As a quick recap, before we get stuck into addition reactions, alkenes are unsaturated hydrocarbons containing a double bond – and it’s this bond that makes them useful in addition reactions.
Addition reactions occur in compounds that have multiple bonds, such as the carbon-carbon double bonds in alkenes.
These alkenes can undergo combustion, so theoretically can be used for fuel. However the high electron density of the double bonds means they have a very high reactivity which simply makes them more useful in other stuff! (For instance to produce alcohol – very useful).
The high electron density in the double bonds makes reactions with electrophiles common. This reaction usually then creates a carbocation, which is susceptible to attack by an anion.
Now for a recap on some important points: Alkenes are unsaturated, meaning they contain a double bond. The presence of this double bond allows alkenes to react in ways that alkanes cannot. This allows us to distinguish alkenes from alkanes using a simple chemical test.
And this test involves using bromine (which is orange). This will become colourless when it is shaken with an alkene – nice and straight forward.
Alkanes on the other hand would not decolourise bromine water, making it nice and simple to tell the difference.