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Not all hydrocarbons are straight chains. Many hydrocarbons have branches of C atoms attached to a chain; they are called branched hydrocarbons. These branched alkanes are isomers of straight-chain alkanes having the same number of C atoms. However, they are different compounds with different physical and chemical properties. As such, they need different names. How do we name branched hydrocarbons?
There are a series of rules for naming branched alkanes (and, ultimately, for all organic compounds). These rules make up the system of nomenclature for naming organic molecules. Worldwide, the International Union of Pure and Applied Chemistry (IUPAC) has developed the system of nomenclature for organic compounds, so these rules are sometimes called the IUPAC rules of nomenclature. By learning and applying these rules, you can name any organic compound when given its structure or determine the unique structure of a molecule from its name. You have already learned the basics of nomenclature—the names of the first 10 normal hydrocarbons. Here, we will add some steps to the procedure so you can name branched hydrocarbons.
First, given the structure of an alkane, identify the longest continuous chain of C atoms; this is known as the parent chain. Note that the longest chain may not be drawn in a straight line. The longest chain determines the parent name of the hydrocarbon. For example, in the molecule shown below, the longest chain of carbons has six C atoms. Therefore, it will be named as a hexane.
However, in this molecule, the longest chain of C atoms is not six, but seven, as shown. So this molecule will be named as a heptane.
The next step is to identify the branches, or substituents, on the main chain. The names of the substituents, or alkyl groups, are derived from the names of the parent hydrocarbons; however, rather than having the ending –ane, the substituent name has the ending –yl (Table 1.2 “Substituent Names for the Five Smallest Substituents.”).
Table 1.2 Substituent Names for the Five Smallest Substituents
| Substituent Formula | Number of C Atoms | Name of Substituent |
|---|---|---|
| CH3 | 1 | methyl– |
| CH3CH2 | 2 | ethyl– |
| CH3CH2CH2 | 3 | propyl– |
| CH3CH2CH2CH2 | 4 | butyl– |
| CH3CH2CH2CH2CH2 | 5 | pentyl– |
| and so forth | and so forth | and so forth |
To name a branched hydrocarbon, the name of the substituent is combined with the parent name of the hydrocarbon without spaces. However, there is likely one more step. The longest chain of the hydrocarbon must be numbered, and the locant (numerical position of the substituent) must be included to account for possible isomers. As with double and triple bonds, the main chain is numbered to give the substituent the lowest possible number. For example, in the alkane shown here, the longest chain is five C atoms long, so it is a pentane.
There is a one-carbon substituent on the third C atom, so there is a methyl group at position 3. We indicate the position using the number, which is followed by a hyphen, the substituent name, and the parent hydrocarbon name—in this case, 3-methylpentane. That name is specific to that particular hydrocarbon and no other molecule. Organic chemistry nomenclature is very specific following the general format shown in Figure 1.3 “IUPAC Nomenclature Guide.”
Figure 1.3 IUPAC Nomenclature Guide
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