Methane Swaps Its Hydrogen Atoms, While Ethene Opens Up Its Double Bond
Prepare methane, ethane, ethene and ethyne with balanced equations, compare complete and incomplete combustion, follow substitution of alkanes by chlorine, see why methane is a greenhouse gas, and write the addition reactions of ethene and ethyne and the uses of all four hydrocarbons.
Why do alkanes, alkenes and alkynes react so differently?
Bubble ethene through reddish-brown bromine solution and the colour vanishes almost at once. Bubble methane through the same solution in the dark and nothing happens at all. Both gases are colourless hydrocarbons, yet one reacts instantly and the other ignores the bromine.
The difference is the double bond.
- Methane and ethane are saturated — every carbon already has four single bonds. To react, they must lose a hydrogen atom and take something else in its place. That is a substitution reaction, and it usually needs energy such as sunlight
- Ethene and ethyne are unsaturated — their double and triple bonds can open up and take on new atoms without losing any. That is an addition reaction, and it happens readily
That single contrast organises this whole part.
- Preparing the four hydrocarbons — methane, ethane, ethene and ethyne
- Reactions of the saturated ones — combustion, and substitution by chlorine
- Why methane matters for the climate — it is a greenhouse gas
- Reactions of the unsaturated ones — addition of hydrogen and halogens
- Uses of all four
These gases are part of everyday life in India.
- Natural gas — compressed for autorickshaws and buses, and piped to many city kitchens — is mainly methane
- Biogas from cattle dung, used for cooking in many villages, is also largely methane
- Ethene is used to ripen fruit evenly in storage chambers
- Ethyne, or acetylene, burns in the flame of welding torches in metal workshops
The link to the previous parts. Part 1 drew these molecules and Part 2 named them. Now the structure tells you how each will react: count the bonds between the carbon atoms, and you can predict whether a reaction will be substitution or addition before writing a single equation.
This page covers the third part of the ICSE Class 10 Chemistry chapter on organic chemistry: preparation of methane, ethane, ethene and ethyne; combustion and substitution; methane as a greenhouse gas; addition reactions; and uses.
The difference is the double bond.
- Methane and ethane are saturated — every carbon already has four single bonds. To react, they must lose a hydrogen atom and take something else in its place. That is a substitution reaction, and it usually needs energy such as sunlight
- Ethene and ethyne are unsaturated — their double and triple bonds can open up and take on new atoms without losing any. That is an addition reaction, and it happens readily
That single contrast organises this whole part.
- Preparing the four hydrocarbons — methane, ethane, ethene and ethyne
- Reactions of the saturated ones — combustion, and substitution by chlorine
- Why methane matters for the climate — it is a greenhouse gas
- Reactions of the unsaturated ones — addition of hydrogen and halogens
- Uses of all four
These gases are part of everyday life in India.
- Natural gas — compressed for autorickshaws and buses, and piped to many city kitchens — is mainly methane
- Biogas from cattle dung, used for cooking in many villages, is also largely methane
- Ethene is used to ripen fruit evenly in storage chambers
- Ethyne, or acetylene, burns in the flame of welding torches in metal workshops
The link to the previous parts. Part 1 drew these molecules and Part 2 named them. Now the structure tells you how each will react: count the bonds between the carbon atoms, and you can predict whether a reaction will be substitution or addition before writing a single equation.
This page covers the third part of the ICSE Class 10 Chemistry chapter on organic chemistry: preparation of methane, ethane, ethene and ethyne; combustion and substitution; methane as a greenhouse gas; addition reactions; and uses.
How are methane and ethane prepared from sodium salts and alkyl halides?
Heating sodium ethanoate or sodium propanoate with soda lime removes carbon dioxide and gives methane or ethane, and reducing iodomethane or bromoethane with nascent hydrogen also gives methane or ethane.
1. From sodium salts — heating with soda lime. Soda lime is a mixture of sodium hydroxide and calcium oxide. The sodium hydroxide reacts; the calcium oxide keeps it dry and stops it attacking the glass.
Methane from sodium ethanoate (sodium acetate):
Ethane from sodium propanoate:
The pattern: the group is removed as sodium carbonate, and the product has one carbon atom fewer than the salt.
Collection: methane and ethane are collected over water, because they are almost insoluble in it.
2. From alkyl halides — reduction with nascent hydrogen. The halide is warmed with a zinc–copper couple and alcohol, which supplies nascent hydrogen, .
