Chemistry-only: chlorine, bromine and iodine addition
| English | 中文 | Pinyin |
|---|---|---|
| halogen addition | 卤素加成 | lǔ sù jiā chéng |
| decolourisation | 褪色 | tuì shǎi |
What would explain this observation?
- Bromine water loses its orange colour when it reacts with an alkene. A displayed product explains where the two bromine atoms went instead of saying that bromine simply disappeared.
- Start with a prediction. State the quantities or features you would compare, then decide what evidence could distinguish two explanations.
Build the model
- Alkenes react with halogens by addition across C=C. A halogen molecule X₂ supplies one atom to each formerly double-bonded carbon, and C=C becomes a single bond. Here X may be Cl, Br or I. Chlorine and bromine addition are readily described at room temperature without a required UV lamp or nickel catalyst; iodine addition can be slower and less favourable, so the drawing represents the stated addition product rather than promising identical test speed for every halogen.
- halogen addition 卤素加成: Addition of one halogen atom to each carbon of a carbon–carbon double bond; decolourisation 褪色: Loss of a reagent’s visible colour during the stated reaction.
What is retained when bromine adds to an alkene?
For ethene plus bromine, CH₂=CH₂ becomes BrCH₂–CH₂Br. Each carbon retains its two hydrogen atoms and gains one bromine, giving four bond contributions. Bromine water changes orange to colourless in the specified reaction; an alkane control does not give the same rapid addition under these conditions. A carbon skeleton and hydrogen count are retained while two halogen atoms are added. The displayed panels distinguish chlorine, bromine and iodine products for all four named starting alkenes.
Match each technical term to its precise meaning.
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- For ethene plus bromine, CH₂=CH₂ becomes BrCH₂–CH₂Br. Each carbon retains its two hydrogen atoms and gains one bromine, giving four bond contributions. Bromine water changes orange to colourless in the specified reaction; an alkane control does not give the same rapid addition under these conditions. A carbon skeleton and hydrogen count are retained while two halogen atoms are added. The displayed panels distinguish chlorine, bromine and iodine products for all four named starting alkenes.
- Draw the original molecule, mark the two double-bonded carbons, change their link to single and attach one halogen to each. Check every carbon and every hydrogen rather than replacing an H with Br. Record actual colour against a blank/control in teacher-approved microscale demonstrations. Halogens need approved containment, eye protection and ventilation; students must not generate chlorine, handle elemental bromine or improvise iodine reaction conditions independently.
Which two habits make the investigation or model in this case more defensible?
Draw the original molecule, mark the two double-bonded carbons, change their link to single and attach one halogen to each. Check every carbon and every hydrogen rather than replacing an H with Br. Record actual colour against a blank/control in teacher-approved microscale demonstrations. Halogens need approved containment, eye protection and ventilation; students must not generate chlorine, handle elemental bromine or improvise iodine reaction conditions independently.
Work from known quantities
- State the known values and their units. Choose the relation because its assumptions fit this case, then rearrange before substitution.
- Known: C₄H₈ + Br₂ → C₄H₈Br₂ accounts for four C, eight H and two Br atoms. Three molecules undergoing one addition each gain six bromine atoms in total. With Cl₂ the corresponding product has two chlorine atoms per molecule; with I₂ it has two iodine atoms. Those supplied product formulae do not describe polymerisation or substitution.
Five alkene molecules each add one Br₂. Find the bromine atoms added in total. Use the same sequence: known quantities → model → relation → substitution → unit and interpretation.
Five alkene molecules each add one Br₂. Find the bromine atoms added in total.
The result is 10 atoms. Known: C₄H₈ + Br₂ → C₄H₈Br₂ accounts for four C, eight H and two Br atoms. Three molecules undergoing one addition each gain six bromine atoms in total. With Cl₂ the corresponding product has two chlorine atoms per molecule; with I₂ it has two iodine atoms. Those supplied product formulae do not describe polymerisation or substitution.
Check the conclusion and its limits
- Do not remove a hydrogen in an addition drawing, or claim UV light is the required catalyst for the alkene test. That confuses addition with alkane substitution. Decolourisation is evidence for unsaturation in the controlled comparison, not unique proof of ethene or universal identification of any unknown liquid. A colourless endpoint alone cannot demonstrate that a sample is safe.
- Return to the original observation. Explain what the result supports, which conditions it assumes, and one way to test a competing explanation.
Bromine addition must replace two of the alkene’s hydrogen atoms. This claim is false: Do not remove a hydrogen in an addition drawing, or claim UV light is the required catalyst for the alkene test. That confuses addition with alkane substitution. Decolourisation is evidence for unsaturation in the controlled comparison, not unique proof of ethene or universal identification of any unknown liquid. A colourless endpoint alone cannot demonstrate that a sample is safe.
Chemistry-only: chlorine, bromine and iodine addition: For ethene plus bromine, CH₂=CH₂ becomes BrCH₂–CH₂Br. Each carbon retains its two hydrogen atoms and gains one bromine, giving four bond contributions. Bromine water changes orange to colourless in the specified reaction; an alkane control does not give the same rapid addition under these conditions. A carbon skeleton and hydrogen count are retained while two halogen atoms are added. The displayed panels distinguish chlorine, bromine and iodine products for all four named starting alkenes.
Bromine addition must replace two of the alkene’s hydrogen atoms.
Do not remove a hydrogen in an addition drawing, or claim UV light is the required catalyst for the alkene test. That confuses addition with alkane substitution. Decolourisation is evidence for unsaturation in the controlled comparison, not unique proof of ethene or universal identification of any unknown liquid. A colourless endpoint alone cannot demonstrate that a sample is safe.
Addition of one halogen atom to each carbon of a carbon–carbon double bond: write the technical term.
halogen addition means Addition of one halogen atom to each carbon of a carbon–carbon double bond.