Chlorine substitution: ultraviolet light and one replacement
Draw the monosubstitution products, account for both chlorine atoms and explain ultraviolet light as the activation-energy source.
Extended only (Supplement) · Chemistry 0620, 2026–2028.
Before you begin
- Identify alkane single bonds and read complete displayed formulas.
- Explain activation energy and preserve atoms in a balanced equation.
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
What happens in substitution?
What you will learn
- Define substitution as replacement of an atom or group by another.
- Describe photochemical alkane chlorination and draw structural or displayed products for monosubstitution.
One hydrogen is replaced, not an entire carbon chain
In substitution, one atom or group of atoms is replaced by another atom or group. The required alkane reaction with chlorine is photochemical: ultraviolet light supplies activation energy. For methane monosubstitution, CH₄+Cl₂→CH₃Cl+HCl, with ultraviolet light stated over the reaction arrow or alongside the equation. The light is an energy input, not an extra chemical atom source.
CH₃Cl has one chlorine replacing one original hydrogen on carbon. The removed hydrogen combines with the other chlorine atom to form HCl. Both chlorine atoms and all four original hydrogens are accounted for across the two products. The displayed diagrams show every individual atom and bond in chloromethane and hydrogen chloride, rather than representing HCl as an unattached label inside the organic molecule.
Keep the stated scope to monosubstitution
Ethane similarly gives CH₃CH₂Cl and HCl on replacing one H: C₂H₆+Cl₂→C₂H₅Cl+HCl under ultraviolet light. Draw the chloroethane structure with three H on one C, two H and one Cl on the other C, a C–C single bond, and H–Cl as a separate product. Rotating that ethane product does not create a different connectivity.
These equations are the specified monosubstitution models, not a claim that real chlorination always stops after one replacement or gives a completely pure product. Further substitution and detailed chain mechanisms are outside this objective. For longer unsymmetrical alkanes, different replacement positions can give different structures; use the specified position rather than assuming one unique structure from a molecular formula alone. The reaction is interpreted as a syllabus model, not an instruction for handling chlorine.
Pause and explain
Which equation is methane monosubstitution under UV?
Worked example
Give methane's chlorine monosubstitution equation and identify the role of ultraviolet light and the two products.
Show the worked solution
- Replace one methane H by Cl to obtain CH₃Cl.
- The displaced H and the second Cl form HCl: CH₄+Cl₂→CH₃Cl+HCl.
- Ultraviolet light provides activation energy in this photochemical reaction; draw the C with three H and one Cl, plus separate H–Cl.
Answer Under ultraviolet light: CH₄+Cl₂→CH₃Cl+HCl. Products are chloromethane and hydrogen chloride; UV supplies activation energy.
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Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
Draw both products of methane chlorine monosubstitution and state the reaction condition.
Give me a hint
Use the displayed models to count carbon's four single bonds.
Compare my reasoning
- Draw one C bonded individually to three H atoms and one Cl atom: CH₃Cl.
- Draw H–Cl as a separate molecule and verify total H₄Cl₂ across both products.
- State ultraviolet light, supplying activation energy: CH₄+Cl₂→CH₃Cl+HCl.
Displayed chloromethane with C–H×3 and C–Cl×1, plus separate H–Cl; ultraviolet light supplies activation energy.
Look for these in your work
- Both products and every bond are shown.
- One replacement and the UV energy role are explicit.
Give the structural products and equation for ethane monosubstitution by chlorine under ultraviolet light.
Give me a hint
Retain the C–C chain and replace just one hydrogen.
Compare my reasoning
- Ethane CH₃CH₃ becomes CH₃CH₂Cl when one H is replaced by Cl.
- The removed H joins the other Cl atom as HCl.
- C₂H₆+Cl₂→C₂H₅Cl+HCl conserves C₂H₆Cl₂; ultraviolet light supplies activation energy.
CH₃CH₂Cl and HCl; C₂H₆+Cl₂→C₂H₅Cl+HCl under ultraviolet light.
Look for these in your work
- The structural product keeps the two-carbon chain.
- Both product formulas, atom conservation and the condition agree.
A proposed methane answer shows C with two H and two Cl, omits a separate product, and calls UV a catalyst. Explain three corrections for the specified monosubstitution model.
Give me a hint
Correct the replacement count, the missing atoms and the energy role independently.
Compare my reasoning
- One replacement gives C with three H and one Cl, CH₃Cl, not CH₂Cl₂.
- Add separate HCl to account for the displaced hydrogen and the second chlorine atom.
- Describe UV as providing activation energy in a photochemical reaction, rather than as a chemical catalyst.
Use CH₃Cl, add separate HCl, and describe ultraviolet light as the activation-energy source.
Look for these in your work
- Each of the three errors is addressed.
- The corrected answer stays within monosubstitution.
Common mistakes
- Putting both chlorine atoms on methane in a monosubstitution answer.
- Omitting HCl and losing the replaced hydrogen and second chlorine atom.
- Calling ultraviolet light a catalyst or a chemical reagent supplying atoms.
- Saying the monosubstitution equation guarantees pure product and no possible later replacement.
What can you explain now?
What provides the activation energy for the required photochemical chlorine substitution?
Compare with the explanation
Ultraviolet light.
UV supplies the energy input; the chemical atom sources remain the alkane and chlorine.
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Make it stick
Draw both methane monosubstitution products without notes, balance the equation and explain the UV condition in one sentence.
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.