Methane from iodomethane:
Ethane from bromoethane:
Here the carbon count stays the same: the halogen atom is simply replaced by hydrogen.
A further route to ethane. Two molecules of iodomethane heated with sodium in dry ether join into one:
This time the carbon count doubles.
Worked check — balancing the sodium propanoate equation.
- Carbon: on the left; on the right
- Hydrogen: on the left; on the right
- Oxygen: on the left; on the right
- Sodium: on the left; on the right
Balanced.
Worked example — volume of methane. What volume of methane at STP is obtained from of sodium ethanoate? C , H , O , Na .
An everyday example. Methane forms naturally wherever plant matter decays without air — in marshes, paddy fields and biogas plants. That is why methane is also called marsh gas, and why a biogas plant fed with cattle dung produces a usable cooking fuel.
The boundary case — predicting the product from the salt. Heating with soda lime always removes one carbon. Sodium propanoate gives ethane, not propane, and sodium ethanoate gives methane, not ethane — a detail that decides the answer in every question of this type.
1. From sodium salts — heating with soda lime. Soda lime is a mixture of sodium hydroxide and calcium oxide. The sodium hydroxide reacts; the calcium oxide keeps it dry and stops it attacking the glass.
Methane from sodium ethanoate (sodium acetate):
Ethane from sodium propanoate:
The pattern: the group is removed as sodium carbonate, and the product has one carbon atom fewer than the salt.
Collection: methane and ethane are collected over water, because they are almost insoluble in it.
2. From alkyl halides — reduction with nascent hydrogen. The halide is warmed with a zinc–copper couple and alcohol, which supplies nascent hydrogen, .
Methane from iodomethane:
Ethane from bromoethane:
Here the carbon count stays the same: the halogen atom is simply replaced by hydrogen.
A further route to ethane. Two molecules of iodomethane heated with sodium in dry ether join into one:
This time the carbon count doubles.
Worked check — balancing the sodium propanoate equation.
- Carbon: on the left; on the right
- Hydrogen: on the left; on the right
- Oxygen: on the left; on the right
- Sodium: on the left; on the right
Balanced.
Worked example — volume of methane. What volume of methane at STP is obtained from of sodium ethanoate? C , H , O , Na .
An everyday example. Methane forms naturally wherever plant matter decays without air — in marshes, paddy fields and biogas plants. That is why methane is also called marsh gas, and why a biogas plant fed with cattle dung produces a usable cooking fuel.
The boundary case — predicting the product from the salt. Heating with soda lime always removes one carbon. Sodium propanoate gives ethane, not propane, and sodium ethanoate gives methane, not ethane — a detail that decides the answer in every question of this type.
How do methane and ethane burn and react with chlorine, and why is methane a greenhouse gas?
In plenty of air, methane and ethane burn completely to carbon dioxide and water with a blue flame; in limited air they give poisonous carbon monoxide or soot; with chlorine in diffused sunlight their hydrogen atoms are replaced one by one; and methane traps heat in the atmosphere.
1. Complete combustion — plenty of air.
A clean, blue, non-luminous flame and a large amount of heat — which is why these gases are good fuels.
2. Incomplete combustion — limited air.
Carbon monoxide is a colourless, odourless, poisonous gas; carbon appears as black soot, giving a yellow, sooty flame.
3. Substitution with chlorine — in diffused sunlight. Hydrogen atoms are replaced by chlorine one at a time, each step releasing hydrogen chloride:
Ethane reacts the same way, the first step giving chloroethane:
In direct bright sunlight the reaction of methane with chlorine can be explosive, giving carbon:
4. Methane as a greenhouse gas. Methane in the atmosphere absorbs heat radiated from the earth's surface and keeps it from escaping to space, so it contributes to global warming.
- Molecule for molecule, methane traps heat more strongly than carbon dioxide
- Sources include flooded paddy fields, cattle and other grazing animals, landfills, decaying organic matter and leaks from natural gas supplies
Worked example — air for complete combustion. What volume of oxygen is needed to burn of ethane completely, and what volume of carbon dioxide forms, under the same conditions?
Worked check — balancing the incomplete ethane equation. Carbon ; hydrogen ; oxygen on the left and on the right. Balanced.
An everyday safety example. A gas water heater or a charcoal stove used in a closed bathroom or room can produce carbon monoxide when the air supply runs short. Because the gas has no smell, good ventilation is essential — the incomplete combustion equations above are the reason.
