How can you distinguish alkenes from alkanes?

Alkenes can be distinguished from alkanes through a chemical test using bromine water, which alkenes decolourise.

Alkenes and alkanes are both hydrocarbons, meaning they are composed of hydrogen and carbon atoms. However, they differ in their structure and reactivity due to the type of bonds they possess. Alkanes are saturated hydrocarbons with single bonds between carbon atoms, while alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond. This difference in bonding leads to different reactions when they are tested with bromine water.

Bromine water is a yellow-brown solution that can be used to test for the presence of a carbon-carbon double bond, which is characteristic of alkenes. When an alkene is added to bromine water, the solution will decolourise, changing from yellow-brown to colourless. This is because the bromine reacts with the carbon-carbon double bond, forming a colourless dibromo compound. This reaction is known as an addition reaction, where the bromine atoms 'add' to the carbon atoms at the double bond, breaking the double bond and forming two new single bonds.

On the other hand, alkanes do not react with bromine water under normal conditions because they lack the carbon-carbon double bond. The bromine water will remain its original yellow-brown colour, indicating that no reaction has taken place. This is a simple and effective way to distinguish between alkenes and alkanes.

In addition to the bromine water test, the physical properties of alkenes and alkanes can also provide clues to their identity. Alkenes generally have lower boiling points than alkanes due to their unsaturated nature. However, this method is less definitive and requires more sophisticated equipment, making the bromine water test a more practical choice for most situations.