Polymerization
a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.
The later polymerizes to give neoprene - a synthetic rubber.
b) Cyclic polymerization
This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.
Occurs in two stages. Acetylene becomes mesitylene.
Showing posts with label Alkynes. Show all posts
Showing posts with label Alkynes. Show all posts
Saturday, January 11, 2020
Wednesday, March 11, 2009
IIT JEE Chemistry - Study Guide - Study Plan - 15. Hydrocarbons
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
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
Sunday, December 28, 2008
Alkynes- Study Guide - IIT JEE
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments); Acidity of alkynes; Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination); Reduction of alkynes; Preparation of alkynes by elimination reactions; Addition reactions of alkynes; Metal acetylides.
Physical properties of alkynes (boiling points, density and dipole moments); Acidity of alkynes; Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination); Reduction of alkynes; Preparation of alkynes by elimination reactions; Addition reactions of alkynes; Metal acetylides.
Sunday, January 27, 2008
IIT JEE Revision Ch.23 Alkynes - Core Chapter Points
syllabus
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);
Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
-----------------
1. Alkynes are hydrocarbons with triple bonds. General formula CnH2n
2. Methods of Preparation of Alkynes;
1. Dehydrohalogenation of vicinal dihalides
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
3. Physical properties
State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
4. Chemical Properties
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
5. Acidic property of acetylene
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
6. Addition of water (hydration of alkynes) (
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
7. Reduction of Alkynes
Reaction Type: Addition
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
8. Preparation of alkynes by elimination reactions; To be posted
9. Formation of metal acetylides
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);
Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
-----------------
1. Alkynes are hydrocarbons with triple bonds. General formula CnH2n
2. Methods of Preparation of Alkynes;
1. Dehydrohalogenation of vicinal dihalides
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
3. Physical properties
State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
4. Chemical Properties
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
5. Acidic property of acetylene
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
6. Addition of water (hydration of alkynes) (
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
7. Reduction of Alkynes
Reaction Type: Addition
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
8. Preparation of alkynes by elimination reactions; To be posted
9. Formation of metal acetylides
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
IIT JEE Revision - Alkynes - Introduction
Alkynes are hydrocarbons having one or more triple bonds.
They are unsaturated.
General formula CnH2n-2
Simplest member of the class
Ethyne C2H2
Other members
Propyne
Butyne
Pentyne
They are unsaturated.
General formula CnH2n-2
Simplest member of the class
Ethyne C2H2
Other members
Propyne
Butyne
Pentyne
IIT JEE Revision Alkynes Isomerism
Alkynes exhibit and chain isomerism and position isomerism
Chain isomerism
The arrangement of the chain is different in these isomers
Ex: Pent-1-yne Linear chain pentyne with triple bond at the terminal carbon.
3-Methylbut-1-yne - branched isomer having a methyl group at the 3rd carbon and triple at the terminal carbon.
Position isomerism - position of triple bond changes. Alkynes having more than four carbon atoms exhibit this isomerism.
1-Alkyne 2 Alkyne
Alkynes do not have geometrical isomerism as the bond angle is 180°
Chain isomerism
The arrangement of the chain is different in these isomers
Ex: Pent-1-yne Linear chain pentyne with triple bond at the terminal carbon.
3-Methylbut-1-yne - branched isomer having a methyl group at the 3rd carbon and triple at the terminal carbon.
Position isomerism - position of triple bond changes. Alkynes having more than four carbon atoms exhibit this isomerism.
1-Alkyne 2 Alkyne
Alkynes do not have geometrical isomerism as the bond angle is 180°
IIt JEE Revision - Alkynes - Preparation
1. Synthesis of carbon and hydrogen
A stream of hydrogen is passed through electric arc struck between carbon electrodes at 3270 K. Ethyne (acytylene) is obtained.
2. From acetylene (ethyne)
Higher alkynes are prepared from ethyne (acytylene) by treating its sodium salt with alkyl halide. Sodium salt of ethyne is prepared from the reaction of ethyne and sodamide (NaNH2).
Sodium salt obtained from ethyne and sodamide is Sodium acetylide (HC≡CNa).
If methyl bromide is added to it, propyne is obtained.
If ethyl bromde is added to it, But-1-yne is obtained.
As higher and higher alkyl bromide are added higher alkynes are obtained in this process.
3. Action of zinc on tetrahalogen derivatives of alkanes
On treatment with zinc, tetrahalides get dehalogated (eliminated from the molecule) and triple bonds forms and alkyne is obtained
1,1,2,2-tetrabromoethane + zinc --> ethyne +zinc bromide
4. Dehydrohalogenation of vicinal dihalides
Vicinal dihalides (having halogen atoms on the adjacent carbon atoms) get dehalogenated on treatment with alcoholic solution of potassium hydroxide.
