Transcript Enthalpy - Career Launcher

Chemistry
AROMATICS
Session objectives
1. Nomenclature of aromatic hydrocarbon
2. Structure of benzene
3. Aromaticity mechanisms of electrophilic of
aromatic substituents and orientation
4. Polynuclear hydrocarbons and their
toxicity.
Nomenclature of aromatic
compound
OMe
Methoxy benzene
Cl
Br
O2N
Br
1, 3-Dibromobenzene
(Correct name)
NO2
1-Chloro-2, 4-dinitrobenzene
(not 4-chloro-1, 3-dinitrobenzene)
Nomenclature of aromatic
compound
Br
Br
Br
Br
Br
Br
o-Dibromobenzene m-Dibromobenzene p-Dibromobenzene
Structure of benzene
Molecular formula C6H6
Raney Ni
C6H6 + 3H2  C6H12
473–523K
shows presence of three double bonds
A
A
1
1
6
2
5
B
3
4
1, 2 - disubstituted
isomer
But both are same
B
2
6
5
3
4
1, 6 - disubstituted
isomer
Structure of benzene
Resonance hybrid
structure of benzene
Support in favour of resonance hybrid of benzene
• The heat of hydrogenation of benzene is 36
kcal/mol.
• Length of carbon-carbon bond in benzene is
(1.39Å), whereas carbon-carbon single bond length
is 1.54Å and carbon-carbon double bond is 1.34Å.
Structure of benzene
Molecular structure of benzene
Structure of benzene
Ladenburg's
prism
Claus structure
Dewar
Armstrong-Bayer
structure castric
Thiele structure
Aromaticity
Class of compounds having a
characteristic stability
Conditions for aromaticity
• Complete delocalization of p-electrons of the ring
system.
• Planarity: Delocalization of p-electrons is possible only
if the ring is planar to allow cyclic overlap of p-orbitals.
• Huckel’s rule for aromaticity: According to Huckel’s rule
for a compound to be aromatic, a molecule must have
(4n + 2) electrons (n=0 to any number)
Aromaticity
Benzene
6 electrons
Phenanthracene
14 electrons
Naphthalene
10 electrons
Anthracene
14  electrons
Aromaticity
..
N
..
S
..
..
O
..
H
Pyrrol
Thiophen
Fura
Preparation of Benzene
Hydroformation or catalytic reforming
CH3  CH2 4 CH3
From neptha
CH3  CH2 5 CH3
From neptha
CH3 (CH2 )6 CH3
From neptha
Pt - Al2O3
C6H6 + 4H2
500°C Benzene
Pt - Al2O3
C6H5CH3 + 4H2
500°C
Toluene
Pt - Al2O3
C6H4(CH3 )2 +
500°C
Xylene
C6H5C2H5
Ethylbenzene
Preparation of Benzene
From acetylene
3CH
CH
red hot
tube
Benzene
From benzenediazonium chloride
+ –
N 2 Cl
+
2[H]
SnCl2/NaOH
+ HCl + N2
Benzene
Preparation of Benzene
+ –
N 2 Cl
+
H3PO2
Cu
Hypophosphorous
acid
+ H3PO3 + HCl + N2
Benzene
Double distillation of coal
Coal
destructive
distillation

 light oil fraction
i) washed with
conc. H2SO4
ii) washed with NaOH
iii) washed with H2O
iv) dried and distilled
Benzene + Toluene +Thiophene
Fractionless
distillation
Benzene (80 – 820C)
Properties
Electrophilic aromatic substitution
reaction of benzene
Halogenation
Cl
+ Cl2
Benzene
Anhd. AlCl3
300 - 320 K
+ HCl
Chlorobenzene
Mechanism of Halogenation
FeCl3 + Cl - Cl  Fe Cl4 + Cl+
H
H
+ slow
+ Cl
Cl
H
H
Cl
+
Resonance hybrid
structure
Cl
Cl
+
+ FeCl4
Cl
+
+
+
H
H
Cl
Abstraction of H+
+ HCl + FeCl3
Chloro benzene
Properties
Nitration
NO 2
H 2SO 4
+ HNO3  330°C
Nitro benzene
Mechanism of Nitration
2H2SO4 + HNO3 
+
NO2 + H3O + 2 HSO-4
+
H
H
+
+ NO2
Nitronium ion
slow
H
NO2
H
NO2
NO2
+
+
+
H
NO2
+
Intermediate
cation
Mechanism of Nitration
H
NO2
NO2
+

