XII Chemistry BIEK Topic - Benzene C6H6

Benzene C6H6
Introduction

Benzene is the basic unit of acromatic hydrocarbon, which is composed of six carbons and six hydrogen atoms. It molecular formula is C6H6. The structural formula of banzene shows that each carbon contains one hydrogen atom and two carbon atoms and alternate single and double bond is present between carbon atoms is a ring.

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Behaviour of Benzene
The chemical analysis and molecular mass determination shows that the molecular formula of benzene is C6H6, which corresponds to alkane (n-hexane) having a molecular formula C6H14. When benzene is treated with chlorine in dark or with KMnO4 solution, no reaction occurs. When the benzene reacts with nitric acid, chlorine and methyl chloride under different conditions then it shows substitution reaction, that is the characteristic property of hydrocarbon.

When benzene reacts with chlorine in presence of sunlight or with Hydrogen in presence of catalyst then addition reaction takes place, that is the characteristics property of unsaturated hydrocarbon.


Benzene As Unsaturated Hydrocarbon

The characteristic reaction of unsaturated hydrocarbon are addition reactions. The following chemical reactions shows the benzene behaves as unsaturated hydrocarbon. In presence of catalyst Nickle when benzene is heated at 150ºC under 10 atm pressure then hydrogenation takes place as a result cyclohexane is formed

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In presence of sunlight when benzene reacts with chlorine at 50ºC under 400 atm pressure then chlorination takes place as a result, hexa cyclo hexane is formed.

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Benzene As Saturated Hydrocarbon

The characteristic reactions of saturated hydrocarbon are substitution reaction. Benzene reacts with different reagents under different conditions and under goes substitution reaction. These reactions show that benzene behaves like a saturated hydrocarbon.

In presence of catalyst concentrated sulphuric acid, when benzene reacts with fuming nitric acid then nitration takes place as a result a substituted product nitro benzene is formed.

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In presence of catalyst Ferric Chloride (FeCl3) when benzene reacts with chlorine then chlorination takes place as a result a substituted product benzene is formed

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In presence of catalyst FeCl3, when benzene reacts with methyl chloride the alkylation takes place as a result methyl benzene or Toluene is formed.

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The above mentioned chemical reaction shows that benzene behaves as a saturated hydrocarbon in spite of a fact that is highly unsaturated hydrocarbon.




Special Character of Benzene
Benzene is not affected by common oxidizing agent such as KMn04 or K2Cr207. Similarly when benzene is treated with chlorine or bromine in dark or with dilute acids no reaction occurs.

However, benzene can easily by oxidized in presence of catalyst Vanadium Pentaoxide (V2O5) to form malcic annydride.

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Structure of Benzene
Benzene is a basic unit of aeromatic cyclic hydrocarbon which is composed of six carbon and six hydrogen atoms. It molecular formula is C6H6. The structural formula shows that each carbon atom is bonded with one hydrogen atom and two carbon atoms, therefore one free electron is present on each carbon atom which is responsible for the aeromatic character and unusual behaviour of benzene.

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Different scientists explain the unusual behaviour of benzene and proposed several structures, the detail of which is given below.


1. Kekul Structure

In 1865, Kekul proposed the structure of benzene in which six carbon atoms bonded together by an alternate C - C double bond to form a ring structure. This structural formula suggest the addition reaction should perform by benzene. The resonating structure of benzene is equally represented as under:

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The above are equivalent and can result by shifting of double bond, which shows that the position of double bond in benzene ring is not fixed so that all C - H position have a partial double bond character.


2. Dewar Structure

Dewar proposed a structure of benzene in which six carbon atoms are bonded together to form hexagonal planer ring in which each carbon attached with one hydrogen and two carbon atoms combined together to form a pi bond as a result the following three resonating structures are formed..

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3. Armstrong - Bayer Structure

In 1887 Armstrong and in 1892 Bayer proposed Armstrong - Bayer centric formula in which the fourth valency or free electron of each carbon atom is directed towards the center of the molecule, as a result centric density is increased.




Modern Concept of Structure of Benzene
The modern concept of structure of benzene can be explained by the help of following two methods:

1. Atomic Orbital Treatment

2. Rasonance

1. Atomic Orbital Treatment

Benzene is a cyclic hydrocarbon, which is composed of six carbon and six hydrogen atoms, each carbon atom is bonded with one hydrogen and two carbon atoms.

Each carbon atom of benzene is Sp2 hyberdized which contain three equivalent partially fill Sp2 hydrid orbitals and one unhyberdized Pz orbital. The Sp2 bybrid orbitals of each carbon atom are arranged at three corners of triangular planer structure. With an angle of 120ºC.

One Sp2 hybrid orbital of each carbon atom overlapped with S atomic orbital of Hydrogen atom to form a sigma bond between C - H due to the overlapping of Sp2 - S orbitals. The remaining two Sp2 hybrid orbitals of each carbon atom overlapped with Sp2 bybrid of two different carbon atoms to form sigma bond between adjacent C - C due to the linearly overlapping of Sp2 - Sp2 orbital.

