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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


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


Nomenclature of aromatic compound


Structure of benzene

Molecular formula C6H6

shows presence of three double bonds

But both are same


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


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)


Aromaticity


Aromaticity


Preparation of Benzene

Hydroformation or catalytic reforming

From neptha

From neptha

From neptha


Preparation of Benzene

From acetylene

From benzenediazonium chloride


Preparation of Benzene


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)


Properties

Electrophilic aromatic substitution reaction of benzene

Halogenation


Mechanism of Halogenation


Properties

Nitration


Mechanism of Nitration


Mechanism of Nitration


Properties

Sulphonation


Mechanism of Sulphonation


Mechanism of Sulphonation


Properties

Friedel-Craft’s reaction

(a) friedel–craft alkylation

(a) friedel–craft acylation


Mechanism of Friedel–Craft’s reaction


Mechanism of Friedel–Craft’s reaction


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.


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.


Addition reaction of benzene

Addition of hydrogen

Addition of halogens


Oxidation


Oxidation


Oxidation


Ozonolysis of benzene


Influence of Substituents on Elecrophilic Aromatic Substitution

Activates

Deactivates


Influence of Substituents on Elecrophilic Aromatic Substitution

or

or


..

..

:

:

Influence of Substituents on Elecrophilic Aromatic Substitution


+

+

+

Influence of Substituents on Elecrophilic Aromatic Substitution


Ortho–Para and meta directors


Polynuclear Hydrocarbon and their Toxicity


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

  • 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)


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)


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)

The compound B has no but other are aromatic compounds.

Solution:

Hence, the answer is (b)


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)


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)


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)


Thank you


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