Polynuclear Aromatic Hydrocarbons Ref. books 1.Organic Chemistry, Vol.1 - I.L. Finar 2.Organic...
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Transcript of Polynuclear Aromatic Hydrocarbons Ref. books 1.Organic Chemistry, Vol.1 - I.L. Finar 2.Organic...
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