Constitutional (structural) isomers for unsaturated hydrocarbons
| saturated hydrocarbons |
| C2H6 |
| C3H8 |
| C4H10 |
| C5H12 |
The box shows that the difference between successive hydrocarbons in a series is CH2.
Hydrocarbons having a 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 hydrocarbons are known as alkenes and have the general formula CnH2n.
When predicting the number of possible isomers for a given carbon skeleton, it is important to take into account that some positions in the chain may be equivalent.
Linear carbon chains are symmetrical about their center.
For a four carbon chain this means that the two end carbons are equivalent and the two middle carbons are equivalent.
Therefore the double bond can be between two blue carbons or a blue and a red carbon.
The carbon skeletons for the two straight chain C4 alkenes is shown below.
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Therefore the double bond can be between two blue carbons or a blue and a red carbon.
The carbon skeletons for the two straight chain C4 alkenes is shown below.
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| CH3CH2CH=CH2 | CH3CH=CHCH3 |
Branched compounds have fewer isomers if two or more branches attached to the same carbon are equivalent.
There is only one C4 branched alkene isomer because introducing a double bond between any of the red carbons and the central carbon gives the SAME compound (2-methylpropene). Put another way, there is only one type of pair of adjacent carbons (red and black).
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The five-carbon skeleton below has only three double bond isomers because a double bond to either of the red carbons gives the same compound (2-methylbut-1-ene). There are three different pairs of adjacent carbons (red and black, black and blue, blue and gold).
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