Naming alkanes is one of the first topics you’re going to cover in your organic chemistry course. Nomenclature of alkanes is not particularly difficult but it can be tricky and tedious. Here are some basic rules we use for the alkane nomenclature.
All names of alkanes are built from the list of parent names. You wanna know the first 10 names of the parent molecules and have those at your fingertips.
Knowing the next ten is not usually required. I still recommend you memorize those as well as your instructor might throw an curved ball at you and give you a very large molecule to name.
The ending -ane in the name means that the molecule belong to the alkane family. Normal alkanes only have single C-C and C-H bonds. These molecules are also open-chain molecules. This means that they don’t have any cycles (rings) in the structure. The cyclic alkanes are called cycloalkanes and they do follow their own naming rules and conventions. We’ll talk about those in a different topic.
How do we construct the branch names?
When you’re dealing with a molecule that is a little more complicated than a simple straight chain, we have a problem. And the problem is that the molecule had branches. So, we no longer can use the simple names based on the number of carbons. Well, how do we name those branches? We count the atoms in the branch and we change the alkane ending -ane to -yl to signify that it is a substituent sitting off the parent chain.
How do we put the rest of the name together?
1. You find the longest continuous chain of carbons:
In this case the longest chain contains 10 carbons. So, the parent name in this case is called decane.
2. Then you wanna identify all the branches/substituents coming off the parent alkane:
3. Number your parent chain the way that would give the lowest possible position numbers (locants) to the groups.
In this example, there are two different ways you can number your chain. The option 1 offers the lower numbers for the substituents unlike option 2.
4. Finally, you put the names of substituents in front of the parent name in an alphabetical order:
Notice, that we add the prefix “tetra” in front of the “methyl” to show that we have four methyl groups in this molecule. As a quick check, always make sure you have the matching number of locations and the numeric prefix in front of the substituent name.
Examples of names of branched alkanes
Give the IUPAC names for the following molecules:
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