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The unhyberdized Pz orbital of each carbon atom is situated perpendicular to the Sp2 plane and parallel to the unhyberidized Pz orbital of other carbon atom. The unhyberdized Pz orbital of two adjacent carbon atoms overlap side by side to form a pi bond between two carbon atoms, therefore three alternate double bonds are formed between carbon atoms in a ring.

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Since the C - C bond lengths in benzene are same i.e. 1.39 Aº, each orbital overlaps with its neighbour equally, therefore all six Pz orbitals overlapped with each other to form a single molecular orbital in such a way an electronic cloud is formed above and below the Sp2 plane (benzene ring) and atrons in benzene is not fixed or they are de localized.

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2. Resonance Or Modern Representation of Structure of Benzene.

Definition

The phenomenon in which position of double bond or pi electroni

1. From Saturated Hydrocarbon

The following two methods show the preparation of benzene from saturated hydrocarbon.


A. From Petroleum or n - Hexane

When n-hexane is heated about 480ºC - 550ºC under 150psi - 300psi pressure and in presence of catalyst such as V2O5 then cyclization takes place as a result cyclo hexame is formed which on dehydrogenation convert into benzene.

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B. From n - Heptane

It presence of catalyst when n-heptane is heated temperature under high pressure then Toluene is formed, which on heating 500ºC - 760ºC in presence of catalyst Co - Mo convert into benzene.

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2. From Unsaturated Hydrocarbon

It presence of catalyst organonickle when ethyne or acetylene is passed throuigh red hot tube then polymerization takes place as a result benzene is formed

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3. From Phenol

When the vapours of Phenol are passed over red-hot zinc dust, then reduction takes place, as a result benzene is formed.

C6H5OH + Zn + C6H6 + ZnO


4. From Sodium Benzene

When sodium benzoate is heated with sodium hydroxide then benzene is formed.

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Reaction of Benzene or Chemical Properties of Benzene
Substitution Reaction of Benzene

Those atoms or molecules or ions, which are electron deficient or contains positive charge are known as electrophile. The chemical reactions in which one electrophile is replaced by another electrophile are called Electrophile Substitution Reaction.

The structure of benzene shows that there is a cloud of pi electrons above and below the plane of benzene molecule. These pi electrons (π) are responsible for electrophile substitution reaction of benzene.

In benzene hydrogen atom act as electrophile. Therefore when any electrophillic reagent reacts with benzene then the hydrogen atom is replaced by attacking molecule to form substituted benzene.

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General Mechanism of Electrophilic Substitution Reactions of Benzene.

The electrophilic substitution reaction in benzene occurs through following mechanism. The π electron which are spread above and below the plane of benzene molecule are responsible for this reaction. An electrophile attacks the pi system of benzene to form a delocalized carbonium ion or sigma complex. The electrophile does this by taking two electrons of Pz orbital to form a sigma bond between it and one carbon atom of benzene ring. This breaks the cyclic system of pi electrons because one carbon becomes Sp3 hyberidized.

This causes instability to the ring, to overcome this instability the benzonium loses a proton from the carbon that bears the electrophile. The loss of proton result in the regeneration of the double bond, which restores the stability of ring and formation of, substituted product.

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Some important electophillic substitution reaction of benzene are given below.


a) Nitration

Introduction of nitro group (NO2)in a compound is called nitration. When benzene reacts with a 1:1 mixture of concentrated nitric acid and concentrated sulphuric acid then nitration takes place as a result nito benzene is formed. In this reaction H2SO4 act as a catalyst.

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b)Sulphonation

Introduction of sulphonic group (HSO3) in a compound is called sulphonation. When benzene reacts with fuming H2SO4 i.e. H2SO4 saturated with SO3, at room temperature then sulphonation takes place as a result benzene sulphonic is formed.

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Various steps of mechanism of sulphonation of benzene are given bwlow

H+HSO4- + SO3 ----> HSO3+ + HSO4-

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c) Halogenation

Introduction of halogen in a compound is called halogenation. In presence of Lewis acid catalyst FeX3, or AlX3, when benzene reacts with halogen than halogenation takes place. As a result halobenzene is formed

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d) Alkylation

Introduction of Alkyle group R in a compound is called alkylation. In presence of catalyst FeX3 or AlX3, when benzene reacts reacts with alkyl halide then alkylation takes place. As result alkyle benzene is formed

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Various steps of mechanism of alkylation are given below.

R+X + FeX3 ----> R+ + FeX4-

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e) Acylation

Introduction of acyl group (R - C = 0) in a compound is called acylation. In presence of catalyst FeX3 or AlX3 when benzene reacts with acyl halide then acylation takes place. As a result, acyl benzene is formed.

