saturated
hydrocarbons |
| C2H6 |
| C3H8 |
| C4H10 |
| C5H12 |
Saturated hydrocarbons have the maximum number of hydrogens for that number of carbons. These are known as
alkanes and have the general formula C
nH
2n+2.
The box shows that the difference between successive hydrocarbons in a series is CH2.
Hydrocarbons having one carbon-carbon double-bond are unsaturated because they have two fewer than the maximum number of hydrogens for a given number of carbon atoms. These are known as
alkenes and have the general formula C
nH
2n.
When deducing the number of alkene isomers for a given carbon skeleton, bear in mind that the double bond can replaces one H at each of two adjacent carbon atoms. Therefore the task is to identify different pairs of adjacent carbons, each of which bear at least one hydrogen.
C – C – C – C
C = C – C – C
C – C = C – C
There are only two different pairs of carbons on a C4 chain.
The carbon skeleton for the two straight chain C4 alkenes:
For branched carbon skeletons, it is important to realise that if a carbon atom bears two or more branches of identical composition, all of the atoms in those branches are the equivalent.
There is only one C4 branched alkene isomer because all carbons adjacent to the central one are equivalent.
Hydrocarbons with one triple bond are also unsaturated. They have the general formula is C
nH
2n-2 and are known as
alkynes.
When deducing
C ≡ C – C – C
C – C ≡ C – C
The carbon skeletons for the two possible C4 alkynes are shown. Note that triple bonds may not appear at branch points because carbons at branch points are already involved in three single bonds to carbon.