The boundary case. Substitution replaces; it never adds. The number of atoms attached to each carbon stays at four throughout, so the carbon skeleton is unchanged — the defining difference from the addition reactions of alkenes.
1. Complete combustion — plenty of air.
A clean, blue, non-luminous flame and a large amount of heat — which is why these gases are good fuels.
2. Incomplete combustion — limited air.
Carbon monoxide is a colourless, odourless, poisonous gas; carbon appears as black soot, giving a yellow, sooty flame.
3. Substitution with chlorine — in diffused sunlight. Hydrogen atoms are replaced by chlorine one at a time, each step releasing hydrogen chloride:
Ethane reacts the same way, the first step giving chloroethane:
In direct bright sunlight the reaction of methane with chlorine can be explosive, giving carbon:
4. Methane as a greenhouse gas. Methane in the atmosphere absorbs heat radiated from the earth's surface and keeps it from escaping to space, so it contributes to global warming.
- Molecule for molecule, methane traps heat more strongly than carbon dioxide
- Sources include flooded paddy fields, cattle and other grazing animals, landfills, decaying organic matter and leaks from natural gas supplies
Worked example — air for complete combustion. What volume of oxygen is needed to burn of ethane completely, and what volume of carbon dioxide forms, under the same conditions?
Worked check — balancing the incomplete ethane equation. Carbon ; hydrogen ; oxygen on the left and on the right. Balanced.
An everyday safety example. A gas water heater or a charcoal stove used in a closed bathroom or room can produce carbon monoxide when the air supply runs short. Because the gas has no smell, good ventilation is essential — the incomplete combustion equations above are the reason.
The boundary case. Substitution replaces; it never adds. The number of atoms attached to each carbon stays at four throughout, so the carbon skeleton is unchanged — the defining difference from the addition reactions of alkenes.
How are ethene and ethyne prepared?
Ethene is made by removing hydrogen bromide from bromoethane with alcoholic potassium hydroxide or by removing water from ethanol with concentrated sulphuric acid; ethyne is made from calcium carbide and water or by removing two molecules of hydrogen bromide from 1,2-dibromoethane.
1. Ethene by dehydrohalogenation. Bromoethane is heated with alcoholic potassium hydroxide. A hydrogen atom and a bromine atom are removed from neighbouring carbons, forming a double bond:
2. Ethene by dehydration. Ethanol is heated with excess concentrated sulphuric acid at about . The acid removes the elements of water:
Ethene is collected over water, being nearly insoluble.
3. Ethyne from calcium carbide. Water is dripped onto lumps of calcium carbide:
The gas is collected over water. Commercial calcium carbide contains impurities, so the ethyne formed has a garlic-like smell unless purified.
4. Ethyne from 1,2-dibromoethane. Heating 1,2-dibromoethane with excess alcoholic potassium hydroxide removes two molecules of hydrogen bromide:
Worked check — balancing the dibromoethane equation.
- Carbon:
- Hydrogen: on the left; on the right
- Bromine: ; potassium: ; oxygen:
Balanced.
Worked example — ethyne from carbide. What volume of ethyne at STP forms from of pure calcium carbide? Ca , C .
How removing atoms creates multiple bonds. Taking one hydrogen and one bromine from neighbouring carbons frees one bonding position on each, and those two positions join to form a second bond between the carbons. Removing two such pairs forms a third bond. Dehydrohalogenation and dehydration are the reverse of addition.
An everyday example — and a warning. Calcium carbide releases ethyne when it meets moisture, and it has been misused to ripen mangoes and bananas quickly. This practice is prohibited in India, because industrial calcium carbide carries harmful impurities such as compounds of arsenic and phosphorus. Safe ripening chambers use controlled ethene gas instead.
The boundary case — alcoholic versus aqueous potassium hydroxide. Alcoholic KOH removes hydrogen bromide from bromoethane to give ethene. Aqueous KOH replaces the bromine with to give ethanol — the subject of Part 4. The solvent decides the product.
1. Ethene by dehydrohalogenation. Bromoethane is heated with alcoholic potassium hydroxide. A hydrogen atom and a bromine atom are removed from neighbouring carbons, forming a double bond:
2. Ethene by dehydration. Ethanol is heated with excess concentrated sulphuric acid at about . The acid removes the elements of water:
Ethene is collected over water, being nearly insoluble.
