15.1 Classification of Hydrocarbons
15.2 Alkanes
15.3 Nomenclature of Alkanes
15.4 Conformations in Hydrocarbons
15.5 Preparation and Properties of Alkanes
Chemistry of Alkenes
15.6 Nomenclature of Alkenes
15.7 Isomerism in Alkenes
15.8 Stability of of Alkenes
15.9
Chemistry of Alkynes
15.10 Isomerism in Alkynes
15.11 Preparation and Properties of Alkynes
Chemistry of Alkadienes
15.12 Dienes
15.13 Stability of Conjugated Dienes
15.14 Delocalization of Electrons
15.15 Electrophilic addition to Conjugated Dienes
Chemistry of Aromatic Hydrocarbons
15.16 Arenes or Aromatic Hydrocarbons
15.17 Nomenclature
15.18 Stability and Structure of Benzene
15.19 Isomerism in Arenes
15.20 Aromaticity (Huckel Rule)
15.21 Sources of Aromatic Hydrocarbons
15.22 Preparation of Benzene and Its Homologues
15.23 Properties of Benzene and Its Homologues
15.24 Mechanism of Electrophilic Substitution Reactions of Benzene
15.25 Directive Influence of Substituents and Their Effect on reactivity
15.26 Polynuclear Hydrocarbons
Chemistry of Petroleum and Petrochemicals
15.27 Petroleum and Composition of Crude Oil
15.28 Fractional Distillation of Crude Oil
15.29 Quality of Gasoline – Octane Number
15.30 LPG and CNG
15.31 Cracking and Reforming
15.32 Petrochemicals
Conceptual Questions with Answers: 15
Additional Numerical Problems for Practice:
Revision Exercises
Very Short Answer questions 45
Short Answer Questions 62
Long Answer Questions 10
Competition File
Numerical Problems
Objective Questions: 65
Fill in the blanks: 15
True or False: 18
Study Plan
Day 1
15.1 Classification of Hydrocarbons
15.2 Alkanes
15.3 Nomenclature of Alkanes
15.4 Conformations in Hydrocarbons
Day 2
15.5 Preparation and Properties of Alkanes (Preparation)
Day 3
15.5 Contd. Properties of alkanes
Day 4
Revision
Practice problems 15.3 to 15.12
Day 5
Chemistry of Alkenes
15.6 Nomenclature of Alkenes
15.7 Isomerism in Alkenes
PP. 15.13, 15.14
Day 6
15.8 Stability of of Alkenes
15.9 Preparation and Properties
Day 7
Revision
PP. 15.15 to 15.20
Conceptual Questions 5,8,9,11,
Day 8
Chemistry of Alkynes
15.10 Isomerism in Alkynes
15.11 Preparation and Properties of Alkynes
day 9
Revision
PP. 15.21 to 15.26
Day 10
Chemistry of Alkadienes
15.12 Dienes
15.13 Stability of Conjugated Dienes
15.14 Delocalization of Electrons
15.15 Electrophilic addition to Conjugated Dienes
Day 11
Chemistry of Aromatic Hydrocarbons
15.16 Arenes or Aromatic Hydrocarbons
15.17 Nomenclature
15.18 Stability and Structure of Benzene
15.19 Isomerism in Arenes
Day 12
15.20 Aromaticity (Huckel Rule)
15.21 Sources of Aromatic Hydrocarbons
15.22 Preparation of Benzene and Its Homologues
Day 13
15.23 Properties of Benzene and Its Homologues
15.24 Mechanism of Electrophilic Substitution Reactions of Benzene
15.25 Directive Influence of Substituents and Their Effect on reactivity
15.26 Polynuclear Hydrocarbons
Day 14
Chemistry of Petroleum and Petrochemicals
15.27 Petroleum and Composition of Crude Oil
15.28 Fractional Distillation of Crude Oil
15.29 Quality of Gasoline – Octane Number
15.30 LPG and CNG
15.31 Cracking and Reforming
15.32 Petrochemicals
Day 15
Examples 15.13 to 15.28
Revision period
Day 16
Conceptual Questions with Answers: 15
Day 17
Revision Exercises: Very Short Answer questions 1 to 30
Day 18
Revision Exercises: Very Short Answer questions 31 to 45
Revision Exercises: Short Answer questions 1 to 15
Day 19
Revision Exercises: Short Answer questions 16 to 45
Day 20
Revision Exercises: Short Answer questions 46 to 62
Competition File-Objective Questions: 1 to 15
Day 21
Competition File-Objective Questions: 16 to 45
Day 22
Competition File-Objective Questions: 46 to 65
Competition File-Fill in the blanks: 15
Day 23
Competition File-True or False: 18
Day 24 - 30
Revision and test paper problem solving
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Alkanes- Study Guide - IIT JEE
Preparation, properties and reactions of alkanes: Homologous series, physical properties of alkanes (melting points, boiling points and density); Combustion and halogenation of alkanes; Preparation of alkanes by Wurtz reaction and decarboxylation reactions.
