A student once asks a perfectly sensible question: “If chlorine reacts with ethane, why doesn’t it just add across a bond the way bromine adds to ethene?” Both compounds contain two carbon atoms, and both reactions involve a halogen. Yet one is an alkane, the other an alkene, and the difference changes the reaction itself.
This is the point where organic reaction types become genuinely useful. If the student can see where a carbon–carbon double bond exists and whether the reaction has ultraviolet-light conditions, they can choose between substitution and addition without guessing from a memorised reagent table.
This H1 Chemistry Tutor Hougang guide explains ethane’s reaction with chlorine in ultraviolet light at room temperature, the formation of chloroethane and hydrogen chloride, why mixtures can appear and how to distinguish that chemistry from halogen addition to ethene. We will also connect the reaction to saturated hydrocarbons, combustion, mole calculations and the revised Singapore–Cambridge A-Level H1 Chemistry 8873 requirements.
The short answer
Ethane is an alkane, C₂H₆. In the familiar school reaction with chlorine under ultraviolet light at room temperature, a hydrogen atom of ethane is replaced by a chlorine atom:
C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
The organic product shown is chloroethane. This is substitution, not addition across a carbon–carbon double bond.
Ethene, C₂H₄, is an alkene. Under suitable non-aqueous halogen-addition conditions it can react across the C=C double bond, producing a dihalogenoalkane. The difference in starting structure and reaction conditions controls the reaction class.
Why ethane is called saturated
An alkane contains carbon–carbon single bonds rather than carbon–carbon double or triple bonds. For a simple open-chain alkane, the familiar general formula is CₙH₂ₙ₊₂.
With n = 2, ethane has formula C₂H₆. Each carbon is connected to the other by a single bond and carries three hydrogens in CH₃–CH₃.
In this context, saturated means the carbon framework has the maximum number of hydrogen atoms possible without introducing a ring or multiple bond. It does not describe how full a bottle is or whether ethane dissolves in water.
Why ethene is unsaturated
Ethene is CH₂=CH₂, with molecular formula C₂H₄. The double bond contains one sigma and one pi bonding component in the standard description.
That pi component creates different reaction opportunities, including addition of halogens or hydrogen across the double bond under suitable conditions.
The two formulas differ by H₂, but their chemical behaviour does not differ only by mass. The connectivity and distribution of bonding electrons are what matter.
First identify the carbon–carbon connection
A carbon–carbon single bond in ethane contains a sigma connection and generally permits rotation of the two carbon groups. A carbon–carbon double bond in ethene contains sigma and pi components and restricts rotation.
This structural contrast is a stronger basis for product prediction than remembering the alphabetic difference between “ethane” and “ethene”.
Draw CH₃–CH₃ and CH₂=CH₂ next to one another. Circle the C–C and C=C features. Then try to explain why an addition reaction can consume ethene’s pi component but there is no equivalent C=C component in ethane.
Chlorine and ultraviolet light
The revised H1 Chemistry 8873 syllabus specifically describes alkanes, exemplified by ethane, undergoing substitution by chlorine in ultraviolet light at room temperature.
The ultraviolet light provides the energy relevant to starting the photochemical radical chemistry in the familiar extended explanation. A chlorine molecule can undergo homolytic cleavage to form chlorine radicals.
A school-level representation is Cl₂ → 2Cl• under appropriate light. This does not mean a household should attempt chlorine experiments. Chlorine gas is toxic, and halogen reactions require professionally controlled facilities. We use written equations and authorised learning resources rather than unsupervised exposure.
What does substitution mean?
A substitution reaction replaces one atom or group in an organic molecule with another. In the first monochlorination of ethane, one C–H bond is replaced by a C–Cl bond in the net product.
The carbon skeleton stays at two carbon atoms. One hydrogen has been removed from the organic product, and one chlorine has been attached. Hydrogen chloride contains the displaced hydrogen atom together with the other chlorine atom from the starting Cl₂ in the overall balanced equation.
This is a different transformation from adding two chlorine atoms across the double bond of ethene.
Check the equation before memorising it
The formula equation C₂H₆ + Cl₂ → C₂H₅Cl + HCl is balanced.
Two carbon atoms appear on each side. The left contains six hydrogen atoms, while the right contains five in chloroethane and one in HCl. Both chlorine atoms on the left are accounted for: one in chloroethane and one in HCl.
The balanced equation is a reliable check even when the compound name is unfamiliar. It is also the foundation for any mole calculation the examiner may attach.