3. Ethyne from calcium carbide. Water is dripped onto lumps of calcium carbide:
The gas is collected over water. Commercial calcium carbide contains impurities, so the ethyne formed has a garlic-like smell unless purified.
4. Ethyne from 1,2-dibromoethane. Heating 1,2-dibromoethane with excess alcoholic potassium hydroxide removes two molecules of hydrogen bromide:
Worked check — balancing the dibromoethane equation.
- Carbon:
- Hydrogen: on the left; on the right
- Bromine: ; potassium: ; oxygen:
Balanced.
Worked example — ethyne from carbide. What volume of ethyne at STP forms from of pure calcium carbide? Ca , C .
How removing atoms creates multiple bonds. Taking one hydrogen and one bromine from neighbouring carbons frees one bonding position on each, and those two positions join to form a second bond between the carbons. Removing two such pairs forms a third bond. Dehydrohalogenation and dehydration are the reverse of addition.
An everyday example — and a warning. Calcium carbide releases ethyne when it meets moisture, and it has been misused to ripen mangoes and bananas quickly. This practice is prohibited in India, because industrial calcium carbide carries harmful impurities such as compounds of arsenic and phosphorus. Safe ripening chambers use controlled ethene gas instead.
The boundary case — alcoholic versus aqueous potassium hydroxide. Alcoholic KOH removes hydrogen bromide from bromoethane to give ethene. Aqueous KOH replaces the bromine with to give ethanol — the subject of Part 4. The solvent decides the product.
What are the addition reactions of ethene and ethyne, and what are these hydrocarbons used for?
Ethene adds one molecule of hydrogen or halogen across its double bond and ethyne adds two across its triple bond, and the decolourisation of bromine is the test for unsaturation; methane and ethane are fuels, ethene makes polythene and ripens fruit, and ethyne is used in welding.
1. Addition of hydrogen — with a nickel catalyst on heating.
2. Addition of halogens to ethene.
3. Addition of halogens to ethyne — in two steps.
Chlorine adds in the same two steps, and iodine adds to give 1,2-diiodoethene:
4. Test for unsaturation. Reddish-brown bromine solution is decolourised by ethene and ethyne, because the bromine adds across the multiple bond. Methane and ethane do not decolourise it in the dark, because they cannot add. Pink alkaline potassium permanganate is also decolourised by unsaturated hydrocarbons.
Worked example — hydrogen and bromine for ethyne. of ethyne is fully saturated. What volume of hydrogen at STP, or what mass of bromine, does it take? Br .
Ethene would take only half as much, because a double bond adds one molecule and a triple bond adds two.
5. Uses.
- Methane: fuel as natural gas, compressed natural gas and biogas; making hydrogen, carbon black and chemicals such as methanol and chloroform
- Ethane: fuel; making ethene
- Ethene: making polythene; ripening fruits in controlled chambers; making ethanol and other chemicals
- Ethyne: the oxy-acetylene flame for welding and cutting metals; making plastics such as PVC and synthetic rubber
An everyday example. The polythene in packaging film and carry bags is made by joining huge numbers of ethene molecules together — an addition reaction repeated over and over, described in the fun fact below.
The boundary case — why ethane gives no addition product. Adding hydrogen to ethane is impossible: every carbon already has four bonds, so there is no multiple bond to open. Saturated compounds substitute; unsaturated compounds add — the rule that separates the two halves of this lesson.
1. Addition of hydrogen — with a nickel catalyst on heating.
2. Addition of halogens to ethene.
3. Addition of halogens to ethyne — in two steps.
Chlorine adds in the same two steps, and iodine adds to give 1,2-diiodoethene:
4. Test for unsaturation. Reddish-brown bromine solution is decolourised by ethene and ethyne, because the bromine adds across the multiple bond. Methane and ethane do not decolourise it in the dark, because they cannot add. Pink alkaline potassium permanganate is also decolourised by unsaturated hydrocarbons.
Worked example — hydrogen and bromine for ethyne. of ethyne is fully saturated. What volume of hydrogen at STP, or what mass of bromine, does it take? Br .
Ethene would take only half as much, because a double bond adds one molecule and a triple bond adds two.
