An alkene contains a carbon–carbon double bond. Addition joins atoms to the carbons of that bond and leaves a carbon–carbon single bond in the product. The carbon skeleton remains connected.
Ethene and bromine form a dibromo product with one bromine on each of the former double-bond carbons. In a supervised alkane/alkene comparison without UV initiation, bromine water is decolourised by the alkene.
AQA profiles also practise hydrogen addition, using a nickel catalyst, and hydration by steam under suitable catalytic conditions. Hydrogen forms an alkane; steam forms an alcohol. Edexcel profiles here assess bromine addition and the test, without inheriting extra AQA requirements.
Write your answer as a condensed structure, such as CH3CH2OH. You can ask to see answer choices if you need help. Drawing every bond and predicting products of larger alkenes need separate practice. Other substances can also decolourise bromine water, so this test alone cannot identify every unknown.
Worked example
CH2=CH2 + Br2 → CH2Br–CH2Br. Each carbon still forms four bonds.
AQA hydration example: ethene and steam can form ethanol. It is addition, not fermentation.
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Explain the change in bonding when bromine adds to ethene.
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Change the double bond to a single bond.
Attach one bromine to each carbon.
Conserve atoms and carbon valency.
Example answer
The carbon–carbon double bond becomes a single bond and a C–Br bond forms at each carbon. Both bromine atoms are incorporated. Each carbon retains four covalent bonds in the condensed product CH2Br–CH2Br.
An answer to improve
Bromine removes a carbon atom and the double bond stays unchanged.
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State the named substances, coefficients and conditions. Replace a general rule with the evidence that actually supports your conclusion.
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