PAH COMPOUNDS

1. Shikshan Prasarak Mandal’s
College of Pharmacy, Akluj
Tal-Malshiras Dist-Solapur
Department of Pharmaceutical Chemistry
UNIT IV: POLYNUCLEAR HYDROCARBONS
Class: Second Year B.Pharmacy (Sem III)
Subject: Pharmaceutical Organic Chemistry II
Prepared By: Mr. R. N. Jalkote ( Assit. Prof)

2. NAPTHALENE
 The aromatic hydrocarbons made up of two or more benzene rings fused together are called
Polynuclear hydrocarbons or Fused – ring hydrocarbons. Naphthalene is the simplest and the
most important member of this class of arenes, in which two benzene rings are fused in ortho
positions. It has structural formula:
 The monosubstituted naphthalene derivatives are usually designated by the prefixes ά and β.

3. • In the IUPAC system, all carbons that may bear substituents are numbered. The numbering
system begins with number 1 at the alpha position.
• A. kekule type formula: naphthalene had molecular formula C10 H8. (elemental analysis)

4. Presence of two fused ring:

5. • Naphthalene is classified as aromatic because its properties resemble those of benzene.
• Its molecular formula c10 h8, might lead one to expect a high degree of unsaturation; yet
naphthalene is resistant to the addition reactions. Characteristics of unsaturated compounds.
• Instead, the typical reactions of naphthalene are Electrophillic substitution reactions, in which
hydrogen is displaced as hydrogen ion and the naphthalene ring system is preserved.
• Like benzene, naphthalene is unusually stable: its heat of combustion is 61 Kcal lower than
that calculated on the assumption that it is aliphatic.
• Naphthalene has the structure required of an aromatic compound: it contains flat six-
membered rings, and consideration of atomic orbital shows that the structure can provide Π
clouds containing six electrons- the aromatic sextet.
 Existence of two monosubstituted derivatives confirms Kekule type formula.

6. RESONANCE MODEL :
1. Resonance hybrid of Three forms.
Thus the pie electrons system of naphthalene is delocalized. The delocalization lowers the
electronic energy.
2. Not All C-C Bond lengths are equal

7. ORBITAL MODEL

8. PREPARATION OF NAPHTHALENE
1. HAWORTH SYNTHESIS:
2. FROM COAL TAR.

9. 3. FROM PETROLEUM
4. FROM 4 PHENYL-1- BUTENE

10. REACTIONS OF NAPHTHALENE
 PHYSICAL PROPERTIES OF NAPHTHALENE:
 Naphthalene is colorless solid which forms shining flaked crystals, mp: 80 degree Celsius.
Bp:217.9. familiar odor as a moth balls. Volatile and sublimes slowly at room temperatures.
Naphthalene is insoluble in water, moderately soluble in alcohol, very soluble in ether and
benzene. It burns with a smoky flame.
 CHEMICAL PROPERTIES:
 Naphthalene gives the usual aromatic Electrophillic substitution reaction. The product of
monosubstituted is either alpha or beta depending on conditions. But alpha product
predominates.

11. Why the alpha product dominates?
 As indicated above, in monosubstitution of naphthalene the alpha product is formed in
preference to beta product. This can be understood if we examine the intermediate carbonium
ion. Two resonance structures can be written for the intermediate carbonium ion obtained
from attack at alpha position. ( without involving other rings), where as only such structure is
possible for the substitution a beta position.

12. 1. Halogenation Reaction
 Naphthalene reacts with chlorine and bromine in CCL4 solution to form α -
chloronaphthalene or α -bromonaphthalene.

13. 2. Nitration
 When warmed with a mixture of concentrated nitric acid and sulfuric
acid, naphthalene forms α -nitronaphthalene.

14. 3. Sulfonation
 On treatment with concentrated sulfuric acid at 40 degree Celsius, it yields α -
naphthalenesulfonic acid. At 165 degree Celsius, β -sulfonic acid is obtained.

15. 4. FRIEDEL-CRAFTS ACYLATION
 Naphthalene reacts with acetyl chloride( or benzyl chloride) and aluminum chloride
in CS2 to give α -acetylnaphthalene. If nitrobenzene is used as solvent, β product is
obtained.

16. 5. Reduction.
 The reduction of naphthalene with sodium and alcohol forms 1,4
dihydronaphthalene. Catalytic hydrogenation gives 1,2,3,4-tetrahydronaphthalene
(tetralin) or decahydronaphthalene (decalin) depending on the catalyst used.

17. 6. Oxidation
 Naphthalene on mild oxidation by chromic acid in acetic acid forms 1,4-
naphthoquinone. Upon oxidation with air or oxygen and vanadium pentoxide, it
yields pthalic anhydride.

18. USES OF NAPHTHALENE
Naphthalene is used as:
1. As moth repellent, in which role it has now been replaced by p-dichlorobenzene.
2. For commercial production of phthalic anhydride
3. For making α -napthol ( insecticides) and β -naphthol (tanning agent).
4. For manufacture of dyes, explosives and synthetic resins.

19. NAPHTHOLS
The monohydroxy derivatives of naphthalene are called naphthols.
Properties:
• they are colorless solids, mp alpha naphthol 123-124 degree Celsius, beta naphthol
96.
• They are insoluble in water, soluble in benzene and very soluble in alcohol and
ether. Naphthol gives all the chemical reactions characteristics of phenols.
• They react with ammonia in the presence of ammonium bisulfite to yield
corrosponding naphthols( Bucherer Reaction)

20. Preparation
 both α and β - naphthols are prepared from the corresponding
naphthalenesulfonic acids by fusion with sodium hydroxide (300
degree Celsius) followed by acidification.

21. REACTIONS
USES
 They are used as insecticides and for making dyes, methyl and
ethyl ethers of β-naphthol are used extensively in perfumery.