5. Uses.
- Methane: fuel as natural gas, compressed natural gas and biogas; making hydrogen, carbon black and chemicals such as methanol and chloroform
- Ethane: fuel; making ethene
- Ethene: making polythene; ripening fruits in controlled chambers; making ethanol and other chemicals
- Ethyne: the oxy-acetylene flame for welding and cutting metals; making plastics such as PVC and synthetic rubber
An everyday example. The polythene in packaging film and carry bags is made by joining huge numbers of ethene molecules together — an addition reaction repeated over and over, described in the fun fact below.
The boundary case — why ethane gives no addition product. Adding hydrogen to ethane is impossible: every carbon already has four bonds, so there is no multiple bond to open. Saturated compounds substitute; unsaturated compounds add — the rule that separates the two halves of this lesson.
Exam tip
What earns full marks on hydrocarbon reactions?
Name the type of reaction, give the conditions, write the structural change, and name every product.
- Say soda lime and heat for methane and ethane from sodium salts, and note one carbon fewer in the product
- Say alcoholic KOH for dehydrohalogenation and concentrated sulphuric acid at about 170 °C for dehydration
- Write calcium carbide and water for ethyne, and excess alcoholic KOH with 1,2-dibromoethane
- Distinguish complete and incomplete combustion by the air supply and the products — CO2 or CO and soot
- Say diffused sunlight for substitution, and write all four chlorination steps of methane with names
- Explain methane as a greenhouse gas and name at least two sources
- Write addition products with structures and names: 1,2-dibromoethane, 1,1,2,2-tetrabromoethane
- Mention nickel catalyst for adding hydrogen
- Give the bromine test and say methane does not decolourise it
- List uses for each of the four hydrocarbons
The misconception to name. Methane does not react with bromine by addition. Methane has no multiple bond, so it can only substitute, and only with energy such as light. **Writing gives carbon six bonds and is always wrong.
A second trap. Writing propane as the product of sodium propanoate with soda lime. The reaction removes the carbon**, so the product is ethane.
- Say soda lime and heat for methane and ethane from sodium salts, and note one carbon fewer in the product
- Say alcoholic KOH for dehydrohalogenation and concentrated sulphuric acid at about 170 °C for dehydration
- Write calcium carbide and water for ethyne, and excess alcoholic KOH with 1,2-dibromoethane
- Distinguish complete and incomplete combustion by the air supply and the products — CO2 or CO and soot
- Say diffused sunlight for substitution, and write all four chlorination steps of methane with names
- Explain methane as a greenhouse gas and name at least two sources
- Write addition products with structures and names: 1,2-dibromoethane, 1,1,2,2-tetrabromoethane
- Mention nickel catalyst for adding hydrogen
- Give the bromine test and say methane does not decolourise it
- List uses for each of the four hydrocarbons
The misconception to name. Methane does not react with bromine by addition. Methane has no multiple bond, so it can only substitute, and only with energy such as light. **Writing gives carbon six bonds and is always wrong.
A second trap. Writing propane as the product of sodium propanoate with soda lime. The reaction removes the carbon**, so the product is ethane.
Did you know
How does a gas made of tiny ethene molecules become a strong polythene bag?
Ethene is a gas of very small molecules, each just two carbon atoms and four hydrogen atoms. Polythene is a tough, flexible solid used for packaging film, bottles and pipes. The solid is made entirely from the gas, using the addition reaction of this lesson again and again.
Under suitable conditions of heat, pressure and a catalyst, the double bond in each ethene molecule opens up. Instead of adding hydrogen or bromine, each opened molecule adds to the next ethene molecule, which opens and adds to the next, and so on:
**Here is a very large number — thousands of ethene units in a single chain. The small molecules are called monomers; the giant molecule they form is a polymer, and this kind of reaction is addition polymerisation.
Why the product is so different from the gas. A few ethene molecules attract one another only weakly, so they fly apart as a gas. Chains thousands of units long tangle together and attract along their whole length, so polythene is a solid that can be stretched into film without falling apart.
Notice that nothing is lost in the reaction. Every atom of every ethene molecule ends up in the polymer; addition only rearranges bonds, it never gives off a by-product.** The empirical formula of polythene, , is the same as that of ethene.
The same idea builds many familiar materials. Replace one hydrogen atom in ethene with chlorine and polymerise, and the product is PVC, used for water pipes. Other substituted ethenes give the plastics in many everyday objects.
And it explains a problem too. The strong carbon–carbon chains that make polythene durable also make it very slow to break down in the environment, which is why discarded plastic bags persist for so long — a consequence of the same addition chemistry that makes them useful.
