Transcript Diapositiva 1 - Rajshahi University Web Page

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
Polynuclear
aromatic
hydrocarbons
composed by two or more benzene rings
are
Polynuclear Hydrocarbons
Benzenoid
Isolated
Non- Benzenoid
Fused rings
Linear
Angular
 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.
Isolated ring
m
3
o
2
1
p 4
5
m
o
2
m
3
1
5
6
6
o
m
o
Biphenyl or diphenyl
4 p
Naphthalene (C10H8)
Shows aromatic properties
Satisfy 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
2.
So naphthalene contains the skeleton
3.
 So nitro group is present in benzene ring
4.
 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
The structure of naphthalene is confirmed by
method of its synthesis
Howarth method
Other way of cyclization
 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.
Synthesis of 1-alkyl naphthalene
From -benzylidene – propenoic acid
Reduction
Oxidation
Addition of Cl2
Electrophilic substitution reaction
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-1
Attack at C-2
At position 1; carbocation intermediate stabilize by
two resonance
So carbocation is more stable position 1 than 2
Sulfonation
 The lower stability of 1-S is attributed to the
steric interaction between the sulfonic
group and the hydrogen atom in the
8-position.
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
Heteronuclear attack
Examples
Examples
Summary of naphthalene reactions
Anthracene (C14H10)

8
9
1
7
2
6
3
5
10

4
Anthracene (C14H10)

1
9
8
2
7
3
6
4
10

5
 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)
Synthesis of anthracene
(b)
(c)
Synthesis of anthracene
(ii) By Haworth synthesis
Synthesis of anthracene
(iii) By Diels-Alder reaction
Chemical reactions
Attack at C-1
Attack at C-2
Leaves naphthalene intact
Loss of RE=84-61=23 kcal
Chemical reactions
Attack at C-9
Leaves two benzene intact
Loss of RE=84-72 =12 kcal
Substitution product
Addition product
Reactions preferentially occur at C-9 & C-10
Chemical reactions
Diels Alder reaction
Addition of one molecule of O2
[HNO3+H2SO4 is not used, leads formation
of 9,10 anthraqunone by oxidation]
Phenanthrene C14H10
6
7
5
4
8
3
2
9
1
10
Phenanthrene C14H10
monosubstitution
(C14H9X) = 5 isomers
3
2
4
5
Disubstitution
(C14H8X2) = 25 isomers
1
6
9
10
7
8
10
8
9
1
7
2
6
5
4
3
Position of double bond
3
2
4
1
5
6
7
10
9
8
 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
2) Posher synthesis
Preparation of 1- alkyl phenanthrene:
Preparation of 2- alkyl phenanthrene:
Oxidation:
Reduction:
 EAS in anthracene or phenanthrene yields mixtures
and is not generally useful.
For example, in
sulfonation:
Diphenyl methane (C13H12)
m
3
p 4
o
2
o
2
1
7
1
m
3
4p
5
6
6
5
m
o
o
m
Biphenyl methane or diphenyl methane
Methods of preparation
1. Friedel- Crafte
2. From benzophenone
Nitration
Halogenation
Oxidation
Stilbene (C6H5-CH=CH-C6H5)
Trans-stilbebe
stable
Cis-stilbebe
unstable
Syntheis of trans-stilbene
(I)
C6H5CHOHCH2C6H5
Syntheis of trans-stilbene
(II)
C6H5CHOHCOC6H5
(III)
-Phenylcinnamic acid
Reactions of trans-stilbene
C6H5CH2CH2C6H5
bibenzyl
Stilbebe dibromide
Dphenyl acetylene
Synthesis of cis-stilbene
Cis-stilbebe is readily converted into transstilbebe under the catalytic influence of traces of
hydrogen bromide and peroxides