Isomers
We know what isomers are.
Coding for Double Bonds
For things like pentene and butene, there are many places you can put the double bonds in.
Pent-1-ene means pentene with the double bond on the first carbon-carbon bond (right).
Pent-2-ene means pentene with the double bond on the second carbon-carbon bond and so on…
But wait! What about pent-4-ene and pent-5-ene? Those don’t exist. Why? Because pent-4-ene is pent-2-ene flipped over, and pent-5-ene is pent-1-ene flipped over!
Flip: C-C-C=C-C and you get C-C=C-C-C!
Methyl and Ethyl Groups
| Code | Meaning |
| Methyl | Has a branch of CH3 coming off one of the bonds. |
| Ethyl | Has a branch of CH3CH2 coming off. |
If the hydrocarbon has a methyl or ethyl group, these two come first, before the coding for the number of carbons in the chain. But before even the ‘methyl’ or the ‘ethyl’ there is a number and hyphen to show which carbon has the methyl or ethyl branch.
For example, this is 2-methylbutane
As you can see, there is a methyl group branching off the second carbon. The rules are similar to double bonds though, there is no such thing as 3-methylbutane because that is basically 2-methylbutane flipped over.
But wait! There are five carbons! Why isn’t it 2-methylpentane? Because remember, these names are based on the longest carbon chain in the hydrocarbon and the longest carbon chain there is 4, hence, butane. This means that 2-methylbutane is an isomer of pentane C5H12.
Some Isomers of Butane C4H10
Some Isomers of Pentane C5H12