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
Fumaric acid is a dicarboxylic acid.
Electrolysis of acqueous solution Potassium fumerate gives ethyne
6. Action of water on calcium carbide
CaC2 + 2H2O gives ethyne and Calcium hydroxide
Calcium carbide required is obtained by heating calcium oxide (from limestone) and coke in an electric furnace at 2275 K.
--------------------
1. Dehydrohalogenation of vicinal dihalides
The process takes place in two stages.
In stage I KOH(alc) reacts with vicinal dihalide and vinylic halide is formed.
second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH2 (sodamide) is used in the reaction to produce the corresponding alkyne.
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
Secondary and tertiary halides cannot be used because eliminatin is the predominant reaction which results in the formation of alkenes.
3. Hydrolysis of CaC two and Mg two C three.
Calcium carbide in reaction with water gives ethyne and calcium hydroxide.(came in X class)
Magnesium carbide in reaction water gives propyne and magnesium hydroxide.
4. Kolbe's Electrolytic Method:
The electrolysis of an acqueous solution of potassium salt of an unsaturated dicarboxylic acid forms alkyne
5. Dehalogenation of vic-tetrahalogen compounds
Tetrahalogenated alkane in reaction with zinc with ethyl alcohol as catalyst gives alkyne and zinc halide.
Preparation of Acetylene or Ethyne
1. By the action of Alcoholic Potash on Ethylene Bromide
BrH two C - CH two Br + 2KOH = C Two H Two + @KBr + 2 H two O
At the end of first state vinyl bromide is formed.
2. By heating tetra-bromoethane with Zinc
C-two H-two Br-four +2Zn = C-two H-two + 2ZnBr-two
3. By the electrolysis of Acqueous solution of Potassium salts of maleic acid (Kolbe's method)
4. By the action of water on calcium carbide (already covered earlier)
5. By heating Iodoform with silver powder
2HCI-three + 6Ag = C-twoH-two + 6AgI
6. By partial oxidation of Methane
2CH-four + 3[O] = C-twoHtwo + 3H-two O
7. By direct synthesis of carbon and hydrogen (Berthelot's synthesis)
2C + H-two = C-twoH-two
------------------------------------------------------------------
1. Action of water on calcium carbide
2. Dehydrohalogenation of vicinal dihalides
3. Action of zinc on tetrahalogen derivatives of alkanes
4. From acetylene
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
6. Synthesis of carbon and hydrogen
Industrial preparation
Ethyne is prepared on an industrial scale by treating calcium carbide with water.
A stream of hydrogen is passed through electric arc struck between carbon electrodes at 3270 K. Ethyne (acytylene) is obtained.
2. From acetylene (ethyne)
Higher alkynes are prepared from ethyne (acytylene) by treating its sodium salt with alkyl halide. Sodium salt of ethyne is prepared from the reaction of ethyne and sodamide (NaNH2).
Sodium salt obtained from ethyne and sodamide is Sodium acetylide (HC≡CNa).
If methyl bromide is added to it, propyne is obtained.
If ethyl bromde is added to it, But-1-yne is obtained.
As higher and higher alkyl bromide are added higher alkynes are obtained in this process.
3. Action of zinc on tetrahalogen derivatives of alkanes
On treatment with zinc, tetrahalides get dehalogated (eliminated from the molecule) and triple bonds forms and alkyne is obtained
1,1,2,2-tetrabromoethane + zinc --> ethyne +zinc bromide
4. Dehydrohalogenation of vicinal dihalides
Vicinal dihalides (having halogen atoms on the adjacent carbon atoms) get dehalogenated on treatment with alcoholic solution of potassium hydroxide.
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
Fumaric acid is a dicarboxylic acid.
Electrolysis of acqueous solution Potassium fumerate gives ethyne
6. Action of water on calcium carbide
CaC2 + 2H2O gives ethyne and Calcium hydroxide
Calcium carbide required is obtained by heating calcium oxide (from limestone) and coke in an electric furnace at 2275 K.
--------------------
1. Dehydrohalogenation of vicinal dihalides
The process takes place in two stages.
In stage I KOH(alc) reacts with vicinal dihalide and vinylic halide is formed.
second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH2 (sodamide) is used in the reaction to produce the corresponding alkyne.
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
Secondary and tertiary halides cannot be used because eliminatin is the predominant reaction which results in the formation of alkenes.
