Preparation, properties and reactions
Reactions of benzene: Structure and aromaticity; Electrophilic substitution reactions: halogenation, nitration, sulphonation, Friedel-Crafts alkylation and acylation; Effect of o-, m- and p-directing groups in monosubstituted benzenes.
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Sabtu, 27 Desember 2008
Sabtu, 26 Januari 2008
IIT JEE Revision - Ch. 24 Benzene - Main chapter points
syllabus
Structure
Aromaticity
Electrophile Substitution Reactions
---Halogenation
---Nitration
--- Sulphonation
--- Friedel-Crafts Alkylation
--- Friedel-Crafts Acylation
Effect of --, m- and p- directing groups in mono-substituted benzenes
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1. Benzene has the molecular formula C6H6. It has hexagonal ring of six carbon atoms with three double bonds in alternate positions.
Arenes are the aromatic hydrocarbons which contain one or more hexagonal rings of carbon atoms with double bonds in alternate positions.
2. Preparation of benzene and its homologues
1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.
3C2H2 gives C6H6
Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.
2 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime
Decarboxylation: Removal carboxyl group
3. From phenol: by distillation of phenol with zinc.
3. Physical properties
i) colour less liquids up to eight carbon atoms
ii) aromatic hydrocarbons are insoluble in water ut soluble in organic solvents.
iii) They are inflammable and burn with sooty flame
4. Chemical properties
Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.
Benzene and other arenes undergo following types of reactions.
1. substitution
2. addition
3. oxidation
5. Halogenation
benzene will react with a mixture of Cl-2 and FeCl-3.
The output is a combination of benzene with Cl, Cl diplacing one hydrogen atom from benzene(Chlorobenzene).
6. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.
7. Sulphonation
The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.
For sulphonation we require excess of H-2SO-4 along with SO-3.
8. Friedel-Crafts Alkylation
Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.
The alkyl group replaces one hydrogen atom in benzene.
9. Friedel-Crafts Acylation
Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.
The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.
10. Effect of o-, m- and p- directing groups in mono-substituted benzenes
In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.
Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.
Structure
Aromaticity
Electrophile Substitution Reactions
---Halogenation
---Nitration
--- Sulphonation
--- Friedel-Crafts Alkylation
--- Friedel-Crafts Acylation
Effect of --, m- and p- directing groups in mono-substituted benzenes
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1. Benzene has the molecular formula C6H6. It has hexagonal ring of six carbon atoms with three double bonds in alternate positions.
Arenes are the aromatic hydrocarbons which contain one or more hexagonal rings of carbon atoms with double bonds in alternate positions.
2. Preparation of benzene and its homologues
1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.
3C2H2 gives C6H6
Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.
2 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime
Decarboxylation: Removal carboxyl group
3. From phenol: by distillation of phenol with zinc.
3. Physical properties
i) colour less liquids up to eight carbon atoms
ii) aromatic hydrocarbons are insoluble in water ut soluble in organic solvents.
iii) They are inflammable and burn with sooty flame
4. Chemical properties
Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.
Benzene and other arenes undergo following types of reactions.
1. substitution
2. addition
3. oxidation
5. Halogenation
benzene will react with a mixture of Cl-2 and FeCl-3.
The output is a combination of benzene with Cl, Cl diplacing one hydrogen atom from benzene(Chlorobenzene).
6. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.
7. Sulphonation
The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.
For sulphonation we require excess of H-2SO-4 along with SO-3.
8. Friedel-Crafts Alkylation
Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.
The alkyl group replaces one hydrogen atom in benzene.
9. Friedel-Crafts Acylation
Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.
The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.
10. Effect of o-, m- and p- directing groups in mono-substituted benzenes
In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.
Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.
IIT JEE Revision Benzene Introduction
Molecular formula C6H6
Gen formula CnH2n-6
Bicyclic arenes CnH2n-12
If m rings are present CnH2n-6m
Gen formula CnH2n-6
Bicyclic arenes CnH2n-12
If m rings are present CnH2n-6m
IIt JEE Revision Aromaticity Huckel Rule
Aromatic compounds are those which resemble benzene in chemical behaviour.