Under suitable conditions of heat, pressure and a catalyst, the double bond in each ethene molecule opens up. Instead of adding hydrogen or bromine, each opened molecule adds to the next ethene molecule, which opens and adds to the next, and so on:
**Here is a very large number — thousands of ethene units in a single chain. The small molecules are called monomers; the giant molecule they form is a polymer, and this kind of reaction is addition polymerisation.
Why the product is so different from the gas. A few ethene molecules attract one another only weakly, so they fly apart as a gas. Chains thousands of units long tangle together and attract along their whole length, so polythene is a solid that can be stretched into film without falling apart.
Notice that nothing is lost in the reaction. Every atom of every ethene molecule ends up in the polymer; addition only rearranges bonds, it never gives off a by-product.** The empirical formula of polythene, , is the same as that of ethene.
The same idea builds many familiar materials. Replace one hydrogen atom in ethene with chlorine and polymerise, and the product is PVC, used for water pipes. Other substituted ethenes give the plastics in many everyday objects.
And it explains a problem too. The strong carbon–carbon chains that make polythene durable also make it very slow to break down in the environment, which is why discarded plastic bags persist for so long — a consequence of the same addition chemistry that makes them useful.
Exam relevance
How do hydrocarbon preparations and reactions appear in JEE and NEET?
This is foundation work for Class 11 Hydrocarbons and Class 12 Haloalkanes and Haloarenes, both examined in JEE Main and NEET Chemistry.
Where the preparations lead. Class 11 Hydrocarbons covers the preparation of alkanes by decarboxylation of sodium salts with soda lime, by reduction of alkyl halides, and by the coupling of alkyl halides with sodium in dry ether. Predicting the product — including the loss of one carbon in decarboxylation and the doubling in coupling — is a recurring question in both exams.
Where substitution leads. The chlorination of methane is used in Class 11 to introduce the free radical mechanism, with initiation, propagation and termination steps. The need for light on this page is the initiation step, and questions on the mechanism and on the mixture of products are standard.
Where the elimination reactions lead. Dehydrohalogenation with alcoholic KOH and dehydration of alcohols reappear in Class 12 Haloalkanes and Haloarenes and Alcohols, Phenols and Ethers. The contrast between aqueous KOH giving substitution and alcoholic KOH giving elimination is a favourite assertion-reason item.
Where addition leads. Class 11 extends the addition of halogens and hydrogen to the addition of hydrogen halides, governed by Markovnikov's rule, and to the ozonolysis of alkenes. The test for unsaturation with bromine is also examined directly.
Where combustion leads. Balancing combustion equations and calculating volumes of oxygen and carbon dioxide are Class 11 Some Basic Concepts numericals, and the general combustion equation for a hydrocarbon is a standard formula.
Where polythene leads. Addition polymers such as polythene and PVC are the starting examples of polymerisation in organic chemistry.
Question types to expect. At this level: preparations with conditions, combustion and substitution equations, addition products and uses. In competitive papers: product prediction from reagents, reaction mechanisms, Markovnikov addition, elimination versus substitution, and combustion stoichiometry.
The single trap that costs marks. Confusing aqueous and alcoholic potassium hydroxide. Aqueous KOH with bromoethane gives ethanol; alcoholic KOH gives ethene, and options are built on exactly that swap.
A second trap. Getting the carbon count wrong in decarboxylation. Sodium propanoate gives ethane, and the same one-carbon loss applies to every sodium salt heated with soda lime.
Board versus competitive emphasis. The ICSE paper marks named preparations with conditions, balanced equations and uses; a competitive paper marks a predicted product, a mechanism step or a rule. The transferable habit is deciding first whether a molecule is saturated or unsaturated — because that single check predicts substitution or addition every time.
Where the preparations lead. Class 11 Hydrocarbons covers the preparation of alkanes by decarboxylation of sodium salts with soda lime, by reduction of alkyl halides, and by the coupling of alkyl halides with sodium in dry ether. Predicting the product — including the loss of one carbon in decarboxylation and the doubling in coupling — is a recurring question in both exams.
Where substitution leads. The chlorination of methane is used in Class 11 to introduce the free radical mechanism, with initiation, propagation and termination steps. The need for light on this page is the initiation step, and questions on the mechanism and on the mixture of products are standard.