3. Hydrolysis of CaC two and Mg two C three.
Calcium carbide in reaction with water gives ethyne and calcium hydroxide.(came in X class)
Magnesium carbide in reaction water gives propyne and magnesium hydroxide.
4. Kolbe's Electrolytic Method:
The electrolysis of an acqueous solution of potassium salt of an unsaturated dicarboxylic acid forms alkyne
5. Dehalogenation of vic-tetrahalogen compounds
Tetrahalogenated alkane in reaction with zinc with ethyl alcohol as catalyst gives alkyne and zinc halide.
Preparation of Acetylene or Ethyne
1. By the action of Alcoholic Potash on Ethylene Bromide
BrH two C - CH two Br + 2KOH = C Two H Two + @KBr + 2 H two O
At the end of first state vinyl bromide is formed.
2. By heating tetra-bromoethane with Zinc
C-two H-two Br-four +2Zn = C-two H-two + 2ZnBr-two
3. By the electrolysis of Acqueous solution of Potassium salts of maleic acid (Kolbe's method)
4. By the action of water on calcium carbide (already covered earlier)
5. By heating Iodoform with silver powder
2HCI-three + 6Ag = C-twoH-two + 6AgI
6. By partial oxidation of Methane
2CH-four + 3[O] = C-twoHtwo + 3H-two O
7. By direct synthesis of carbon and hydrogen (Berthelot's synthesis)
2C + H-two = C-twoH-two
------------------------------------------------------------------
1. Action of water on calcium carbide
2. Dehydrohalogenation of vicinal dihalides
3. Action of zinc on tetrahalogen derivatives of alkanes
4. From acetylene
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
6. Synthesis of carbon and hydrogen
Industrial preparation
Ethyne is prepared on an industrial scale by treating calcium carbide with water.
IIT JEE REvision - Preparation of Alkynes by Elimination Reactions
Partial oxidation of Methane
2CH-4 + 3[O] = C2H2 + 3H2O
Methane on thermal decomposition by pyrolysis gives Ethyne + Hydrogen at around 1500 degree Celsius in an electric arc.
2CH-4 + 3[O] = C2H2 + 3H2O
Methane on thermal decomposition by pyrolysis gives Ethyne + Hydrogen at around 1500 degree Celsius in an electric arc.
IIT JEE Revision - Alkynes - Physical Properties
State:
First three members are gases at room temperature.
The next eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water but are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
Melting point
Slightly higher compared to alkanes and alkenes. This is because of linear structure which allows more close packing. The magnitude of attractive forces are higher among them.
Melting points increase with increase in molecular mass among alkynes.
B.P.
The behaviour is similar to melting point.
First three members are gases at room temperature.
The next eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water but are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
Melting point
Slightly higher compared to alkanes and alkenes. This is because of linear structure which allows more close packing. The magnitude of attractive forces are higher among them.
Melting points increase with increase in molecular mass among alkynes.
B.P.
The behaviour is similar to melting point.
IIT JEE Revision - Alkynes - Chemical Properties and Reactions
Reactions specifically mentioned in the syllabus are posted in more detail as separate topics.
Oxidation
a) Combustion
b) Oxidation with alkaline potassium permanganate
c) Ozonolysis
Alkynes react with ozone to form ozonides.
These ozonides on decomposition with water in the presence of zinc give diketones(two carbonyl groups).
Ethyne gives glyoxal on reacting with ozone. Glyoxal also has two carbonyl groups.
III. Polymerization
a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.
The later polymerizes to give neoprene - a synthetic rubber.
b) Cyclic polymerization
This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.
Occurs in two stages. Acetylene becomes mesitylene.
IV. Isomerization
1-alkyne gets converted to 2-alkyne and vice versa.
Oxidation
a) Combustion
b) Oxidation with alkaline potassium permanganate
c) Ozonolysis
Alkynes react with ozone to form ozonides.
These ozonides on decomposition with water in the presence of zinc give diketones(two carbonyl groups).
Ethyne gives glyoxal on reacting with ozone. Glyoxal also has two carbonyl groups.
III. Polymerization
a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.
The later polymerizes to give neoprene - a synthetic rubber.
b) Cyclic polymerization
This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.
Occurs in two stages. Acetylene becomes mesitylene.
IV. Isomerization
1-alkyne gets converted to 2-alkyne and vice versa.
IIT JEE Revision - Acidity of Alkynes
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
Acetylides react with alkyl halides to give higher alkynes.
Explanation for the acidic character of alkynes
The acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes.
In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by the nucleus and are quite electronegative.
Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.