Selasa, 29 Januari 2008
IIT JEE Revision Ch.21 Alkanes - Core Chapter Points
Syllabus
Preparation, properties and reactions of alkanes:
Homologous series,
Preparation of alkanes by Wurtz reaction
Preparation of alkanes decarboxylation reactions.
physical properties of alkanes (melting points, boiling points and density); Combustion and halogenation of alkanes;
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Alkanes: Introduction
Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds in their molecules.
Thye are also called paraffins (meaning little affinity or reactivity, we will see later why it is so).
Alkanes are divided into 1. Open chain or acyclic Alkanes and 2. CycloAlkanes or cyclic alkanes.
The general formula of alkanes is CnH2n+2
Preparation of alkanes
1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts
1. From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes (Note that this point will come in alkenes and alkynes chapter as reactions of them).
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.
Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.
2. Wurtz reaction (From alkyl halides)
When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing both twice the number of carbon atoms of alkyl halide is obtained.
3. Decarboxylation reaction
When sodium salt of a monocarboxylic acid is heated with soda lime (amixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
Physical properties of alkanes
1. State: CH4 to C4H10 are gases, C5H12 to C17H36 are liquids and higher ones are solids
2. Boiling point: Boiling point increases with molecular mass. Branched isomers have a lower boiling point than normal alkanes.
3. Melting point
4. Solubility: Being nonpolar, these are insoluble in water.
5. Density: Liquid alkanes lighter than water
Combustion
Large quantity of heat generated in the combustion of alkanes
Halogenation of alkanes
This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2). The mechanism of chlorination and bromination involves free radicals.
Preparation, properties and reactions of alkanes:
Homologous series,
Preparation of alkanes by Wurtz reaction
Preparation of alkanes decarboxylation reactions.
physical properties of alkanes (melting points, boiling points and density); Combustion and halogenation of alkanes;
-------------
Alkanes: Introduction
Alkanes are saturated hydrocarbons containing only carbon-carbon single bonds in their molecules.
Thye are also called paraffins (meaning little affinity or reactivity, we will see later why it is so).
Alkanes are divided into 1. Open chain or acyclic Alkanes and 2. CycloAlkanes or cyclic alkanes.
The general formula of alkanes is CnH2n+2
Preparation of alkanes
1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts
1. From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes (Note that this point will come in alkenes and alkynes chapter as reactions of them).
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.
Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.
2. Wurtz reaction (From alkyl halides)
When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing both twice the number of carbon atoms of alkyl halide is obtained.
3. Decarboxylation reaction
When sodium salt of a monocarboxylic acid is heated with soda lime (amixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
Physical properties of alkanes
1. State: CH4 to C4H10 are gases, C5H12 to C17H36 are liquids and higher ones are solids
2. Boiling point: Boiling point increases with molecular mass. Branched isomers have a lower boiling point than normal alkanes.
