How to Identify Multiple or Complex Substituents in a Branched Alkane

When searching for the substituents in a branched alkane, you may run across some situations where you aren't sure how to proceed. For example, what do you do when there are two or more instances of the same substituent? And what if a complex substituent appears on one of the branches?

When naming a branched alkane, you first need to locate and number the parent (longest) chain. You must then locate and identify all the substituents that stick off of the parent chain, including multiple and complex branches. Finally, you order the substituents alphabetically in front of the parent chain to construct the name of the branched alkane.

Identify and name multiple instances of the same constituent

What if the molecule contains more than one of the same substituent? For example, what if the compound has two methyl-group substituents on the molecule? In such a scenario, you don't name each of the methyl groups individually. Instead, you put a prefix in front of a single substituent name to indicate the number of these substituents that the molecule contains. (The following table contains a list of prefixes.) For example, if the molecule has two separate methyl groups, the prefix di– goes in front of the name methyl, making the substituent name dimethyl. In addition, two numbers are placed in front of the substituent name to show the locations of the two methyl groups on the parent chain.

Students often forget that for every substituent you must explicitly list a number, even if the substituents are attached to the same carbon. If the molecule has three methyl groups coming off the parent chain, the substituent name is trimethyl, if it has four methyls, the name is tetramethyl, if it has five methyls, the name is pentamethyl, and so on.

These prefixes are ignored when placing the substituents in alphabetical order (dimethyl would be alphabetized under m, for example). This figure shows two examples of molecules that have identical substituents. Note the use of commas to separate the numbers preceding the substituent name.

Identify and name complex substituents

Sometimes, substituents will branch and will have no common name, like the substituent shown here: In such a case, you name the substituent just as if it were a separate alkane, but with the parent name of the substituent ending with the suffix –yl instead of –ane, indicating that this portion of the molecule is a substituent off the main chain.

The main catch in naming complex substituents, however, is that the carbon that attaches the substituent to the parent chain must be the number-one carbon; thus, when you number the carbons in the substituent chain, you force the carbon that attaches to the main chain to be carbon number one. (The next figure shows an example of how this works.)

Because the complex substituent shown here is three carbons long and has methyl groups at carbons one and three, the complex substituent is named 1,2-dimethylpropyl.

Because you're naming a substituent off the parent chain, the name must end with the suffix –yl, not –ane.

Now you can put all the parts of the name together. By convention, when a complex substituent is included in a name, the name of the complex substituent is set off with parentheses. So the name of the molecule in this example is 5-(1,2-dimethylpropyl)-nonane.