Where the elimination reactions lead. Dehydrohalogenation with alcoholic KOH and dehydration of alcohols reappear in Class 12 Haloalkanes and Haloarenes and Alcohols, Phenols and Ethers. The contrast between aqueous KOH giving substitution and alcoholic KOH giving elimination is a favourite assertion-reason item.
Where addition leads. Class 11 extends the addition of halogens and hydrogen to the addition of hydrogen halides, governed by Markovnikov's rule, and to the ozonolysis of alkenes. The test for unsaturation with bromine is also examined directly.
Where combustion leads. Balancing combustion equations and calculating volumes of oxygen and carbon dioxide are Class 11 Some Basic Concepts numericals, and the general combustion equation for a hydrocarbon is a standard formula.
Where polythene leads. Addition polymers such as polythene and PVC are the starting examples of polymerisation in organic chemistry.
Question types to expect. At this level: preparations with conditions, combustion and substitution equations, addition products and uses. In competitive papers: product prediction from reagents, reaction mechanisms, Markovnikov addition, elimination versus substitution, and combustion stoichiometry.
The single trap that costs marks. Confusing aqueous and alcoholic potassium hydroxide. Aqueous KOH with bromoethane gives ethanol; alcoholic KOH gives ethene, and options are built on exactly that swap.
A second trap. Getting the carbon count wrong in decarboxylation. Sodium propanoate gives ethane, and the same one-carbon loss applies to every sodium salt heated with soda lime.
Board versus competitive emphasis. The ICSE paper marks named preparations with conditions, balanced equations and uses; a competitive paper marks a predicted product, a mechanism step or a rule. The transferable habit is deciding first whether a molecule is saturated or unsaturated — because that single check predicts substitution or addition every time.
Key takeaways
What must you be able to do from this part?
Six preparations, two kinds of combustion, one substitution sequence, a greenhouse gas and a set of addition reactions.
- Methane: with soda lime;
- Ethane: ;
- Soda lime removes one carbon; of sodium ethanoate gives of methane
- Complete combustion: , blue flame
- Incomplete combustion: CO or soot in limited air; carbon monoxide is poisonous and odourless
- Substitution in diffused sunlight: , each step releasing HCl
- Methane is a greenhouse gas, from paddy fields, cattle, landfills and gas leaks
- Ethene: bromoethane with alcoholic KOH; ethanol with concentrated sulphuric acid at about
- Ethyne: ; 1,2-dibromoethane with excess alcoholic KOH
- Alcoholic KOH gives ethene; aqueous KOH gives ethanol
- Addition to ethene: gives ethane; , , give 1,2-dihaloethanes
- Addition to ethyne: two molecules — ethyne to ethene to ethane; bromine gives 1,2-dibromoethene then 1,1,2,2-tetrabromoethane
- ** of ethyne** takes of hydrogen or of bromine
- Bromine decolourisation tests for unsaturation
- Uses: methane and ethane as fuels; ethene for polythene and fruit ripening; ethyne for welding
The sharpest self-test is four gas jars. Imagine methane, ethane, ethene and ethyne in unlabelled jars, and plan the tests and reactions that would identify each — then write one equation for every jar that proves your choice.
- Methane: with soda lime;
- Ethane: ;
- Soda lime removes one carbon; of sodium ethanoate gives of methane
- Complete combustion: , blue flame
- Incomplete combustion: CO or soot in limited air; carbon monoxide is poisonous and odourless
- Substitution in diffused sunlight: , each step releasing HCl
- Methane is a greenhouse gas, from paddy fields, cattle, landfills and gas leaks
- Ethene: bromoethane with alcoholic KOH; ethanol with concentrated sulphuric acid at about
- Ethyne: ; 1,2-dibromoethane with excess alcoholic KOH
- Alcoholic KOH gives ethene; aqueous KOH gives ethanol
- Addition to ethene: gives ethane; , , give 1,2-dihaloethanes
- Addition to ethyne: two molecules — ethyne to ethene to ethane; bromine gives 1,2-dibromoethene then 1,1,2,2-tetrabromoethane
- ** of ethyne** takes of hydrogen or of bromine
- Bromine decolourisation tests for unsaturation
- Uses: methane and ethane as fuels; ethene for polythene and fruit ripening; ethyne for welding
The sharpest self-test is four gas jars. Imagine methane, ethane, ethene and ethyne in unlabelled jars, and plan the tests and reactions that would identify each — then write one equation for every jar that proves your choice.