3. Melting point
4. Solubility: Being nonpolar, these are insoluble in water.
5. Density: Liquid alkanes lighter than water
Combustion
Large quantity of heat generated in the combustion of alkanes
Halogenation of alkanes
This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2). The mechanism of chlorination and bromination involves free radicals.
IIT JEE Revision Alkanes - Introduction
Alkanes are also called paraffins
Gen formula CnH2n+2
Open alkanes
Cyclo alkanes
They are saturated.
Under normal conditions of temperature and pressure they do not react with reagents like acids, bases, oxidizing agents and reducing agents
Gen formula CnH2n+2
Open alkanes
Cyclo alkanes
They are saturated.
Under normal conditions of temperature and pressure they do not react with reagents like acids, bases, oxidizing agents and reducing agents
IIT JEE Revision Alkanes Nomenclature
1. The unbranched alkanes are named according to the number of alkanes of carbons. Examples: Methane, Ethane, Propane etc.
2. For alkanes containing branched carbon chains, the principal chain of the compound needs to be determined. This is the longest continuous carbon chain in the molecule.
3. In determining the principal chain sometimes, two or more chains in the molecule may have same number of carbons in their chains. In such a case, the chain having the greater number of branches is chosen as the principal chain of the molecule.
4. Numbers have to be given for the carbon atoms, to which the branches are attached. To give numbers, principal chain of the carbon atoms is numbered from the direction that gives the lower number to the first branching point.
5. Branching groups are in general termed as substituents. Each group is to be given an IUPAC name (prefix name) and is prefixed to the name of the principal chain of the molecule.
6. Compound is named according to the pattern “number- alkyl group prefix name principal chain name”. Name of the group (prefix) and name of the principal chain are written together as one word.
7. Where there are more than one substituent groups, each substituent group must be given its own number depending on the carbon to which it was attached.
8. In case of identical substituents, prefixes di, tri, tetra etc., are used before the group prefix name to indicate number of identical groups.
9. Substituent groups are written in alphabetical order regardless of their location in the principal chain but the prefixes di, tri… as well as the prefixes tert- and sec- are ignored in deciding the alphabetical order, and the prefixes iso, neo, and cyclo are considered.
10. If the number of carbon atoms of the principal chain from either gives identical numbers to the substituent attachment carbons, the direction which gives the lower number to the first written group (according to the alphabetical order) is chosen.
Examples
2,2,4,4-Tetramethylhexane
3-Ethyl-2,2-dimethylpentane
2. For alkanes containing branched carbon chains, the principal chain of the compound needs to be determined. This is the longest continuous carbon chain in the molecule.
3. In determining the principal chain sometimes, two or more chains in the molecule may have same number of carbons in their chains. In such a case, the chain having the greater number of branches is chosen as the principal chain of the molecule.
4. Numbers have to be given for the carbon atoms, to which the branches are attached. To give numbers, principal chain of the carbon atoms is numbered from the direction that gives the lower number to the first branching point.
5. Branching groups are in general termed as substituents. Each group is to be given an IUPAC name (prefix name) and is prefixed to the name of the principal chain of the molecule.
6. Compound is named according to the pattern “number- alkyl group prefix name principal chain name”. Name of the group (prefix) and name of the principal chain are written together as one word.
7. Where there are more than one substituent groups, each substituent group must be given its own number depending on the carbon to which it was attached.
8. In case of identical substituents, prefixes di, tri, tetra etc., are used before the group prefix name to indicate number of identical groups.
9. Substituent groups are written in alphabetical order regardless of their location in the principal chain but the prefixes di, tri… as well as the prefixes tert- and sec- are ignored in deciding the alphabetical order, and the prefixes iso, neo, and cyclo are considered.
10. If the number of carbon atoms of the principal chain from either gives identical numbers to the substituent attachment carbons, the direction which gives the lower number to the first written group (according to the alphabetical order) is chosen.
