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benzene

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Benzene is an organic chemical compound with the molecular formula C6H6. Benzene is a colorless and highly flammable liquid with a sweet smell and a relatively high melting point.

What is BENZENE?

Resonance Energy of BENZENE:

Br2/

CCl4

•No ReactionCold

KMnO4

•No ReactionH2O

/ H+

•No Reaction

BENZENE does not behave like Alkenes or Alkynes:

Molecular Structure of BENZENE:

Kekule’s structure of BENZENE:

Aromaticity

Aromaticity is a property of the sp2 hybridized planar rings in which the p-orbital, one on each atom, allow cyclic delocalization of π electrons.

CRITERIA:

RULE 1: An aromatic compound is cyclic and planar

RULE 2: Each atom in an aromatic ring has a p-orbital

which is parallel so that a continuous overlap is possible

around the ring

RULE 3: The cyclic pi-molecular orbital formed by overlap of

p-orbital must contain ( 4n+2) pi electrons.

Cycloheptatriene•Cyclic & Planar•Has a 3 double bonds & 6 pi-electrons •1 saturated Carbon without a p-orbital

Cyclooctatetriene•Cyclic & Planar•Has a p-orbital on each atom of the ring•Huckel Rule not satisfied

Benzene•Cyclic & Planar•Has a p-orbital on each atom of the ring•Obeys Huckel Rule

PHYSICAL PROPERTIES

1. Benzene is a colorless liquid of boiling point 80.10C, melting point 5.50C2. It is immiscible with water but miscible with alcohol, ether and chloroform3. Benzene itself is a good solvent for many organic and inorganic substance4. Its vapour is highly toxic; inhalation over a period of time leads to anemia.5. It burns with a luminous, sooty flame6. Benzene and its derivatives shows characteristics IR spectrum

Method of Preparation of BENZENE:

Large scale preparation (From Petroleum):

Small scale preparation:

• By passing Acetylene through red hot tube at 5000C

• By heating Phenol with Zinc dust

Principle types of reactions of BENZENE:

• Electrophilic Substitution reaction• Addition reactions• Oxidation reactions

Electrophilic Substitution Reaction:

General mechanism:

+ E+HE

H+slow, rate

determiningStep 1:

Step 2:E

H+

fast + H+E

Electro- phile

Resonance-stabilized cation intermediate

Comparison between Alkene & Benzene:

Alkene Benzene

The Carbonium ion produced from the Alkene usually combines with a Nucleophile to give the overall addition product.

Whereas in Benzene the Nucleophile removes a proton from the Carbonium ion intermediate. The loss of proton allows the electrons from the C-H bond to go back into the ring & re-generate the aromatic pi-system.

Both are sucseptible to Electrophilic attack because of their exposed pi-electrons.

Chemical Reactions:

+

Benzenesulfonic acid

Sulfonation:

H S O 3 HS O 3

H2 S O 4

++

An alkylbenzene

Alkylation:

RR XA l X3 H X

++

Acylation:

An acylbenzene

H R C X A l X3 H X

O

C R

O

H

+ +

Chlorobenzene

Halogenation:

H ClCl2FeCl3 HCl

++

Nitrobenzene

Nitration:

H NO2HNO3H2 SO4

H2 O

Friedel-Crafts alkylation:

ClAlCl3

HCl+

Benzene 2-Chloropropane(Isopropyl chloride)

Cumene(Isopropylbenzene)

+

Friedel-Crafts Alkylation

Step 1: formation of an alkyl cation as an ion pair.

Step 2: attack of the alkyl cation on the ring.

Step 3: proton transfer regenerates aromaticity.

R Cl ClAlCl

Cl

R Cl

Cl

ClAl Cl R+ AlCl4

-

An ion pair containing a carbocation

+-

+

A molecular complex

+ R+

R

H

R

H

R

H

A resonance-stabilized cation

+

+

+

H

RCl AlCl3 R AlCl3 HCl+ ++

Addition Reactions:

Addition of Hydrogen:

Addition of Halogens:

The above two reactions involve a Free Radical Mechanism

Uses:

• Benzene is an important industrial solvent and precursor in the production of drugs, plastics, synthetic rubber, and dyes. • It is a natural constituent of crude oil, and may be synthesized from other compounds present in petroleum. •Benzene is widely used in the United States; it ranks in the top 20 chemicals for production volume. •Some industries use benzene to make other chemicals which are used to make plastics, resins, and nylon and synthetic fibers. •Benzene is also used to make some types of rubbers, lubricants, dyes, detergents, drugs, and pesticides. •Natural sources of benzene include volcanoes and forest fires. •Benzene is also a natural part of crude oil, gasoline, and cigarette smoke.

Summary:

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