What is chloroethane?
Chloroethane is CH₃CH₂Cl, a halogenoalkane. The prefix chloro- identifies chlorine bonded to the organic carbon chain, while ethane identifies the two-carbon saturated skeleton.
It is not ethylene chloride’s familiar addition product, 1,2-dichloroethane, which has two chlorine atoms attached to the chain.
This is a useful naming test because one extra word or number in a product name often signals a completely different chemical transformation.
Why ethane has only one monochloro position in the simple molecule
Both carbon atoms in ethane are equivalent, and all six hydrogens are chemically equivalent in the ordinary unsubstituted molecule.
Replacing any one hydrogen with chlorine therefore yields the same constitutional product, chloroethane. There is not a separate stable “1-chloroethane” versus “2-chloroethane” constitutional pair.
Once the molecule is substituted or the carbon chain becomes more complicated, distinct substitution positions can emerge. That is why the ethane example is an especially clear place to begin.
Substitution can continue
Chloroethane still contains C–H bonds. Under suitable continued chlorination conditions, further substitutions may occur, forming more highly chlorinated organic products.
The first substitution product is therefore not necessarily the only possible product in an actual uncontrolled mixture. Multiple substitution is one reason selective manufacture and purification can be challenging.
A student should distinguish the idealised first substitution shown in the syllabus from the practical possibility of a mixture. The initial balanced reaction still provides useful chemistry, but it is not a guarantee of one isolated product under every exposure.
Why the syllabus starts with ethane
Ethane’s two equivalent carbon atoms simplify the discussion: one monochloro constitutional product is possible. A larger alkane may offer several non-equivalent substitution sites.
This lets JC1 students focus first on the reaction type, chlorine reagent, ultraviolet-light condition and product formula before adding positional isomerism.
The learning sequence matters. Introducing a complicated radical mechanism and several isomers before the student understands substitution can make a simple H1 requirement unnecessarily difficult.
How the radical explanation fits in
A radical is a species with an unpaired electron. In a broader photochemical account, ultraviolet light can split Cl₂ homolytically, and radical reactions can propagate a substitution chain.
That is useful conceptual enrichment, but detailed initiation, propagation and termination mechanism equations are not specified as compulsory outcomes for the revised H1 alkane example. Students following H2 Chemistry may encounter deeper treatment.
The H1 priority is recognising the correct reaction, reagent, essential light condition and major named product—not presenting a full advanced radical-chain mechanism unless an unfamiliar data question supplies what is needed.
Don’t confuse ultraviolet light with heat alone
The named H1 reaction condition is chlorine with ultraviolet light at room temperature. A student should not replace it with “hot acidified dichromate” simply because both involve some form of energy input.
Different reactions require different conditions. Heated oxidising agents are part of the syllabus’s ethanol and aldehyde oxidation routes, not the standard ethane monochlorination example.
The best study aid connects each reagent and condition to the bond change it enables rather than storing a list of apparently interchangeable laboratory words.
Ethene addition: a contrasting equation
Ethene can undergo addition of chlorine in an appropriate reaction context. A simplified non-aqueous addition equation is CH₂=CH₂ + Cl₂ → ClCH₂CH₂Cl.
The product is 1,2-dichloroethane. The C=C double bond becomes a C–C single bond and one chlorine atom attaches to each carbon.
The revised H1 syllabus explicitly names ethene addition with **bromine in CCl₄** and with hydrogen using a nickel catalyst; chlorine addition is a chemically analogous illustration, not a reason to replace the named bromine example in a short recall question.
Ethene with bromine, the named H1 comparison
For the specified non-aqueous bromine example, CH₂=CH₂ + Br₂ → BrCH₂CH₂Br is the standard net addition equation.
The product is 1,2-dibromoethane. Both bromine atoms add across the original double-bond carbons.
If a question instead says *bromine water*, solvent participation can lead to bromohydrin products as well, so do not promise 1,2-dibromoethane is always the exclusive major product of every aqueous test. Read the actual medium.
An easy side-by-side reaction map
Draw ethane, CH₃CH₃, on the left and ethene, CH₂=CH₂, on the right. Above ethane write “Cl₂, UV light, room temperature” and the substitution product CH₃CH₂Cl.
Above ethene write “Br₂ in CCl₄” and the addition product BrCH₂CH₂Br. In one case one hydrogen is replaced and HCl is formed; in the other both bromine atoms attach while the C=C becomes a single C–C connection.
The distinction should be visible from the structures even if you cover the reaction labels.