They contain alternate single and double bonds in a cyclic structure.
They undergo substitution reactions rather than addition reactions
Criteria for Aromaticity- Contribution by Huckel
1. Delocalisation: The molecule should contain a cyclic cloud of delocalized Pi electrons above and below the plane of the molecule.
2. Planarity:For a molecule to be aromatic, the ring must be planar.
3. Huckel Rule: the pi electron cloud must contain a total of 4n+2 pi electrons whre n is an integer equal to 0,1,2,3.
Benzene has 6 pi electrons, napthalene has 10.
They contain alternate single and double bonds in a cyclic structure.
They undergo substitution reactions rather than addition reactions
Criteria for Aromaticity- Contribution by Huckel
1. Delocalisation: The molecule should contain a cyclic cloud of delocalized Pi electrons above and below the plane of the molecule.
2. Planarity:For a molecule to be aromatic, the ring must be planar.
3. Huckel Rule: the pi electron cloud must contain a total of 4n+2 pi electrons whre n is an integer equal to 0,1,2,3.
Benzene has 6 pi electrons, napthalene has 10.
IIT JEE Revision Benzene Preparation
Preparation of benzene and other aromatic compounds or arenes
1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.
3C2H2 gives C6H6
Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.
2. From aryl halides:
Benzene is obtained from chlorobenzene by reducing it with Ni-Al alloy in the presence of sodium hydroxide
Arenes are obtained by reaction fo aryl halide, sodium metal and alkyl halide in dry ether.
Bromobenzene + sodium + Ethylbromide give Ethylbenzene and Sodiumbromide
3. Arenes from Benzene and alkyl halides
Arenes can also be obtained from benzene and alkyl halides in the presence of anhydrous aluminium chloride. This is called Friedel Craft's reaction.
4.From Grignard reagent (Phenyl magnesium halide)
Arenes are also prepared by reacting aromatic Grignard reagent and alkyl halide.
5. From phenol: by distillation of phenol with zinc.
6 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime. Decarboxylation: Removal carboxyl group
7. From diazonium salts: Benzene diazonium is reduced by hypophosphorus acid.
1. From alkynes: acetylene and other alkynes polymerise at high temperatures to give benzene and other arenes.
3C2H2 gives C6H6
Benzene was first synthesized by Berthelot by passing acetylene through red hot iron tube.
2. From aryl halides:
Benzene is obtained from chlorobenzene by reducing it with Ni-Al alloy in the presence of sodium hydroxide
Arenes are obtained by reaction fo aryl halide, sodium metal and alkyl halide in dry ether.
Bromobenzene + sodium + Ethylbromide give Ethylbenzene and Sodiumbromide
3. Arenes from Benzene and alkyl halides
Arenes can also be obtained from benzene and alkyl halides in the presence of anhydrous aluminium chloride. This is called Friedel Craft's reaction.
4.From Grignard reagent (Phenyl magnesium halide)
Arenes are also prepared by reacting aromatic Grignard reagent and alkyl halide.
5. From phenol: by distillation of phenol with zinc.
6 Decarboxylation of aromatic acids: by heating sodium benzoate with soda lime. Decarboxylation: Removal carboxyl group
7. From diazonium salts: Benzene diazonium is reduced by hypophosphorus acid.
IIT JEE Revision Benzene Physical Properties
is colourless, volatile liquid with burning state and characteristic smell.
Melting point is 278.5 K and boiling point is 353 K (80 C)
Insoluble in water but soluble in organic solvents.
Highly Inflammable and burns with sooty flame
It is lighter than water and its specific gravity is 0.878
Benzene itself is a good solvent for fats, oil, resin etc.
Benzene is toxic in nature
Melting point is 278.5 K and boiling point is 353 K (80 C)
Insoluble in water but soluble in organic solvents.
Highly Inflammable and burns with sooty flame
It is lighter than water and its specific gravity is 0.878
Benzene itself is a good solvent for fats, oil, resin etc.
Benzene is toxic in nature
IIT JEE Revision - Benzene Chemical Properties and Reactions
Even though double bonds are present, benzene is quite stable and does not undergo common addition reactions undergone by alkenes.