Various steps of mechanism of acylation are given below.

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f) Friedal and Crafts Reaction

General chemist friedal and crafts first introduced alkyl group and acyle group in benzene in presence of catalyst FeX3 or AlX3, Therefore, the alkylation and acylation reaction are collectively known as Friedal and Crafts reactions.


2. Addition Reaction of Benzene

Under general circumstances, Benzene under goes addition reaction. As a result of these reactions, the aeromatic character of ring is lost and benzene is reduced to saturated cyclic compound.

Some important addition reactions of benzene are


a) Hydrogenation

Introduction of hydrogen in a compound is called hydrogenation. In presence of catalyst nickle. When benzene is heated at about 150ºC under 10 atm pressure, then hydrogenation takes place. As a result cyclo hexane is formed

C6H6 - 3H2 ----> C6H12 (150ºC, 10 atm, Ni)


b) Halogenation

Introduction of halogen in a compound is called halogenation. In presence of sunlight, when benzene is heated with chlorine at about 50ºC under 400 atm pressure, then halogenation (chlorination) takes place. As a result hexa chloro cyclo hexane is formed.

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3. Oxidation

Benzene do not oxidize by common oxidizing agents such as aqueous alkaline solution of KmnO4 or acidic solution of K2Cr2O7. But in pressure of catalyst Vanadium Pentaoxide (V2O5) benzene is oxidized by oxygen to form a Maleic anhydride.

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Orientation In Benzene
Introduction

Benzene is a basic unit of aeromatic hydrocarbon, which is composed of six carbons and six hydrogen atoms. It molecular formula is C6H6. The structural formula of benzene shows that each carbon contains one hydrogen atom and two carbon atoms and alternate single and double is present between carbon atoms is a ring. Therefore, all carbons atoms and Hydrogen atoms of benzene ring are identical.

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Explanation

Benzene shows an electrophillic substitution reaction in which one hydrogen atom of benzene is replaced by attacking electrophile E+ as a result, a stable substituted benzene is formed.

C6H6 + E+ ----> C6H5E + H+

In substituted benzene all carbon atoms are not equivalent, therefore the carbon number 1 and 4 are called Para Positions, carbon number 2 and 6 are called Ortho Positions and carbon number 3 and 5 are called Meta Positions.

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1, 4 - Para Positions

2, 6 - Ortho Position

3, 5 - Meta Position

In presence of first substituent E+ the incoming second electrophile Y+ may occupie any of Orth, Para or Meta Position.




Classification of First Substituent Group
Since the incoming second electrophile i.e. Y+ occupie the position as it given by the first electrophile E+. Therefore the first substituent group E+ can be classified into two main groups according to their influence on the reactivity of the ring.

1. Meta Directing Group

2. Ortho Directing Group


1. Meta Directing Group

NO2, HSO3, COOH, COOR, CHO, COR, .... are meta directing group, because they orient or direct the incoming second substituent group Y+ to Meta Position.


Examples


a). Chlorination of Nitro Benzene

In presence of catalyst, FeCl3 when nitro benzene reacts with chlorine then chlorination takes place as a result first Meta chloro nitro benzene and meta dichloro nitro benzene are formed.

C6H5NO2 + Cl2 ----> C6H4NO2Cl + HCl

C6H5NO2 + Cl2 ----> C6H4NO2Cl2 + HCl


b). In presence of catalyst concentrated sulphuric acid, when nitro benzene is heated with nitric acid then nitration takes place, as a result Meta trinitro benzene is formed

C6H5NO2 + NO2OH ----> C6H4(NO2)2 + H2O

C6H4(NO2)2 + NO2OH ----> C6H3(NO2)3 + H2O


c). Nitration of Benzoic Acid

In presence of catalyst concentrated sulphuric acid when benzoic reacts with fuming nitric acid (HNO3), then nitration takes place as a result meta di nitro benzoic acid is formed.

C6H5COOH + NO2OH ----> C6H4NO2COOH + H2O

C6H4NO2COOH + NO2OH ----> C6H3(NO2)2COOH + H2O


2. Ortho Directing Group

X, R, OH, NH2, NR2, NHCOR ... are ortho para directing group, because they orient or direct the incoming second substituent group Y+ to ortho para position. As a result a mixture of ortho and para substituted product is formed.


Examples


a). Nitration of Chloro Benzene

In presence of catalyst, concentrated sulphuric acid when chloro benzene reacts with Nitric Acid then Nitration takes place as a result a mixture of ortho nitro chloro benzene and para nitro chloro benzene is formed.

C6H5Cl + NO2OH ----> C6H4CINO2


b). Nitration of Methyl Benzene

In presence of catalyst concentrated sulphuric acid, when methyl benzene or Toluene reacts with nitric acid then nitration takes place, as a result a mixture of ortho nitro toluene and para nitro toluene are formed

C6H5CH3 + NO2OH ----> C6H4CH3NO2