In the case of nonterminal alkynes, no hydrogen atom is attached directly to the triple bonded carbon atom (triple bond to carbon and one single bond to another carbon) and hence hydrogen atoms are not released easily.
Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.
acidic charater
HC≡HC > CH2=CH2 > CH3-CH3
The relative acidity of acetylene is more than that of ammonia but less than that of water.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
Acetylides react with alkyl halides to give higher alkynes.
Explanation for the acidic character of alkynes
The acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes.
In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by the nucleus and are quite electronegative.
Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.
In the case of nonterminal alkynes, no hydrogen atom is attached directly to the triple bonded carbon atom (triple bond to carbon and one single bond to another carbon) and hence hydrogen atoms are not released easily.
Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.
acidic charater
HC≡HC > CH2=CH2 > CH3-CH3
The relative acidity of acetylene is more than that of ammonia but less than that of water.
IIT JEE Revision - Acid Catalysed Hydration of Alkynes
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K.
The catalyst in this reaction is HgSO4 (Mercuric sulphate).
The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess.
Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Example:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
The catalyst in this reaction is HgSO4 (Mercuric sulphate).
The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess.
Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Example:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
IIT JEE REvision - Reduction of Alkynes
Reaction Type: Addition
Summary
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
The stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
The reaction proceeds via single electron transfer from the Na with H coming from the NH3
These reaction conditions do not reduce alkenes, hence the product is the alkene.
MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.
Summary
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
The stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
The reaction proceeds via single electron transfer from the Na with H coming from the NH3
These reaction conditions do not reduce alkenes, hence the product is the alkene.
MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.
IIT JEE Revision Addition reactions of alkynes
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
I. Addition reactions (specially given in jee syllabus)
1. Addition of Hydrogen
If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.
2. Addition of halogens
chlorine and bromine add on alkali
3. Addition of hydrogen halides
This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).
4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Examples:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
5. Addition of hypohalous acid (HOX)
Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.
6. Addition of H2SO4
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.
In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.
They readily take part in addition reactions and can also be easily oxidized.
I. Addition reactions (specially given in jee syllabus)
1. Addition of Hydrogen
If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.
2. Addition of halogens
chlorine and bromine add on alkali
3. Addition of hydrogen halides
This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).
4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Examples:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
5. Addition of hypohalous acid (HOX)
Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.
6. Addition of H2SO4
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.
In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.
IIT JEE Revision - Metal Acetylides
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
Wednesday, January 16, 2008
Ch.23 Alkynes - Core Points for Revision
syllabus
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);
Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
-----------------
1. Alkynes are hydrocarbons with triple bonds. General formula CnH2n
2. Methods of Preparation of Alkynes;
1. Dehydrohalogenation of vicinal dihalides
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
3. Physical properties
State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
4. Chemical Properties
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
5. Acidic property of acetylene
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
6. Addition of water (hydration of alkynes) (
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
7. Reduction of Alkynes
Reaction Type: Addition
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
8. Preparation of alkynes by elimination reactions; To be posted
9. Formation of metal acetylides
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);
Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
-----------------
1. Alkynes are hydrocarbons with triple bonds. General formula CnH2n
2. Methods of Preparation of Alkynes;
1. Dehydrohalogenation of vicinal dihalides
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
3. Physical properties
State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
4. Chemical Properties
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
5. Acidic property of acetylene
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
6. Addition of water (hydration of alkynes) (
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
7. Reduction of Alkynes
Reaction Type: Addition
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
8. Preparation of alkynes by elimination reactions; To be posted
9. Formation of metal acetylides
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
Saturday, September 1, 2007
TMH-JEE-chemistry-Study Guide - ch 23 Alkynes Part 2
Chemical Properties
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
I. Addition reactions (specially given in jee syllabus)
1. Addition of Hydrogen
If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.
2. Addition of halogens
chlorine and bromine add on alkali
3. Addition of hydrogen halides
This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).
4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Examples:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
5. Addition of hypohalous acid (HOX)
Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.
6. Addition of H2SO4 (Ref: Standard XI Chemistry by Khan et al, Uttam Prakashan)
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.
In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.
II. Oxidation
a) Combustion
b) Oxidation with alkaline potassium permanganate
c) Ozonolysis
Alkynes react with ozone to form ozonides. These ozonides on decomposition with water in the presence of zinc give diketones(two carbonyl groups).
Ethyne gives glyoxal on reacting with ozone. Glyoxal also have two carbonyl groups.
III. Polymerization
a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.
The later polymerizes to give neoprene - a synthetic rubber.
b) Cyclic polymerization
This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.