Benzene and other arenes undergo following types of reactions.
1. substitution
2. addition
3. oxidation
Substitution reactions are covered in different posts
Addition reactions
1. hydrogen
2. halogens
3. ozone
Oxidation
combustion
Oxidation of benzene
Oxidation of alkyl side chain
Benzene and other arenes undergo following types of reactions.
1. substitution
2. addition
3. oxidation
Substitution reactions are covered in different posts
Addition reactions
1. hydrogen
2. halogens
3. ozone
Oxidation
combustion
Oxidation of benzene
Oxidation of alkyl side chain
IIT JEE Revision Benzene - Halogenation
Halogenation
Benzene will react with a mixture of Cl-2 and FeCl-3 (catalyst).
The output is a combination of benzene with Cl, Cl displacing one hydrogen atom from benzene(Chlorobenzene).
AlCl3 can also be used as catalyst.
Bromine also combines with benzene with AlCl3 as catalyst and forms bromobenzene.
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At a slightly elevated temperature, chlorination of benzene in presence of a catalyst gives rise to a mixture of orthochlorobenzene and para chlorobenzene.
On prolonged chlorination, benzene forms hexachlorobenzene.
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Reaction mechanism
Reaction is carried out in the presence of ferric salts i.e, FeCl3 in case of chlorine and FeBr3 in case of bromine.
The metal catalyst is a Lewis acid (Fe is electron deficient) and hence polarises the halogen molecule.
Cl2 + FeCl3 --> FeCl4ˉ + Cl+
Cl+ attacks the benzene molecule.
It has been suggested by some that free chloronium ion (Cl+) may not have actual existence. The electrophile is supposed to be made available by a complex between FeCl3 and Cl2
The electrophile attack creates a carbocation.
The carbocation loses a proton(H+) to FeCl4ˉ and the formation of aryl halide takes place.
Benzene will react with a mixture of Cl-2 and FeCl-3 (catalyst).
The output is a combination of benzene with Cl, Cl displacing one hydrogen atom from benzene(Chlorobenzene).
AlCl3 can also be used as catalyst.
Bromine also combines with benzene with AlCl3 as catalyst and forms bromobenzene.
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At a slightly elevated temperature, chlorination of benzene in presence of a catalyst gives rise to a mixture of orthochlorobenzene and para chlorobenzene.
On prolonged chlorination, benzene forms hexachlorobenzene.
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Reaction mechanism
Reaction is carried out in the presence of ferric salts i.e, FeCl3 in case of chlorine and FeBr3 in case of bromine.
The metal catalyst is a Lewis acid (Fe is electron deficient) and hence polarises the halogen molecule.
Cl2 + FeCl3 --> FeCl4ˉ + Cl+
Cl+ attacks the benzene molecule.
It has been suggested by some that free chloronium ion (Cl+) may not have actual existence. The electrophile is supposed to be made available by a complex between FeCl3 and Cl2
The electrophile attack creates a carbocation.
The carbocation loses a proton(H+) to FeCl4ˉ and the formation of aryl halide takes place.
IIT JEE Revision - Benzene Nitration
6. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.
Heating benzene with the nitrating mixture consisting of concentrated nitric acid and sulphuric acid to about 330 K.
Product is nitrobenzene
Benzene + HNO3---> Nitrobenzene + H2O (H2SO4 and 330 K)
A mixture of nitric acid and sulphuric acid is the nitrating agent.
Heating benzene with the nitrating mixture consisting of concentrated nitric acid and sulphuric acid to about 330 K.
Product is nitrobenzene
Benzene + HNO3---> Nitrobenzene + H2O (H2SO4 and 330 K)
IIT JEE Revision Benzene Sulphonation
The product is a combination Benzene and SO-3H that displaced one hydrogen atom from benzene.
