Generating possible carbon skeletons
When drawing all of the possible carbon skeletons (The carbon skeleton shows the arrangement of carbon atoms.) for a hydrocarbon containing a given number of carbon atoms, use a systematic approach. Bear in mind that
When deciding how many different branched isomers are possible
- one of the possible carbon skeletons is a linear chain with the given number of carbons (the given number of carbons is specified by a subscript on carbon - a C5 compound has five carbons).
- other carbon skeletons have their main chain shorter than the maximum, with some carbons in branches.One set of possible isomers has a chain one carbon shorter, and one branch containing one carbon atom.
Another set has a chain two carbons shorter, and two branches each containing one carbon atom; and so on.
When deciding how many different branched isomers are possible
write the main chain as carbons separated by dashes.Place your mouse over the chain at the right above to see how the dashes can be converted into H's.
attach C1 branch(es) to carbon atoms other than those at the end of the chain.C
|C — C — C
As shown at the right, attaching carbons to the carbons at the end of the chain gives an extended (not branched) chain. There is only one possible branched carbon skeleton for a C4 (four carbon) chain. This has the branch at the centre carbon (2-methyl propane).
- be aware that chains longer than C3 have some carbons in the same bonding environment that are equivalent
C – C- 
-C – C
For example, in a four-carbon chain, due to symmetry, the center two carbons are equivalent, and the same skeleton is generated by attaching CH3– to either middle carbon. Therefore there is one carbon skeleton that has a four carbon chain with one branch (2-methylbutane).
- Repeat the analysis, using successively shorter chains and more one-carbon branches to determine the total number of isomers for a given number of carbons.