 2 - sulfonic acid

FeCl

3

CH

3

COCl

AlCl 3

CS 2

COCH

3

1 - Acetylnaphthalene

CH 3

COCl

AlCl 3

PhNO 2

COCH

3

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