For sulphonation we require excess of conc. H-2SO-4 along with SO-3(Fuming sulphuric acid or oleum)
Sulphonation can also be carried by treating benzene with chlorosulphonic acid(ClSO3H). The product is benzene sulphonic acid
For sulphonation we require excess of conc. H-2SO-4 along with SO-3(Fuming sulphuric acid or oleum)
Sulphonation can also be carried by treating benzene with chlorosulphonic acid(ClSO3H). The product is benzene sulphonic acid
IIT JEE Revision Friedel-Crafts Alkylation
Benzene reacts with a combination of alkyl halide and AlCl-3. AlCl-3 acts as a Lewis acid.
The alkyl group replaces one hydrogen atom in benzene.
Benzene + Methyl chloride --> Toluene + HCl
Toluene = Benzene with CH3 substitution
The alkyl group replaces one hydrogen atom in benzene.
Benzene + Methyl chloride --> Toluene + HCl
Toluene = Benzene with CH3 substitution
IIT JEE Revision Friedel-Crafts Acylation
Acylation is the term given to substituting an acyl group such as CH-3CO- into another molecule. An acyl group is a hydrocarbon group attached to a carbon-oxygen double bond.
The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.
On treatment with an acid chloride (Acyl chloride) in the presence of anyhydrous aluminium chloride, acylation occurs.
Benzene + Acetyl chloride --> Acetophenone + HCl
Mechanism
The attacking electrophile an acyl carbocation RCO is supplied by acid chloride RCOCl in the presence of anhydrous AlCl3
The most commonly used example of an acyl group is the ethanoyl group, CH3CO-.
On treatment with an acid chloride (Acyl chloride) in the presence of anyhydrous aluminium chloride, acylation occurs.
Benzene + Acetyl chloride --> Acetophenone + HCl
Mechanism
The attacking electrophile an acyl carbocation RCO is supplied by acid chloride RCOCl in the presence of anhydrous AlCl3
IIT JEE Revision Benzene
Effect of o-, m- and p- directing groups in mono-substituted benzenes
In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.
Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.
the Pattern of Orientation in Aromatic Substitution
The reaction most studied in connection with the orientation problem is nitration, but the principles established also apply for the msot part ot the related reactions of halogenation, sulfonation, alkylation and acylation.
The group present on the benzene ring affect the incoming attacking groups.
Two effects: orientation, reactivity
Orientation effect: The three possible disubstituted products -ortho, meta and para are not formed in equal amounts.
some substituents groups are ortho directors and some others or meta directors or para directors
Reactivity:some activate the ring and make it more reactive. Some deactivate and make it (benzene) less reactive.
Three groups are classified
1. Ortho and para directing activators: Groups release electrons and activate the benzene ring
-OH, -OCH3, NH2, -NHCOCH3, -CH3
2. Meta directing deactivating groups: They withdraw electrons from the benzene ring and deactivate it
-NO2, -CN, -CHO
3. Ortho and para directing deactivating groups
Halogens -F, Cl, Br, I
In planning syntheses based on substitution reactions of mono-substituted benzenes, you must be able to predict in advance which of the available positions of the ring are most likely to be substituted.
Basically, three problems are involved in the substitution reactions of aromatic compounds: (a) proof of the structures of the possible isomers, o, m, p, that are formed; (b) the percentage of each isomer formed, if the product is a mixture; and (c) the reactivity of the compound being substituted relative to some standard substance, usually benzene.
the Pattern of Orientation in Aromatic Substitution
The reaction most studied in connection with the orientation problem is nitration, but the principles established also apply for the msot part ot the related reactions of halogenation, sulfonation, alkylation and acylation.
The group present on the benzene ring affect the incoming attacking groups.
Two effects: orientation, reactivity
Orientation effect: The three possible disubstituted products -ortho, meta and para are not formed in equal amounts.
some substituents groups are ortho directors and some others or meta directors or para directors
Reactivity:some activate the ring and make it more reactive. Some deactivate and make it (benzene) less reactive.
Three groups are classified
1. Ortho and para directing activators: Groups release electrons and activate the benzene ring
-OH, -OCH3, NH2, -NHCOCH3, -CH3
2. Meta directing deactivating groups: They withdraw electrons from the benzene ring and deactivate it
-NO2, -CN, -CHO
3. Ortho and para directing deactivating groups
Halogens -F, Cl, Br, I
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