HSO
4
+
Abstraction of H+
+
Nitrobenzene
H 2SO4
Properties
Sulphonation
SO3H
+
Benzene
Conc. H 2SO4
 330 K
+ H2O
Benzene
sulphonic acid
Mechanism of Sulphonation
2H2SO4  H3O+ + SO3 + HSO-4
H
H SO
3
+ SO3
+
+
H 3O
H SO H
3
+
+
+
H SO H
3
+
Resonance hybrid structure
H SO H
3
H SO H
3
Mechanism of Sulphonation
H SO H
3
+
SO3H
HSO4
+
Benzene
sulphonic acid
H 2SO4
Properties
Friedel-Craft’s reaction
(a) friedel–craft alkylation
+ CH 3Cl
CH 3
Anhyd AlCl3
+
HCl
Toluene
(a) friedel–craft acylation
+ CH 3COCl
Anhyd AlCl3
COCH 3
+
HCl
Acetyl benzene
(Aceto phenone)
Mechanism of Friedel–Craft’s
reaction
CH3Cl + AlCl3  AlCl-4 + CH+
3
CH3COCl + AlCl3  AlCl-4 + CH3CO+
H

+ C H3
CH 3
H
+
+
+
H
CH 3
+
Resonance hybrid
structure
H
CH 3
CH 3
Mechanism of Friedel–Craft’s
reaction
H
CH 3
CH 3
+
+
AlCl4
+ HCl + AlCl3
Toluene
Limitations of Friedel Craft
alkylation
1. The danger of poly substitution.
2. The possibility that the alkyl group will
rearrangement.
3. Aryl halides cannot take place of alkyl halides.
4. An aromatic ring less reactive than that of the
halobenzenes does not undergo the Friedel Crafts
reaction.
Limitations of Friedel Craft
alkylation
5. Aromatic rings containing the
— NH2, — NHR or — NR2 group
do not undergo Friedel Crafts
alkylation because the strongly
basic nitrogen ties up the Lewis
acid.
H
–
C6H5NH2 + AlCl3
C6H5 N + AlCl3
H
Addition reaction of benzene
Addition of hydrogen
+ 3H2
Ni
475 - 500 K
Cyclohexane
(non aromatic
compound)
Benzene
(Aromatic
compound)
Addition of halogens
+ 3Cl2
Cl
Cl
Cl
Cl
Cl
h
Cl
Benzene Hexa Chloride (B.H.C)
Oxidation
COOH
CH 3
+ 3 [O]
KMnO4 / H 2O
+ H2O
375 - 385 K
Benzoic acid
Toluene
COOH
CH 2CH 3
+ 6 [O]
KMnO4 / H 2O
375 - 385 K
+ 2H2O + CO2
Benzoic acid
Oxidation
COOH
CH 3
KMnO 4 / H 2O
+ H2O
 385 K, 6[O]
COOH
CH 3
P-xylene
Terephthalic acid
CH 3
CH 3
COOH
KMnO4 / H 2O
COOH
+ 2H2O
 385 K, 6[O]
O-xylene
Phthalic acid
Oxidation
CH 3
C
CH 3
CH 3
CH 3
KMnO4
CH 3
C
CH 3
COOH
Ozonolysis of benzene
CHO
(i) O3
3
CHO
(ii) Zn dust/H2O
Glyoxal
CH 3
CHO
(i) O3
(ii) Zn dust/water
+
2
CHO
Glyoxal
CH 3
C
CHO
O
Methyl glyoxal
Influence of Substituents on
Elecrophilic Aromatic Substitution
Substituent
NH2 , OH, OCH3
NHCOCH3 , C6H5 , CH3
Effects on reactivity
Activates

