Testing for Alkenes with Bromine Water

What is the test for an alkene?

Add bromine water and observe whether it changes from orange to colourless.

What is the original colour of bromine water?

Orange.

What happens when bromine water is added to an alkene?

It changes from orange to colourless.

What happens when bromine water is added to an alkane under normal test conditions?

It remains orange.

Why do alkenes decolourise bromine water?

Bromine undergoes an addition reaction across the C

What happens to the double bond when bromine reacts with an alkene?

The C

What type of reaction occurs between an alkene and bromine?

Addition.

What is the word equation for ethene reacting with bromine?

Ethene + bromine → dibromoethane.

What is the balanced symbol equation for ethene reacting with bromine?

C₂H₄ + Br₂ → C₂H₄Br₂.

Why is bromine water useful for distinguishing alkanes from alkenes?

Alkenes decolourise bromine water, while alkanes generally do not under the test conditions.

What does a positive bromine-water test indicate?

The presence of a carbon–carbon double bond, consistent with an unsaturated compound.

Why should the bromine-water test be carried out without strong ultraviolet light?

UV light can allow substitution reactions with alkanes, complicating the test.

Does the bromine-water test identify the exact alkene present?

No, it indicates unsaturation rather than a specific compound.