Hi and welcome back to my channel. Today we're going to be looking at the test for unsaturation. First of all, let's define the words saturated and unsaturated. A saturated molecule is one which contains only carbon to carbon single bonds. For example, the alkanes and the cycloalkanes.
An unsaturated molecule contains carbon-to-carbon multiple bonds, for example, the carbon-to-carbon double bond in the alkenes. To be able to tell apart saturated and unsaturated molecules, we test them with bromine water. Bromine water is decolourised rapidly by unsaturated compounds. When bromine water is added to an unsaturated compound, the carbon to carbon double bond can open.
The bromine to bromine bond breaks and the bromine atoms add across the double bond. Bromine is a brown colour when bromine is... found in its elemental state Br2.
When the bromine to bromine bond breaks the brown colour disappears and thus it becomes decolourised. When you react bromine with a saturated molecule such as ethane there is no carbon to carbon double bond to break. to allow bromine to add on to the molecule.
This means that the bromine to bromine bond stays intact and the bromine is not decolourised. Let's have a look at this in action. Thank you. Pause the video now and decide which of these will show a positive test for unsaturation with bromine water. This first molecule here is methane.
We cannot add bromine to methane so this will not show a positive test. Here we have propene and as it has a carbon to carbon double bond it will show a positive test for unsaturation. Cyclopropane has only single carbon to carbon bonds and is saturated therefore will not show a positive result.
2-methylbutytoene is an alkene and therefore will show a positive result with bromine water. Thank you for watching my video, I hope that you've found it helpful. Please remember to subscribe and follow me on Twitter at Miss Adams Chem for regular updates on new videos and ring the bell to be notified.
Follow me on Instagram at Miss Adams Chemistry for flashcards throughout the year. Bye for now!