N  CH3 3 , NO2
CN, SO3H, COOH
CHO, COR, halogens
Deactivates
Influence of Substituents on
Elecrophilic Aromatic
Substitution
Cl
Cl
Cl/FeCl
3

Cl
Cl
Cl
or
Cl
or
Cl
Influence of Substituents on
Elecrophilic Aromatic
Substitution
..
: OH
..
: OH
OH
OH
OH
Influence of Substituents on
Elecrophilic Aromatic
Substitution
O
O
O–
N
–
O
O
O
+
O
N
N
+
+
O
O
N
N
O
–
Ortho–Para and meta directors
Ortho-para directors
Meta directors
–F, –Cl, –Br, –I
–NO2
–OH
–SO3H
–OCH3, –OR
–CN
–NH2
–COOH
–NHR, –NR2
–CHO
–NHCOCH3
–N+ (CH3)3
–CH3, –C 2H5
O
||
–C — OR
Polynuclear Hydrocarbon and
their Toxicity
Naphthalene
Anthracene
Phenanthracen
e
CH 3
1, 2-benzanthracene
20-methyl cholanthracene
Class exercise
Class exercise 1
In the reaction of C6H5Y, the major
product (> 60%) is m-isomer, so the
group Y is
(a) — COOH
(b) — Cl
(c) — OH
(d) — NHCOCH3
Solution:
— COOH is m-directing group.
Hence, the answer is (a)
Class exercise 2
Which of the following is the
strongest o, p-directing group?
(a) –OH
(b) –Cl
(c) –OCH3
(d) –CH3
Solution:
Because the lone pair of electrons of OCH3 takes
part in resonance with benzene ring. Hence it is
stronger ortho-para directing group.
Hence, the answer is (c)
Class exercise 3
Aromatic compounds burn with a
sooty flame because
1. they have a ring
2. they have a relatively high
percentage of hydrogen
3. they have a relatively high
percentage of carbon
4. Of structure of carbon atoms
Solution:
Aromatic compounds have relatively
high percentage of carbon.
Hence, the answer is (c)
Class exercise 4
Benzene is less reactive than ethene and
ethyne towards addition reactions due to
1. the presence of 3-bonds
2. the cyclic nature
3. the sp2 hybridization of carbon
atoms
4.the delocalization of -electrons
Solution:
Delocalization of -electrons
reduce the reactivity of benzene.
Hence, the answer is (d)
Class exercise 5
Which of the following has the
highest melting point?
(a) o-xylene
(b) m-xylene
(c) p-xylene
(d) toluene
Solution:
p-xylene being symmetrical packs
closely in the crystal lattice.
Hence, the answer is (c)
Class exercise 6
Which of the following types of
compounds are expected to have
the highest octane number?
(a) Straight chain alkanes
(b) Cycloalkanes
(c) Branched chain alkanes
(d) Aromatic hydrocarbons
Solution:
Aromatic hydrocarbons have high octane number.
Hence, the answer is (d)
Class exercise 7
The chemical system that is
non-aromatic is
(a)
(b)
(c)
(d)
+
Solution:
The compound B has no -electrons
but other are aromatic compounds.
Hence, the answer is (b)
Class exercise 8
What is the ratio of  and 
bonds in benzene?
(a) 1 : 4
(b) 2 : 1
(c) 1 : 1
(d) 2 : 2
Solution:
In benzene
Number of -bonds = 3
Number of s-bonds = 12
Ratio = 3 : 12 = 1 : 4
Hence, the answer is (a)
Class exercise 9
Which one of the following is not
compatible with arenes?
(a) Greater stability
(b) Delocalization of -electrons
(c) Electrophilic addition
(d) Resonance
Solution:
Arenes do not give electrophilic addition because
they have stable compounds.
Hence, the answer is (c)
Class exercise 10
Which one of the following is
not aromatic?
(a) Benzene
(b) Cyclopentadienyl anion
(c) Tropyllium cation
(d) Cyclopentadienyl cation
Solution:
Cyclopentadienyl cation has only four -electrons
.
Hence, the answer is (d)
Thank you