Examples
2,2,4,4-Tetramethylhexane
3-Ethyl-2,2-dimethylpentane
Revision - Conformations in Alkanes
Isomerism
Conformations
The different arrangements of atoms in a molecule which can be obtained due to rotation about carbon-carbon single bond are called conformations
To present ocnformations, the diagrams used by chemists a. Saw horse representation and (b) Newman projection can be used.
Conformations of Ethane
Of all the conformations for ethane, only two extreme conformations are important and these are
Staggered conformation
Eclipsed conformation
The staggered conformation is more stable than the eclipsed conformation.
Conformations of Butane
In butane (CH3CH2CH2CH3) the rotation about the single bond between the two inner carbon atoms (C2 and C3) is examined.
The lowest energy conformation will be theo one, in which the two methyl groups are as far apart as possible i.e., 180° away from each other. This conformation will be maximum staggered and is called anti conformation.
Order of stability
Anti>Skew or Gauche>Eclipsed>Fully eclipsed
Conformations
The different arrangements of atoms in a molecule which can be obtained due to rotation about carbon-carbon single bond are called conformations
To present ocnformations, the diagrams used by chemists a. Saw horse representation and (b) Newman projection can be used.
Conformations of Ethane
Of all the conformations for ethane, only two extreme conformations are important and these are
Staggered conformation
Eclipsed conformation
The staggered conformation is more stable than the eclipsed conformation.
Conformations of Butane
In butane (CH3CH2CH2CH3) the rotation about the single bond between the two inner carbon atoms (C2 and C3) is examined.
The lowest energy conformation will be theo one, in which the two methyl groups are as far apart as possible i.e., 180° away from each other. This conformation will be maximum staggered and is called anti conformation.
Order of stability
Anti>Skew or Gauche>Eclipsed>Fully eclipsed
IIT JEE Revision - Preparation of alkanes
General methods
1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts
From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes.
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.
Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.
2. From alkyl halides
Wurtz reaction: When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing twice the number of carbon atoms of alkyl halide is obtained.
Reducing agents can be used to add hydrogen to the halide and remove the halogen atom.
i) Zinc + HCl is one reducing agent.
ii) Catalytic hydrogenation using Pd or Pt as catalyst
iii) Hydrogen iodide (halogen acid) in the presence of red phosphorous also acts as reducing agent. In this reaction phosphorous combines with iodine to form phosphorous triiodide.
iv) zinc copper couple and alcohol
By the use of Grignard reagent
Alkyl halides react with magnesium metal in diethyl ether to form alkyl magnesium halides which are called as Grignard reagents.
Grignard reagents are highly reactive and are easily decomposed by water or alcohol to form alkanes
RMgX + HOH (H2O) ---> RH + Mg(OH)X
3. From carboxylic acids
i) Reduction of carboxylic acid: Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group.
ii) When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
RCOONa + NaOH -->RH + Na2CO3
In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.
iii) Kolbe's reaction
When an acqueous solution of sodium or potassium salt of carboxylic acid is eletrolysed alkane is evolved at the anode.
Kolbe's reaction can also be used like wurtz reaction for preparing alkanes with even number of carbon atoms.
The methods in this section can be summarised as
R-COONa ---> RH
R-COOK ---> R-R - Kolbe's reaction
R-COOH---> R-CH3
Industrial method: Petroleum provides the natural source of alkanes.
1. From unsaturated hydrocarbons (alkenes and alkynes)
2. From alkyl halides
3. From carboxylic acids and their salts
From unsaturated hydrocarbons (alkenes and alkynes)
By catalytic hydrogenation alkenes and alkynes are converted into alkanes.
Ni, Pt or Pd in the form of fine powder are used as catalysts. A temperature of 523-573 K needs to be employed.
Methane cannot be prepared by this method because alkenes or alkynes will have two carbons at their lowest level.
2. From alkyl halides
Wurtz reaction: When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing twice the number of carbon atoms of alkyl halide is obtained.
