Hydrocarbons (SQA National 5 Chemistry): Revision Note

Saturated & unsaturated compounds

What are hydrocarbons?

• Hydrocarbons are compounds that contain only carbon and hydrogen atoms

• They can be classified into two main types based on the bonding between their carbon atoms

1. Saturated hydrocarbons

• A hydrocarbon is saturated if the carbon atoms are joined together by single C-C bonds only

• The two homologous series of saturated hydrocarbons you need to know are:

  • Alkanes

  • Cycloalkanes

2. Unsaturated hydrocarbons

• A hydrocarbon is unsaturated if it contains at least one double C=C bond

• The homologous series of unsaturated hydrocarbons you need to know is the alkenes

Unsaturation testing - the bromine solution test

• All alkanes are saturated and alkenes are unsaturated

• The presence of the C=C double bond allows alkenes to react in ways that alkanes cannot

• This means that a simple chemical test can distinguish between a saturated and an unsaturated hydrocarbon

  • Bromine solution is used in the test for alkenes as it is safer and easier to handle than bromine

  • A simple chemical test can tell the difference between a saturated and an unsaturated hydrocarbon.

Method

• Place a few drops of hydrocarbon in a test tube

• Add a few drops of orange-brown bromine solution to the test tube

• Add a bung / stopper

• Gently shake the test tube

The results

• Unsaturated compounds, e.g., alkenes, decolourise the bromine solution

  • It turns from orange-brown to colourless

• Saturated compounds, e.g. alkanes, show no visible change

  • The mixture remains orange/brown

Explaining the results

• The alkene performs an addition reaction

  • This is where the C=C double bond breaks and the bromine atoms add onto the carbon atoms

  • This uses up the bromine

  • So, the colour disappears

• Saturated compounds cannot do addition reactions