Occurs in two stages. Acetylene becomes mesitylene.
IV. Isomerization
1-alkyne gets converted to 2-alkyne and vice versa.
V. Formation of metal acetylides (specially given in jee syllabus)
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
VI. Reaction with Grignard reagent or alkyl-lithium
The acytylinic hydrogen on reacting with R'MgBr or R'Li produces the alkane R'H and metal acetylide
Acidic property of acetylene (specially given in jee syllabus)
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
Acetylides react with alkyl halides to give higher alkynes.
Explanation for the acidic character of alkynes
the acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes. In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by thenucleus and areq uite electronegative. Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.
In the case of nonterminal alkynes, hydrogen atom is not attached directly to the triple bonded carbon atom and hence it is not released easily.
Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.
acidic charater
HC≡HC > CH2=CH2 > CH3-CH3
The relative acidity of acetylene is more than that of ammonia but less than that of water.
Dissolving Metal Reduction of Alkynes (specially given in jee syllabus)
Reaction Type: Addition
Summary
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
Note that the stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
The reaction proceeds via single electron transfer from the Na with H coming from the NH3
These reaction conditions do not reduce alkenes, hence the product is the alkene.
MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.
------------------
Source for reduction
http://library.tedankara.k12.tr/carey/ch9-7.html
------------------
The alkynes have at least one triple bond in them, therefore, they are quite reactive chemically.
They readily take part in addition reactions and can also be easily oxidized.
I. Addition reactions (specially given in jee syllabus)
1. Addition of Hydrogen
If the triple bond is not present at the end of the chain of the molecule (it is not a terminal alkyne), its reduction (addition of hydrogen) produces either a cis alkene or a trans alkene depending upon the choice of reducing agent.
2. Addition of halogens
chlorine and bromine add on alkali
3. Addition of hydrogen halides
This addition takes place in accordance with Markonikov's rule(do you remember the rule?).
Peroxides have the same effect on addition of the HBr to acetylenes (alkynes) as they have on alkenes (do you remember the effect?).
4. Addition of water (hydration of alkynes) (specially given in jee syllabus)
In the presence of acid (H2SO4) and HgSO-4, a molecule of water adds to the triple bond at 348K. The catalyst in this reaction is HgSO4 (Mercuric sulphate). The final products of this reaction are carbonyl compounds aldehydes and ketones.
Initially enol is formed which is raidly converted into an equilibrium mixture containing keto form in excess. Enol is so called because it contains 'ene' (double bond) and an alcoholic group (ol).
Examples:
Addition of water to Ethyne or acytelene: Acetylene is passed into water (at about 330K) containing 60% H2SO4 and about 1% mercuric sulphate (HgSO4) as a catalyst, acetaldehyde is formed.
In the first step 'ethenol' is formed and in the second step the rearrangement of it takes place and its isomer 'acetaldehyde' is formed.
The conversion of enol form into keto form is termed tautomerism
5. Addition of hypohalous acid (HOX)
Alkynes react with two molecules of hypohalous acids in two stages.
For example take ethyne or acytelene and HOCl.
In the first stage HO gets added to one carbon and Cl adds to the other carbon.
In the second stage one more HOCl gets added to the intermediate product which has a double bond. The addition now follows markonikov's rule. OH gets added to HC-OH and Cl gets added to CH-Cl. Two OHs create instability and H2O molecule gets removed.
An aldehyde 2,2-Dichloroethanal (Dichloroacetaldehyde) is formed.
6. Addition of H2SO4 (Ref: Standard XI Chemistry by Khan et al, Uttam Prakashan)
Acetylene adds two molecules of concentrated H2SO4 in two stages and forms ethylidene hydrogen sulphate as the final product.
In the first stage Vinylhydrogen sulphate H2C=CH-OSO3H is formed. (H gets added to one CH and OSO3H gets added to the other CH).
The addition of second molecule follows markownikov's rule. H gets added to CH2 and OSO3 gets added to the other carbon. Thus two functional groups OSO3H gets added to one carbon.
The final product is Ethylidine hydrogen sulphate.
II. Oxidation
a) Combustion
b) Oxidation with alkaline potassium permanganate
c) Ozonolysis
Alkynes react with ozone to form ozonides. These ozonides on decomposition with water in the presence of zinc give diketones(two carbonyl groups).
Ethyne gives glyoxal on reacting with ozone. Glyoxal also have two carbonyl groups.
III. Polymerization
a) Acytylene dimerizes (two molecules come together as one molecule) in the presence of cuprous chloride and ammonium chloride to give vinyl acetylene which on reacting with HCl gives chloroprene.