Reducing agents can be used to add hydrogen to the halide and remove the halogen atom.
i) Zinc + HCl is one reducing agent.
ii) Catalytic hydrogenation using Pd or Pt as catalyst
iii) Hydrogen iodide (halogen acid) in the presence of red phosphorous also acts as reducing agent. In this reaction phosphorous combines with iodine to form phosphorous triiodide.
iv) zinc copper couple and alcohol
By the use of Grignard reagent
Alkyl halides react with magnesium metal in diethyl ether to form alkyl magnesium halides which are called as Grignard reagents.
Grignard reagents are highly reactive and are easily decomposed by water or alcohol to form alkanes
RMgX + HOH (H2O) ---> RH + Mg(OH)X
3. From carboxylic acids
i) Reduction of carboxylic acid: Carboxylic acids are reduced to alkanes by hydroiodic acid (HI). In this reaction COOH group in the carboxylic acid is reduced to CH3 group.
ii) When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
RCOONa + NaOH -->RH + Na2CO3
In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.
iii) Kolbe's reaction
When an acqueous solution of sodium or potassium salt of carboxylic acid is eletrolysed alkane is evolved at the anode.
Kolbe's reaction can also be used like wurtz reaction for preparing alkanes with even number of carbon atoms.
The methods in this section can be summarised as
R-COONa ---> RH
R-COOK ---> R-R - Kolbe's reaction
R-COOH---> R-CH3
Industrial method: Petroleum provides the natural source of alkanes.
Revision - Preparation of alkanes by Wurtz reaction
When an alkyl halide (usually bromide or iodide) is treated with sodium in dry ether, a symmetrical alkane containing both twice the number of carbon atoms of alkyl halide is obtained.
RX + 2Na + XR ---> R-R + 2NaX (catalyst in dry ether)
There is a possibility, in the reaction to use different alkyl halides instead of a single halide. If two different halides are taken with the aim of preparing an alkane with odd number of carbon atoms, a mixture of products is obtained in stead of a single alkane. This is because in this case three reactions takes place and three different products are obtained.
RX + 2Na + XR ---> R-R + 2NaX (catalyst in dry ether)
There is a possibility, in the reaction to use different alkyl halides instead of a single halide. If two different halides are taken with the aim of preparing an alkane with odd number of carbon atoms, a mixture of products is obtained in stead of a single alkane. This is because in this case three reactions takes place and three different products are obtained.
Senin, 28 Januari 2008
IIT JEE Revision Preparation of Alkanes by Decarboxilation
Decarboxylation reaction
When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
RCOONa + NaOH -->RH + Na2CO3
In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.
When sodium salt of a monocarboxylic acid is heated with soda lime (a mixture of NaOH and Cao in the ratio of 3:1) at about 630 K, alkane is formed.
RCOONa + NaOH -->RH + Na2CO3
In this reaction a CO2 group is removed from carboxylic acid and therefore the reaction is called decarboxylation.
IIT JEE Revision Alkanes Physical Properties
1. State: CH4 to C4H10 are gases, C5H12 to C17H36 are liquids and higher ones are solids
2. Boiling point: Boiling point increases with molecular mass(carbon atoms).
Branched isomers have a lower boiling point than normal alkanes.
3. Melting point: Melting points increase with molecular mass. But the increase is not a regular variation. In general, the alkanes with even number of carbon atoms have a higher melting points as compared to the immediately next lower alkanes with odd number of carbon atoms.
4. Solubility: Alkanes are almost non-polar molecules. Being nonpolar, these are insoluble in water. They dissolve in non-polar solvents such as ether, benzene, carbon tetrachloride etc. The solubility generally decreases with increase in molecular mass.
5. Density: Liquid alkanes lighter than water. the density increases with the increase in the number of carbon atoms.
2. Boiling point: Boiling point increases with molecular mass(carbon atoms).
Branched isomers have a lower boiling point than normal alkanes.
3. Melting point: Melting points increase with molecular mass. But the increase is not a regular variation. In general, the alkanes with even number of carbon atoms have a higher melting points as compared to the immediately next lower alkanes with odd number of carbon atoms.
