JEE 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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Main Topics Covered in the TMH Book
STRUCTURE OF BENZENE
MECHANISM OF ELECTROPHILIC SUBSTITUTION REACTIONS
THEORY OF ORIENTATION
ACTIVATION OF BENZENE VIA RESONANCE
RELUES FOR PREDICTING ORIENTATION N DISUBSTITUTED BENZENES
ARENES (AROMATIC-ALIPHATIC HYDROCARBONS)
ALKENYL BENZENES
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Important themes of the Chapter/Topic
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.
I. Structure
1. Benzene is a liquid hydrocarbon of formula C-6H-6.
2. It is a cyclic molecule in the shape of a flat hexagon.
3. In the bonds among six carbon atoms three have to be double bonds so that tetravalent bonding arrangement is preserved and the double bonds have to be alternate bonds in the ring.
4. The resonance energy of benezene is about 150.6 kJ per mol.
5. All carbon-carbon bond distances in benzene are equal (139 pm) and are intermediate in length between single (154 pm) and double bonds (134 pm)
6. Benzene is a planaer molecule where each carbon is sp^2 hybridized.
7. Of the three hybrid orbitals, two are used in sigma-bonding with two other carbon atoms and the third is used in sigma-bonding with hydrogen atom.
8. In addition to hybrid orbitals, eahc carbon has one p orbital occupied by one electron. This orbital lies perpendiculars to the plane of benzene ring, and hence this electron is of pi-type.
9. The p orbital of each carbon atom can overlap with the adjacent p orbitals making additional bond of pi-type. But this bond is not localized between two carbon atoms. But forms two continuous doughnut-shaped electon cloud one lying above and the other below the plane of cyclic carbon skeleton. The delocalization of pi-electrons gives rise to resonance energy and makes the molecule more stable.
10. The overlap of p orbitals in both directions gives rise to resonance hybrid of two strctures known as Kekule structures.
(August Kekule presented the structure of Benzene on 27 January 1865)
II. Nomenclature
III. Aromaticity
Aromatic compound are those which resemble benzene om chemical behaviour. These compounds contain alternate double and singe carbon-carbon bonds in a cyclic structure. They are more stable compared to aliphatic hydrocarbons having double bonds and undergo substitution reactions rather than addition reactions. These two characteristic behaviours in combination is aromatic character or aromaticity.
1. A molecule acquires aromatic characteristics provided it has cyclic clouds of delocalized pi-electrons above and below the plane of the molecule and this pi- clouds have a total of (4n+2) pi-electrons. This requirement of 4n+2 pi-electrons is known as 4n+2 rule or Huckel rule.
2. Examples are benzene (n = 1), naphthalene (n = 2) and anthracene (n = 3).
The modern theory of aromaticity was advanced by Eric Huckel. For aromaticity, the molecule must be planar, cyclic system having delocalised (4n+2) pi elctrons.
IV. Isomerism
V. 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.
4.From diazonium salts reduction of benzene diazonium with hypophosphorus acid (H3PO2)
5. From aryl halides
6. From Friedel Craft's reaction
Benzene is treated with alkyl halide in the presence of anhydrous aluminium chloride. For example when the alkyl halide is monochloromethane, Toluene(methylbenzene - methyl group substituted for one H in benzene is obtained)
7. From Grignard's reagent: reacting aromatic Grignard reagent with alkyl halide
Ex: Phenyl magnesium bromide + Isopropyl bromide in ether give isopropyl benzene.
Va. 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
iv) M.P. and B.P. increase with increasing molecular mass.
v) they are toxic and carcinogenic in nature.
VI. 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
VIa. Substitution Reactions in Benzene
1. The typlical reactions of the benzene ring are those in which the pi-electrons serve as a source of electrons for electrophylic (acidic) reagents. In these substitution reactions, hydrogen atoms attached to carbon atoms are replaced by another atom or group of atoms.
2. Because of delolcalization of pi-electrons, benzene does not show addition reactions as in the case of alkenes(presence of double bond).
3. 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).
4. Nitration
A mixture of nitric acid and sulphuric acid is the nitrating agent.
(a) The reaction between nitric acid and sulphuric acid gives nitronium ion. Nitric acid acts as base in this reaction and donate OH giving No-2^+
(b) the nitronium ion makes the electrophylic attack on benzene. Benzonium ion is formed. This step is a slow reaction or step. Hence this is a rate determining step. Benzonium ion is a carbocation. It is a resonance hybrid.
(c) From the benzonium ion the proton is removed by the HSO-4^- which is a product of step (a). Thus a combination benzene and NO-2 that displaces one hydrogen atom from benzene is formed.
5. 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.
6. 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.
7. 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-.
If you react benzene with ethanoyl chloride in the presence of an aluminium chloride catalyst, the equation for the reaction is:
AlCl-3
C-6H-6 + CH-3CoCl ------> C-6H-5-COCH-3 + HCl
In the simplified formula for the product, the phenyl group is usually written on the left-hand side and the alkyl group to the right of the carbon-oxygen double bond.
The aluminium chloride is acting as a catalyst.
The product is called phenylethanone (old name, acetophenone).
Source: http://www.chemguide.co.uk/organicprops/acylchlorides/fc.html
Mechanism of electrophilic substitution reactions of benzene
VIb. Addition reactions
VIc. Oxidation reactions
VII. Effect of --, 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.
a. 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.
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web sites
Notes on the Structure and Nomenclature-naming of Aromatic Compounds
http://www.docbrown.info/page07/Aromatics.htm
Updated 23 Nov 2015, 14 Oct 2007
Showing posts with label Benzene. Show all posts
Showing posts with label Benzene. Show all posts
Monday, November 23, 2015
Saturday, December 27, 2008
Benzene- Study Guide - IIT JEE
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.
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.
Saturday, January 26, 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
---------
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.
---------
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.
-----------
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
Wednesday, January 16, 2008
Ch.24 Benzene - Core Points for Revision
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
---------
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.
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