The later polymerizes to give neoprene - a synthetic rubber.
b) Cyclic polymerization
This takes place when alkyne is passed through red hot iron tube at 400 degress celsius.
Occurs in two stages. Acetylene becomes mesitylene.
IV. Isomerization
1-alkyne gets converted to 2-alkyne and vice versa.
V. Formation of metal acetylides (specially given in jee syllabus)
Acytelene reacts with Na and LI liberating H-2 gas and forming metal acetylide. Therefore acetylene has chemical behaviour similar to acids.
Heavy metal ions mainly, Ag+ and Cu+ react with acetylinic hydrogen (hydrogen atom in acetylene) to form insoluble acetylides.
VI. Reaction with Grignard reagent or alkyl-lithium
The acytylinic hydrogen on reacting with R'MgBr or R'Li produces the alkane R'H and metal acetylide
Acidic property of acetylene (specially given in jee syllabus)
Acetylene and other terminal alkynes (1-alkynes) are weakly acidic in character.
They react with strong bases like NaNH2 ( sodium in liquid ammonia) to form sodium acetylide derivatives known as acetylides or alkynides.
Acetylides react with alkyl halides to give higher alkynes.
Explanation for the acidic character of alkynes
the acidic character of 1-alkynes can be explained on the basis of sp hybridisation state of the carbon atoms in alkynes. In sp hybridisation, s-character is 50% and due to this large s-character, the electons in sp hybrid orbitals are held more tightly by thenucleus and areq uite electronegative. Consequently the eletron pair of H-C≡C bond gets displaced more towards the carbon atom and helps in release of H+ ion.
In the case of nonterminal alkynes, hydrogen atom is not attached directly to the triple bonded carbon atom and hence it is not released easily.
Alkynes are weakly acidic but alkenes and alkanes do not show acidic behaviour.
acidic charater
HC≡HC > CH2=CH2 > CH3-CH3
The relative acidity of acetylene is more than that of ammonia but less than that of water.
Dissolving Metal Reduction of Alkynes (specially given in jee syllabus)
Reaction Type: Addition
Summary
Alkynes can be reduced to trans-alkenes using Na in NH3 (l)
This reaction is stereospecific giving only the trans-alkene via an anti addition.
Note that the stereochemistry of this reaction complements that of catalytic hydrogenation (syn)
The reaction proceeds via single electron transfer from the Na with H coming from the NH3
These reaction conditions do not reduce alkenes, hence the product is the alkene.
MECHANISM FOR THE REDUCTION OF ALKYNES WITH Na / NH3
Step 1:
Sodium transfers an electron to the alkyne giving a radical anion.
Step 2:
The radical anion removes a proton from the ammonia in an acid / base reaction.
Step 3:
A second atom of sodium transfers another electron to the alkyne giving an anion.
Step 4:
The anion removes a proton from the ammonia in an acid / base reaction.
------------------
Source for reduction
http://library.tedankara.k12.tr/carey/ch9-7.html
------------------
Study Guide TMH JEE Ch.23 Alkynes
JEE syllabus
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);
Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
----------------------
Main Topics Covered in the TMH Book
METHODS OF PREPARATION
CHEMICAL PROPERTIES
--------------
From X class book
General formula: C(n)H(2n-2)
================================================
Structural--------------Common------------IUPAC
Formula:----------------name--------------name
------------------------------------------------
C two H two ---------Acetylene------------------Ethyne
C three H four------ Methyl acetylene-----------Propyne
C four H six *------ ethyl acetylene------------1-Butyne
--------------------------------------(Triple bond is the
--------------------------------------first carbon bond)
C four H six*--------Dimethyl acetylene---2-Butyne
---------------------------------------(Triple bond is the
----------------------------------------2nd carbon bond)
=========================================================
Preparation of Ethyne:
1. Calcium Carbide + Water = Ethyne + Calcium Hydroxide
(Cold water is added dropwise through a thistle funnel into conical flask containing calcium carbide at room temperature)
2. By dehydrohalogenation reaction
1,2 dibromoethane + KOH(alcoholic) = Ethyne + KBR + water (equation not balanced)
3. Methane on thermal decomposition by pyrolysis gives Ethyne + Hydrogen at around 1500 degree Celsius in an electric arc.
-------------------
Methods of Preparation of Alkynes; From TMH Book
1. Dehydrohalogenation of vicinal dihalides (came in X class book).
What are vicinal dihalides?
term used to describe the location of two identical chemical groups or atoms which are bonded to adjacent carbon atoms.
