Branched alkanes are alkanes in which at least one carbon atom is bonded to three or four other carbon atoms. unlike the linear (or unbranched) alkanes, where carbon atoms are only bonded to one or two other carbon atoms.
The rules described in this section for the nomenclature of branched alkanes serve as the foundation for the nomenclature of all other types of organic molecules – therefore, it is recommended to become comfortable with these rules.
The structures of branched alkanes can be divided into two parts: the parent chain, and the substituents. The parent chain is the longest continuous chain of carbon atoms, while the substituents are the groups branching off of the parent chain. To name branched alkanes, you must first identify the parent chain.
Here are the structures of two branched alkanes:
The carbons of the parent chain (the longest continuous chain of carbons) are indicated with yellow circles, and the carbons of the substituents are indicated with blue circles. Be careful – the parent chain might not always be drawn left to right.
Branched alkanes are named by first listing the names of the substituents, then finishing with the name of the parent chain.
For branched alkanes, the substituents are known as alkyl substituents. These are saturated alkane chains which branch off of the parent chain. Molecules with other functional groups have other kinds of substituents.
The names of alkyl substituents are based on the names of linear alkanes. When naming an alkyl substituent, the –ane suffix of the linear alkane is replaced with the –yl suffix of the alkyl substituent. For example, if an alkyl substituent has one carbon atom, it is a methyl substituent (or more commonly, a methyl group). Here are the names of some common alkyl substituents:
The name given to the parent chain of a branched alkane is the same as the name of the corresponding linear alkane. So, for example, a six carbon parent chain is named hexane. Once again, it is very important to learn the names of the linear alkanes.
The name of a branched alkane must also describe the positions of the parent chain which are connected to the substituents. To do this, we must assign numbers to the carbons of the parent chain. Start numbering from the end of the parent chain that gives the smallest numbers possible to the carbons of the parent chain which are bonded to substituents. The parent chains in the above examples would be numbered as follows:
In the first example, we start numbering the parent chain from the right, so that the two methyl groups (blue) are on carbons 2 and 4. If we numbered starting from the left, the methyl groups would be on carbons 3 and 5, not the smallest numbers possible. In the second example, the ethyl group is on carbon 4 regardless of where we start numbering. Thus, we start numbering from the left, so that the methyl group is on carbon 3 instead of carbon 5.
Now that we have covered how to identify and number the parent chain, we can start to put together the name of a branched alkane. These names can be assigned according to the following steps:
- Identify the parent chain.
- Number the carbons of the parent chain.
- List the names of the substituents in alphabetical order.
- Finish with the name of the parent chain.
While we have already gone through the first two steps, it is easiest to go through steps 3 and 4 by way of example. Let’s revisit the structure from above:
We have already identified the parent chain (highlighted in yellow), and numbered the carbons of the parent chain such that the carbons bonded to substituents are given the lowest numbers possible.
In this molecule, there is a methyl substituent on carbon 3, and an ethyl substituent on carbon 4. Therefore, we say that it has a 3-methyl substituent and a 4-ethyl substituent, with hyphens separating the numbers and names. We then list the substituents in alphabetical order (and separate them with a hyphen), giving: 4-ethyl-3-methyl. Finally, we add the name of the parent chain immediately after the names of the substituents (with no spaces or hyphens). The parent chain of this molecule is seven carbons long, which corresponds to heptane. Therefore, the name of this molecule is: 4-ethyl-3-methylheptane.
Some branched alkanes have more than one of a particular substituent… Now, let’s go through these steps again for another branched alkane:
The parent chain (the longest continuous chain of carbons) is already highlighted in yellow. As this parent chain is ten carbons long, this is a decane.
Prefixes are added to the name of a substituent if If a molecule has two or more of a particular substituent, prefixes are used to indicate the number of . The prefix di– is used if there are two of a particular substituent, tri- is used for three, tetra– for four, and so. The numbers of the parent chain carbons bonded to these substituents are Note that, when alphabetizing substituents, these prefixes are ignored.
Now, we are ready to assign names to these branched alkanes.
Next, branched substituents