Work out the hydrogen chloride amount
Suppose a hypothetical question assumes that 0.0400 mol ethane undergoes only the single monochlorination step and conversion is complete.
According to C₂H₆ + Cl₂ → C₂H₅Cl + HCl, one mole ethane produces one mole HCl. Therefore the idealised HCl amount is 0.0400 mol.
The one-to-one relationship applies to that particular balanced equation. A real chlorination mixture can include additional substitution, so the stated assumption is essential to the numerical answer.
Another mole-ratio check
Suppose a student writes C₂H₆ + 2Cl₂ → C₂H₅Cl + HCl as the equation for monochlorination. The chlorine count is wrong: four atoms on the left and only two on the right.
You cannot repair the mismatch by saying “more chlorine makes the reaction faster”. Stoichiometric coefficients must conserve atoms in the exact net reaction being written.
First balance the intended product equation, then discuss reactant excess or kinetics separately. The physical amount of excess reagent in a beaker is not the same thing as a coefficient in the balanced net transformation.
Ethane combustion: the other syllabus reaction
Alkanes can also undergo combustion in sufficient oxygen. For ethane the balanced complete combustion equation is:
2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
This is not a halogen substitution. Carbon-containing reactant molecules are transformed into carbon dioxide and water.
A question using the word “combustion” is therefore asking about oxygen and energy release rather than replacement of one hydrogen with chlorine.
Why combustion is exothermic
Breaking the original bonds in fuel and oxygen requires energy, while forming the strong bonds in CO₂ and H₂O releases energy. The overall enthalpy change is negative in the familiar complete-combustion process.
Do not say “bonds give out energy when broken”. A reaction releases net energy when product-bond formation contributes more energy release than the breaking steps require in the simple bond-energy picture.
This is a cross-topic connection to Chemical Energetics. The same ethane formula can appear in an organic reaction question and an enthalpy calculation for different reasons.
A short gas-volume exercise
If 0.100 mol ethane combusts completely according to the balanced equation, the amount of CO₂ formed is 0.200 mol because the ethane-to-CO₂ ratio is 1:2.
If the problem supplies molar gas volume 24.0 dm³ mol⁻¹ at its stated conditions, the predicted CO₂ volume is 0.200 × 24.0 = 4.80 dm³.
Do not use that molar volume at unspecified temperatures and pressures without checking the question. The reaction coefficients give mole ratios; gas-volume conversion requires appropriate measurement conditions.
What happens when oxygen is insufficient?
Incomplete combustion can produce carbon monoxide, soot or other products depending on conditions. The precise product amounts cannot be inferred merely from the phrase “oxygen is limited” without a specified reaction model.
That is different from UV substitution, where chlorine reacts with the hydrocarbon by replacing a hydrogen. Keeping the two processes separate prevents a learner from using a combustion equation to answer a halogenation problem.
When writing a chemical explanation, identify the reagent, reaction class and products before doing calculations.
Compare substitution with addition in one sentence
Substitution replaces an atom or group in a molecule. Addition joins atoms or groups across a multiple bond in the ordinary alkene example.
For ethane plus chlorine under UV light, substitution replaces H with Cl. For ethene plus bromine in CCl₄, addition changes C=C to C–C and adds bromine to the two carbon atoms.
A student who can say those two sentences accurately already has a strong foundation for a reaction-map question.
Why the word “chlorination” can mislead
Chlorination describes incorporation of chlorine in a chemical transformation. It does not uniquely identify addition, substitution or every possible product.
A chlorine reagent interacting with ethane in UV light is not equivalent to chlorine adding to ethene. The starting hydrocarbon’s bonding and the specified conditions determine which pathway is relevant.
Do not learn reagent names as if each has only one use in the entirety of Chemistry. Learn how reagent, substrate and conditions work together.
What if a question gives propane instead?
Propane is CH₃CH₂CH₃. Its end carbon hydrogens and central carbon hydrogens are not all equivalent.
Monochlorination can yield 1-chloropropane and 2-chloropropane as distinct constitutional products under appropriate photochemical conditions. Their proportions depend on relative reaction rates and conditions.
This is useful enrichment and a strong transfer test. It shows why a product mixture can arise even when we stop after one substitution per molecule.
Use the carbon skeleton as a checksum
Ethane has two carbon atoms. Chloroethane still has two. The proposed first substitution product should not be chloromethane with one carbon or chloropropane with three.
