Which of the following substances would decolourise bromine? .

Question

BlackTom AI · Accepted Answer

The question asks which substance would decolourise bromine. This is a classic test for unsaturation in organic compounds.
First, consider the mechanism: bromine water is orange/brown in colour, and it fades to colourless when an alkene (a compound with a C=C double bond) undergoes an addition reaction with Br2, forming a vicinal dibromo compound. This happens because the electrons in the carbon–carbon double bond polarise Br2 and lead to a rapid electrophilic addition, consuming Br2 and removing its colour.
Now apply this to the provided answer note: 'd. ethene' is listed as the correct option. Ethene (C2H4) contains a C=C double bond, so it readily reacts with bromine water to form 1,2-dibromoethane, decolourising the solution. This aligns with the standard observation that alkenes decolourise bromine.
What would be the expected explanation for the other potential substances (if they were present as options)? In general:
- Alkanes (no C=C) do not react with bromine under mild conditions; bromine water remains orange/brown, so no decolourisation occurs.
- Alkynes (C≡C) can also decolourise bromine, though under different conditions (and often more rapidly) due to their higher unsaturation.
- Benzene rings (aromatics) typically resist bromine decolourisation in the absence of a catalyst or high-energy conditions; they require radical halogenation conditions to show noticeable decolourisation.
- Halogens or other oxidising agents may oxidise or react with bromine differently, but they do not simply mimic the alkene test.
In summary, the statement that ethene would decolourise bromine fits the well-established test for unsaturation through electrophilic addition to a C=C bond, producing a colourless product as Br2 is consumed.

Which of the following substances would decolourise bromine? .单项选择题

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