The process takes place in two stages.
In stage I KOH(alc) reacts with vicinal dihalide and vinylic halide is formed.
what is vinylic halide?
A vinyl halide in chemistry is any alkene with at least one halide substituent bonded directly on one of the unsaturated carbons.
This first stage requires mild conditions.
second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH two is used in the reaction to produce the corresponding alkyne.
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
Secondary and tertiary halides cannot be used because eliminatin is the predominant reaction which results in the formation of alkenes.
3. Hydrolysis of CaC two and Mg two C three.
Calcium carbide in reaction with water gives ethyne and calcium hydroxide.(came in X class)
Magnesium carbide in reaction water gives propyne and magnesium hydroxide.
4. Kolbe's Electrolytic Method:
The electrolysis of an acqueous solution of potassium salt of an unsaturated dicarboxylic acid forms alkyne
what is a dicarboxylic acid?
Dicarboxylic acids are organic compounds that are substituted with two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R is usually an alkyl, alkenyl, or akynyl group. Dicarboxylic acids can be used to prepare copolymers such as nylon and polyethylene terephthalate.
In general, dicarboxylic acids show the same chemical behaviour and reactivity as monocarboxylic acids. The ionization of the second carboxyl group occurs less readily than the first one. This is because more energy is required to separate a positive hydrogen ion from the doubly charged anion than from the single charged anion.
A mnemonic to aid in remembering the order of the common nomenclature for the first six dicarboxylic acids is "Oh my, such great apple pie!" (oxalic, malonic, succinic, glutaric, adipic, pimelic).
When one of the carboxy groups is replaced with an aldehyde group, the resulting structure is called a "aldehydic acid".
5. Dehalogenation of vic-tetrahalogen compounds
Tetrahalogenated alkane in reaction with zinc with ethyl alcohol as catalyst gives alkyne and zinc halide.
-------------------
From the Book by Madan and Bisht: preparation of Acetylene or Ethyne
1. By the action of Alcoholic Potash on Ethylene Bromide
BrH two C - CH two Br + 2KOH = C Two H Two + @KBr + 2 H two O
At the end of first state vinyl bromide is formed.
2. By heating tetra-bromoethane with Zinc
C-two H-two Br-four +2Zn = C-two H-two + 2ZnBr-two
3. By the electrolysis of Acqueous solution of Potassium salts of maleic acid (Kolbe's method)
4. By the action of water on calcium carbide (already covered earlier)
5. By heating Iodoform with silver powder
2HCI-three + 6Ag = C-twoH-two + 6AgI
6. By partial oxidation of Methane
2CH-four + 3[O] = C-twoHtwo + 3H-two O
7. By direct synthesis of carbon and hydrogen (Berthelot's synthesis)
2C + H-two = C-twoH-two
------------------------------------------------------------------
From Jauhar's book
1. Action of water on calcium carbide
2. Dehydrohalogenation of vicinal dihalides
3. Action of zinc on tetrahalogen derivatives of alkanes
4. From acetylene
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
6. Synthesis of carbon and hydrogen
Industrial preparation
Ethyne is prepared on an industrial scale by treating calcium carbide with water.
-------------------------
Physical properties
State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
M.P. and B.P.
-------------------
Simple questions on methods of preparing alkynes are given in practice questions for review of concepts of this topic.
http://iit-jee-chemistry-ps.blogspot.com/
-------------------
Preparation, properties and reactions of alkynes:
Physical properties of alkynes (boiling points, density and dipole moments);
Acidity of alkynes;
Acid catalysed hydration of alkynes (excluding the stereochemistry of addition and elimination);
Reduction of alkynes;
Preparation of alkynes by elimination reactions;
Addition reactions of alkynes;
Metal acetylides.
----------------------
Main Topics Covered in the TMH Book
METHODS OF PREPARATION
CHEMICAL PROPERTIES
--------------
From X class book
General formula: C(n)H(2n-2)
================================================
Structural--------------Common------------IUPAC
Formula:----------------name--------------name
------------------------------------------------
C two H two ---------Acetylene------------------Ethyne
C three H four------ Methyl acetylene-----------Propyne
C four H six *------ ethyl acetylene------------1-Butyne
--------------------------------------(Triple bond is the
--------------------------------------first carbon bond)
C four H six*--------Dimethyl acetylene---2-Butyne
---------------------------------------(Triple bond is the
----------------------------------------2nd carbon bond)
=========================================================
Preparation of Ethyne:
1. Calcium Carbide + Water = Ethyne + Calcium Hydroxide
(Cold water is added dropwise through a thistle funnel into conical flask containing calcium carbide at room temperature)
2. By dehydrohalogenation reaction
1,2 dibromoethane + KOH(alcoholic) = Ethyne + KBR + water (equation not balanced)
3. Methane on thermal decomposition by pyrolysis gives Ethyne + Hydrogen at around 1500 degree Celsius in an electric arc.