Similarly, ethene’s bromine addition product retains two carbons. The reaction changes bonding and adds atoms without extending the carbon skeleton.
Counting carbons is an excellent quick audit in Paper 1. It cannot solve every organic question by itself, but it can reject an incorrect product instantly.
An original MCQ-style challenge
Which conditions best match the conversion of ethane to chloroethane: (A) aqueous NaOH and heat, (B) Cl₂ and ultraviolet light at room temperature, (C) H₂/Ni, or (D) concentrated H₃PO₄ and heat?
The correct answer is B. A is associated with appropriate halogenoalkane substitution, C with hydrogenation of multiple bonds or carbonyl compounds under relevant conditions, and D with ethanol dehydration in the named H1 example.
The trick is not memorising one letter. It is recognising that ethane is saturated and the conversion replaces a hydrogen with chlorine.
A second challenge: determine the reaction class
Suppose the net organic equation is CH₂=CH₂ + H₂ → CH₃CH₃ under a nickel catalyst. Is this substitution, addition or elimination?
It is addition. Two hydrogen atoms become attached across the original double bond, and the carbon–carbon connection becomes a single bond.
Now compare C₂H₆ + Cl₂ → C₂H₅Cl + HCl under ultraviolet light. That is substitution. The equations themselves show the difference, even if you forget the labels for a moment.
A third challenge: why can’t ethane simply add Br₂?
Ethane has no C=C pi-bond component available for the familiar bromine addition pathway. It is a saturated hydrocarbon.
This does not imply ethane is absolutely chemically inert. It can undergo combustion and certain radical substitutions under suitable conditions.
A good answer should use qualified language: “under the typical mild alkene-addition test conditions” rather than an unjustified claim that ethane cannot react with any halogen under any conditions.
Understanding a failed bromine-water test
In a classroom-style controlled comparison, a typical alkene rapidly decolourises bromine under appropriate conditions while an ordinary alkane generally does not under mild dark conditions.
However, bromine-water observations are not unique proof of one exact structure; other reactive groups may also decolourise bromine. And the aqueous product distribution can differ from non-aqueous dibromide addition.
The right conclusion identifies what the observation supports and which assumptions are needed. That is scientific reasoning rather than a universal “orange becomes colourless, therefore ethene” slogan.
A practical reason for selective chemistry
If a production process needs chloroethane rather than a mixture of multiply chlorinated products, the possibility of continued substitution presents a selectivity challenge.
Chemists and engineers have to consider the desired product, competing reactions, processing costs and separation. The balanced school equation captures one desired step, but the real chemical system can include several parallel or sequential transformations.
This is a useful connection to industrial chemistry without giving unsafe laboratory procedures for chlorine handling.
The role of light is not magic
Ultraviolet photons carry energy. Photochemical reactions can use this energy to access pathways that are not prominent under otherwise comparable ordinary dark conditions.
In the extended chlorine example, homolytic bond cleavage gives reactive radical species. But a photon does not provide a mysterious extra chlorine atom, and light is not itself written as a chemical reactant in the balanced net stoichiometric equation.
The chemistry still obeys conservation of atoms, electrons and charge where relevant. Energy input and material input are different quantities.
How Paper 2 might make this harder
An integrated question may give ethane, chlorine, an unknown organic product and a mass or gas-volume measurement. You may be asked to infer the substitution product, identify a hydrogen chloride by-product and calculate a reaction amount.
Start with the balanced equation. Interpret any specified limitations or assumptions about monochlorination. Then convert the measured amount to moles and apply the coefficients.
The question may feel like two chapters, but the method is one chain: structure → reaction type → balanced equation → mole calculation.
Seven-day revision sequence
Day one: draw ethane and ethene correctly and distinguish saturated from unsaturated. Day two: learn ethane’s chlorine substitution equation and UV-light condition. Day three: practise organic product naming and atom conservation.
Day four: compare alkane substitution with ethene addition of bromine in CCl₄. Day five: balance ethane combustion and do a gas-volume question. Day six: interpret a question describing continued substitution or a propane isomer mixture. Day seven: complete mixed Paper 1 and Paper 2 practice without chapter labels.
If the student confuses C₂H₆ with C₂H₄, revisit the molecular structure first. A different reagent chart will not repair that problem.
How a tutor should diagnose the first error
I would ask a student to draw ethane. If the student draws C=C, we have a structure problem. If the structure is correct but the student predicts addition, we have a reaction-class problem. If the reaction is right but the product has three carbons, we have an atom-accounting problem.