4. Solubility: Alkanes are almost non-polar molecules. Being nonpolar, these are insoluble in water. They dissolve in non-polar solvents such as ether, benzene, carbon tetrachloride etc. The solubility generally decreases with increase in molecular mass.
5. Density: Liquid alkanes lighter than water. the density increases with the increase in the number of carbon atoms.
IIT JEE Revision Alkanes Chemical Reactions
Chemical properties or reactions
1. Substitution reactions of alkanes
a)Halogenation
Involves the replacement of one or more atoms of hydrogen by the halogen atoms.
b) Nitration
Involves the replacement of a hydrogen atom of alkane with -NO2 group.
Nitration of higher alkanes (hexane or higher) is carried out by boiling alkane with nitric acid.
Nitration of lower alkanes can be carried out in vapour phase by heating alkane and nitric acid to very high temperatures in the range of 723-773 K. At such higher temperature C-C bonds of alkane breakes, and hence a mixture of nitroalkanes may obtained.
For example in the reaction between ethane and nitric acid at very high temperature, both nitroethane and nitromethane(due to breaking of one C-C bond in ethane) are obtained as mixture.
c) sulphonation
This involves the replacement of a hydrogen atom by -SO3H group.
Fuming sulphuric acid reacts with alkane at higher temperature.
Higher alkanes (hexane and above) only give this reaction.
Lower alkanes particularly methane, ethane do not give this reaction.
2. Oxidation
a) Complete combustion
b) Incomplete combustion
c) Controlled oxidation
3. Action of steam
This reaction is used for the production of hydrogen from natural gas.
On passing a mixture of steam and methane over heated nickel(over alumina Al2O3) catalyst at 1273 K, methane gets oxidized to carbon monoxide and all hydrogen atoms get released.
4. Isomerisation
branched isomers of alkanes are obtained by heating alkanes with anhydrous aluminium chloride (AlCl3) and hydrogen chloride at 573 K under a pressure of about 30-35 atmosphere.
5. Aromatization
Alkanes containing 6 or more carbon atoms get converted to aromatic compounds, when heated at about 773 K under higher pressures of the order of 10-20 atm in the presence of catalysts - like oxides of chromium, molybdenum or vanadium supported on alumina gel.
6. Thermal decomposition or fragmentation
When higher alkanes are heated to high temperatures (700-800 K) in the presence of alumna or silica catalysts, they break down to lower alkanes and alkenes.
If methane is heated to high temperature up to 1500 K, it breaks down to its elements (carbon and hydrogen)
1. Substitution reactions of alkanes
a)Halogenation
Involves the replacement of one or more atoms of hydrogen by the halogen atoms.
b) Nitration
Involves the replacement of a hydrogen atom of alkane with -NO2 group.
Nitration of higher alkanes (hexane or higher) is carried out by boiling alkane with nitric acid.
Nitration of lower alkanes can be carried out in vapour phase by heating alkane and nitric acid to very high temperatures in the range of 723-773 K. At such higher temperature C-C bonds of alkane breakes, and hence a mixture of nitroalkanes may obtained.
For example in the reaction between ethane and nitric acid at very high temperature, both nitroethane and nitromethane(due to breaking of one C-C bond in ethane) are obtained as mixture.
c) sulphonation
This involves the replacement of a hydrogen atom by -SO3H group.
Fuming sulphuric acid reacts with alkane at higher temperature.
Higher alkanes (hexane and above) only give this reaction.
Lower alkanes particularly methane, ethane do not give this reaction.
2. Oxidation
a) Complete combustion
b) Incomplete combustion
c) Controlled oxidation
3. Action of steam
This reaction is used for the production of hydrogen from natural gas.
On passing a mixture of steam and methane over heated nickel(over alumina Al2O3) catalyst at 1273 K, methane gets oxidized to carbon monoxide and all hydrogen atoms get released.
