Alkanes & Alkenes
Hydrocarbons can be classified depending on the presence of double carbon bonds. Molecules with only single bonds (saturated) are known as alkanes and molecules with a double carbon bonds (unsaturated) are known as alkenes.
Homologous series
Alkanes and alkenes are what we call a homologous series, which is basically a group of molecules which all have the same general formula, show trends in their physical properties (such as boiling point) and undergo the same types of chemical reactions.
Alkanes have the general formula CnH2n+2, which means for every carbon atom there are double the number, plus an additional two hydrogen atoms. Alkanes form bonds with as many other atoms as they possibly can so they are referred to as saturated hydrocarbons. There are only single bonds connecting the atoms together.
Alkenes have the general formula CnH2n, which means for every carbon atom there are double the number of hydrogen atoms. Alkenes are different to alkanes because they contain at least one double carbon bond. We refer to them as unsaturated because they aren't yet bonded to as many atoms as possible. The double bond can open up and allow the two carbons to bond with more atoms in addition reactions (see below).
Combustion reactions
All hydrocarbons can be burnt to release heat energy, which is why they make good fuels. Remember that combustion is simply reacting the molecule with oxygen. The products of a combustion reaction are carbon dioxide and water.
If we try to carry out the same reaction in an environment where there is a lack of oxygen, we’ll get incomplete combustion. Instead of producing carbon dioxide (CO2) where two oxygen atoms are bonded to each carbon, carbon monoxide (CO) is formed where only one oxygen atom can bind to each carbon, due to the limited availability of oxygen. Carbon monoxide is poisonous because it binds to haemoglobin in our red blood cells, preventing it from carrying oxygen. Less oxygen in our blood means that our cells don’t have the oxygen they need to respire, making carbon monoxide poisoning deadly.
Sometimes, if oxygen is really limited, we may get no oxygen atoms bonding at all to the carbon atoms. In this case carbon is produced as a single product and appears as fine black particles which we know as soot.
Reaction of alkenes with bromine
Only hydrocarbons with a double carbon bond (i.e. alkenes) can undergo addition reactions with bromine. The double bond breaks and bromine is added to the carbon atoms which made up the double bond. Bromine water is orange when unbound to another molecule and loses its colour when bound to the hydrocarbon. The orange to colourless colour change can be used as a test to detect the presence of an unsaturated compound.
This is called an addition reaction because the double carbon bonds breaks apart and the bromine atoms are added to the carbons.
Reaction of alkanes with bromine
Alkanes can also react with bromine, but in only in the presence of UV light. Since alkanes are already bonded to as many hydrogens as possible, one of the hydrogens needs to be swapped for a bromine. We call these substitution reactions.