Only after these are secure would I ask about the light condition, side products, further substitutions and timed calculations.
This is the value of careful diagnosis in a small-group lesson. Three students may share a table, but each may need a different first correction.
A 90-minute lesson structure
A focused 90-minute session might begin with a retrieval challenge on formulas and bond types, proceed to one short conceptual explanation, and then ask every student to complete new reaction diagrams alone.
Next we would practise a mixed problem linking substitution and gas stoichiometry. The final stage would be an independent unseen question and a short correction log identifying the student’s first error.
eduKate uses a three-student small-group model in its established teaching approach. Families should verify which H1 Chemistry programmes, locations and schedules are currently offered rather than assume every Hougang search result represents a physical centre in Hougang.
For parents planning JC Chemistry support
Ask your child why ethane reacts with chlorine under UV light by substitution, while ethene adds bromine under the named non-aqueous conditions. Ask which molecule contains a double bond.
Then ask your child to balance the ethane–chlorine reaction without notes. If the formula, reaction class and balance are all correct, the student has developed a coherent foundation.
A tutor’s most useful contribution is not another colourful reaction chart. It is making sure the learner can reconstruct the chemistry when the compound or wording changes.
Scope and A-Level readiness
The revised H1 Chemistry syllabus 8873 specifies that alkanes, exemplified by ethane, are generally unreactive except for combustion and substitution with chlorine in ultraviolet light at room temperature. Alkenes, exemplified by ethene, undergo the named addition reactions with Br₂ in CCl₄ and H₂ with Ni.
Detailed radical-chain mechanisms are not listed as a compulsory recall requirement for the H1 ethane example. Understand the science, but prioritise the published reagents, conditions, product identity and reaction type.
This distinction matters for JC1 students using H2 enrichment worksheets and JC2 students deciding how to allocate final revision time.
Connections to further studies
Hydrocarbon chemistry underpins fuels, petrochemical manufacturing, materials, polymer production and chemical analysis. The difference between substitution and addition is foundational to understanding how industrial feedstocks are transformed.
H1 Chemistry is a valuable scientific literacy subject, but many university chemistry and engineering programmes have specific subject-level prerequisites, sometimes requiring H2 Chemistry or Mathematics. Check the actual course and intake requirements directly.
For now, a reliable H1 result begins with a reliable concept: look at the carbon bonds first, then the reagent and its conditions.
Frequently asked questions
What is the equation for ethane and chlorine under UV light?
For the first monochlorination step: C₂H₆ + Cl₂ → C₂H₅Cl + HCl.
What is the organic product of ethane chlorination?
Chloroethane, CH₃CH₂Cl, in the idealised monosubstitution equation.
What type of reaction is it?
Substitution: one hydrogen in ethane is replaced by chlorine.
Why is ultraviolet light required?
It provides the photochemical energy needed to initiate the familiar radical substitution pathway under suitable conditions.
Is ethane saturated?
Yes. It is an alkane with a C–C single bond and formula C₂H₆.
Why does ethene react differently?
Ethene contains a C=C double bond with a pi component that can undergo addition under appropriate conditions.
Can further chlorination occur after chloroethane forms?
Yes. Other hydrogens remain, so further substitution is possible and actual mixtures may include additional chlorinated products.
Does ethane undergo combustion?
Yes. In excess oxygen, 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
Are full free-radical mechanism steps compulsory for H1?
The named revised H1 learning outcome requires combustion and UV-light chlorine substitution; detailed radical-chain mechanisms are not specified as a separate compulsory recall outcome for that example.
Can I perform ethane and chlorine experiments at home?
No. Chlorine and photochemical halogenation pose significant hazards. Learn the reaction through equations, models and authorised educational demonstrations.
Continue learning in the Hougang H1 Chemistry library
To understand the double-bond contrast, read H1 sigma and pi bonds in ethene. To examine the named bromine test and its conditions, use H1 bromine water, alkenes and addition reactions. For a broader set of organic conversions, follow H1 reaction pathways, reagents and conditions.
The official 2027 SEAB A-Level syllabus directory identifies Chemistry 8873; the revised H1 Chemistry learning outcomes list the named reactions. Families can consult JC1 H1 Chemistry tutorials, JC2 H1 Chemistry tutorials and eduKate’s consultation page for current arrangements.
Take the structures of ethane and ethene, and choose one halogen reaction for each. Draw the products and explain exactly which bonds changed. When your answers differ for a chemically sound reason, you have learned the difference between substitution and addition.