4. Isomerisation
branched isomers of alkanes are obtained by heating alkanes with anhydrous aluminium chloride (AlCl3) and hydrogen chloride at 573 K under a pressure of about 30-35 atmosphere.
5. Aromatization
Alkanes containing 6 or more carbon atoms get converted to aromatic compounds, when heated at about 773 K under higher pressures of the order of 10-20 atm in the presence of catalysts - like oxides of chromium, molybdenum or vanadium supported on alumina gel.
6. Thermal decomposition or fragmentation
When higher alkanes are heated to high temperatures (700-800 K) in the presence of alumna or silica catalysts, they break down to lower alkanes and alkenes.
If methane is heated to high temperature up to 1500 K, it breaks down to its elements (carbon and hydrogen)
IIT JEE Revision - Combustion of Alkanes
Complete combustion
Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.
C3H8 + 5O2 --> 3CO2 + 4H2O
2C4H10 + 13O2 --> 8CO2 + 10H2O
Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.
A simple explanation is that, hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over.
The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke.
Carbon monoxide is produced as a colourless poisonous gas.
Controlled combustion
a) When a mixture of methan and oxygen in the molar ratio of 9:1 is compresed to abut 1100 atmospheres and passed through copper tubes at 575 K, methane is oxidized to methanol - Output is methanol
b) When methane and oxygen are passed through heated molybdenum oxide (Mo2O3), it is oxidized to methanal (formaldehyde).
Alkanes having teriary hydrogen atom can be oxidized to alcohols in the presence of potassium permangante.
Alkanes are oxidized to carboxylic acids by silver oxide (Ag2O)
Complete combustion (given sufficient oxygen) of any hydrocarbon produces carbon dioxide and water.
C3H8 + 5O2 --> 3CO2 + 4H2O
2C4H10 + 13O2 --> 8CO2 + 10H2O
Incomplete combustion (where there isn't enough oxygen present) can lead to the formation of carbon or carbon monoxide.
A simple explanation is that, hydrogen in the hydrocarbon gets the first chance at the oxygen, and the carbon gets whatever is left over.
The presence of glowing carbon particles in a flame turns it yellow, and black carbon is often visible in the smoke.
Carbon monoxide is produced as a colourless poisonous gas.
Controlled combustion
a) When a mixture of methan and oxygen in the molar ratio of 9:1 is compresed to abut 1100 atmospheres and passed through copper tubes at 575 K, methane is oxidized to methanol - Output is methanol
b) When methane and oxygen are passed through heated molybdenum oxide (Mo2O3), it is oxidized to methanal (formaldehyde).
Alkanes having teriary hydrogen atom can be oxidized to alcohols in the presence of potassium permangante.
Alkanes are oxidized to carboxylic acids by silver oxide (Ag2O)
IIT JEE Revision Alkanes Halogenation
Halogenation of alkanes
This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2).
The mechanism of chlorination and bromination involves free radicals.
Step 1 (Initiation)
Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.
Step 2 (Propagation)
(a) A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
(b) The methyl radical abstracts a bromine atom from another molecule of Br2 to form the methyl bromide product and another bromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.
Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2 or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle. There will be equilibrium and reaction terminates.
This involves substitution o fhydrogen atom by halogen atom. The order of reactivity is F2>Cl2>Br2(>I2).
The mechanism of chlorination and bromination involves free radicals.
Step 1 (Initiation)
Heat or uv light cause the weak halogen bond to undergo homolytic cleavage to generate two bromine radicals and starting the chain process.
Step 2 (Propagation)
(a) A bromine radical abstracts a hydrogen to form HBr and a methyl radical, then
(b) The methyl radical abstracts a bromine atom from another molecule of Br2 to form the methyl bromide product and another bromine radical, which can then itself undergo reaction 2(a) creating a cycle that can repeat.
Step 3 (Termination)
Various reactions between the possible pairs of radicals allow for the formation of ethane, Br2 or the product, methyl bromide. These reactions remove radicals and do not perpetuate the cycle. There will be equilibrium and reaction terminates.
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