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Chemistry. AROMATICS. Session objectives. Nomenclature of aromatic hydrocarbon Structure of benzene Aromaticity mechanisms of electrophilic of aromatic substituents and orientation Polynuclear hydrocarbons and their toxicity. Nomenclature of aromatic compound.

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Chemistry


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AROMATICS


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Session objectives

  • Nomenclature of aromatic hydrocarbon

  • Structure of benzene

  • Aromaticity mechanisms of electrophilic of aromatic substituents and orientation

  • Polynuclear hydrocarbons and their toxicity.


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Nomenclature of aromatic compound


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Nomenclature of aromatic compound


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Structure of benzene

Molecular formula C6H6

shows presence of three double bonds

But both are same


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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.


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Structure of benzene

Molecular structure of benzene


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Structure of benzene


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Aromaticity

Class of compounds having a characteristic stability

Conditions for aromaticity

  • Complete delocalization ofp-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)


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Aromaticity


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Aromaticity


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Preparation of Benzene

Hydroformation or catalytic reforming

From neptha

From neptha

From neptha


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Preparation of Benzene

From acetylene

From benzenediazonium chloride


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Preparation of Benzene


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Double distillation of coal

Coal

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)


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Properties

Electrophilic aromatic substitution reaction of benzene

Halogenation


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Mechanism of Halogenation


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Properties

Nitration


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Mechanism of Nitration


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Mechanism of Nitration


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Properties

Sulphonation


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Mechanism of Sulphonation


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Mechanism of Sulphonation


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Properties

Friedel-Craft’s reaction

(a) friedel–craft alkylation

(a) friedel–craft acylation


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Mechanism of Friedel–Craft’s reaction


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Mechanism of Friedel–Craft’s reaction


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Limitations of Friedel Craft alkylation

  • The danger of poly substitution.

  • The possibility that the alkyl group will rearrangement.

  • Aryl halides cannot take place of alkyl halides.

  • An aromatic ring less reactive than that of the halobenzenes does not undergo the Friedel Crafts reaction.


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Limitations of Friedel Craft alkylation

  • 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.


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Addition reaction of benzene

Addition of hydrogen

Addition of halogens


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Oxidation


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Oxidation


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Oxidation


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Ozonolysis of benzene


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Influence of Substituents on Elecrophilic Aromatic Substitution

Activates

Deactivates


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Influence of Substituents on Elecrophilic Aromatic Substitution

or

or


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..

..

:

:

Influence of Substituents on Elecrophilic Aromatic Substitution


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+

+

+

Influence of Substituents on Elecrophilic Aromatic Substitution


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Ortho–Para and meta directors


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Polynuclear Hydrocarbon and their Toxicity


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Class exercise


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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)


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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)


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Class exercise 3

  • Aromatic compounds burn with a sooty flame because

  • they have a ring

  • they have a relatively high percentage of hydrogen

  • they have a relatively high percentage of carbon

  • Of structure of carbon atoms

Solution:

Aromatic compounds have relativelyhigh percentage of carbon.

Hence, the answer is (c)


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Class exercise 4

  • Benzene is less reactive than ethene and ethyne towards addition reactions due to

  • the presence of

  • the cyclic nature

  • the sp2 hybridization of carbon atoms

  • the delocalization of

Delocalization of reduce the reactivity of benzene.

Solution:

Hence, the answer is (d)


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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)


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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)


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Class exercise 7

The chemical system that is non-aromatic is

(a)

(b)

(c)

(d)

The compound B has no but other are aromatic compounds.

Solution:

Hence, the answer is (b)


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Class exercise 8

What is the ratio of bonds in benzene?

(a) 1 : 4(b) 2 : 1(c) 1 : 1(d) 2 : 2

In benzene

Number of -bonds = 3

Number of s-bonds = 12

Ratio = 3 : 12 = 1 : 4

Solution:

Hence, the answer is (a)


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Class exercise 9

Which one of the following is not compatible with arenes?

(a) Greater stability(b) Delocalization of (c) Electrophilic addition(d) Resonance

Solution:

Arenes do not give electrophilic addition because they have stable compounds.

Hence, the answer is (c)


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Class exercise 10

Cyclopentadienyl cation has only four .

Which one of the following is not aromatic?

(a) Benzene(b) Cyclopentadienyl anion(c) Tropyllium cation(d) Cyclopentadienyl cation

Solution:

Hence, the answer is (d)


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Thank you


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