-------------------
Methods of Preparation of Alkynes; From TMH Book
1. Dehydrohalogenation of vicinal dihalides (came in X class book).
What are vicinal dihalides?
term used to describe the location of two identical chemical groups or atoms which are bonded to adjacent carbon atoms.
The process takes place in two stages.
In stage I KOH(alc) reacts with vicinal dihalide and vinylic halide is formed.
what is vinylic halide?
A vinyl halide in chemistry is any alkene with at least one halide substituent bonded directly on one of the unsaturated carbons.
This first stage requires mild conditions.
second stage: Vinyl halide is unreactive and hence vigorous conditions are required and a strong base NaNH two is used in the reaction to produce the corresponding alkyne.
2. Reaction of metal acetalides with primary alkyl halides. This method can be used to generate large alkyne from the smaller one.
Secondary and tertiary halides cannot be used because eliminatin is the predominant reaction which results in the formation of alkenes.
3. Hydrolysis of CaC two and Mg two C three.
Calcium carbide in reaction with water gives ethyne and calcium hydroxide.(came in X class)
Magnesium carbide in reaction water gives propyne and magnesium hydroxide.
4. Kolbe's Electrolytic Method:
The electrolysis of an acqueous solution of potassium salt of an unsaturated dicarboxylic acid forms alkyne
what is a dicarboxylic acid?
Dicarboxylic acids are organic compounds that are substituted with two carboxylic acid functional groups. In molecular formulae for dicarboxylic acids, these groups are often written as HOOC-R-COOH, where R is usually an alkyl, alkenyl, or akynyl group. Dicarboxylic acids can be used to prepare copolymers such as nylon and polyethylene terephthalate.
In general, dicarboxylic acids show the same chemical behaviour and reactivity as monocarboxylic acids. The ionization of the second carboxyl group occurs less readily than the first one. This is because more energy is required to separate a positive hydrogen ion from the doubly charged anion than from the single charged anion.
A mnemonic to aid in remembering the order of the common nomenclature for the first six dicarboxylic acids is "Oh my, such great apple pie!" (oxalic, malonic, succinic, glutaric, adipic, pimelic).
When one of the carboxy groups is replaced with an aldehyde group, the resulting structure is called a "aldehydic acid".
5. Dehalogenation of vic-tetrahalogen compounds
Tetrahalogenated alkane in reaction with zinc with ethyl alcohol as catalyst gives alkyne and zinc halide.
-------------------
From the Book by Madan and Bisht: preparation of Acetylene or Ethyne
1. By the action of Alcoholic Potash on Ethylene Bromide
BrH two C - CH two Br + 2KOH = C Two H Two + @KBr + 2 H two O
At the end of first state vinyl bromide is formed.
2. By heating tetra-bromoethane with Zinc
C-two H-two Br-four +2Zn = C-two H-two + 2ZnBr-two
3. By the electrolysis of Acqueous solution of Potassium salts of maleic acid (Kolbe's method)
4. By the action of water on calcium carbide (already covered earlier)
5. By heating Iodoform with silver powder
2HCI-three + 6Ag = C-twoH-two + 6AgI
6. By partial oxidation of Methane
2CH-four + 3[O] = C-twoHtwo + 3H-two O
7. By direct synthesis of carbon and hydrogen (Berthelot's synthesis)
2C + H-two = C-twoH-two
------------------------------------------------------------------
From Jauhar's book
1. Action of water on calcium carbide
2. Dehydrohalogenation of vicinal dihalides
3. Action of zinc on tetrahalogen derivatives of alkanes
4. From acetylene
5. By electrolysis of aqueous solution of potassium salt of fumaric acid
6. Synthesis of carbon and hydrogen
Industrial preparation
Ethyne is prepared on an industrial scale by treating calcium carbide with water.
-------------------------
Physical properties
State; first three members are gases at room temperature. thenext eight are liquids while the higher ones are solids.
Solubulity: are mostly insoluble in water nbut are soluble in organic solvents such as petroleum, ether, carbon tetrachlorde. benzene etc.
M.P. and B.P.
-------------------
Simple questions on methods of preparing alkynes are given in practice questions for review of concepts of this topic.
http://iit-jee-chemistry-ps.blogspot.com/
-------------------
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