- Molecules are named systematically using the IUPAC system of nomenclature
- The longest continuous carbon chain determines the root
- The suffix for alkanes is -ane
- Branched substituents are identified at the front of the name in alphabetical order
We will begin nomenclature by looking at IUPAC nomenclature of hydrocarbons, which are molecules that contain only carbon and hydrogen. All IUPAC hydrocarbon nomenclature is structured to include three fundamental sections: the root of the molecule, the hydrocarbon suffix, and the branching substituents. The central part of the name is the root, which identifies the longest, unbroken chain of carbon atoms in the molecule. The hydrocarbon suffix identifies whether the molecule has no multiple bonds (alkane), double bonds (alkene), or triple bonds (alkyne). The branching substituent(s) identify any carbon branches off the main root chain.
The tables below provide the fundamentals of all three components of hydrocarbon nomenclature, including the format for branching substituent nomenclature, the root name, as well as the suffixes for alkanes, alkenes, and alkynes.
The grey box below outlines the six basic rules when constructing a name for a simple hydrocarbon. Examples of how to apply each of the rules are included below.
- Find the longest continuous chain of carbon atoms (parent chain). This chain determines the parent name (root) of the alkane.
- Identify the hydrocarbon suffix.
- Number the parent chain. For simple alkanes, begin the numbering at end of the chain that is nearest to a branch. For alkenes and alkynes, begin numbering at the end that is nearest to the multiple bond.
- Name the substituent groups (branches). Each substituent name has two components: a root and a suffix. Use the table above to determine the root name based on the longest chain of carbons in the branch. The suffix for alkyl substituent groups is “-yl”.
- If an alkyl substituent appears more than once, use the prefixes: di, tri, tetra, penta, hexa, etc.
- Complete the name. Use the numbers from step 3 to designate the location of the substituents groups. Numbers are separated from letters by a dash (-) and two numbers are separated by a comma (,). If two or more substituents are present on the same carbon atom, use the number twice. List the groups alphabetically (alphabetize based on the root group from step 4, not the prefixes from step 5).
Applying rules 1 and 2:
The simplest, unbranched hydrocarbons can be named using only the first two rules. In the example below, there are eight carbons in the longest continuous chain. Therefore, the root is “oct”. There are no double or triple bonds, so the molecule is an alkane, which uses the hydrocarbon suffix “ane”. Putting the two parts together, the name of the molecule below is octane.
Applying rules 3, 4, and 6:
If a molecule has a branch, such as in the example below, we also need rules 3,4, and 6. The first step is to determine the longest continuous carbon chain, which is seven. As there are no double bonds, the root and suffix combine to make a “heptane”.
This name does not account for the branching group, so we next need to go to rules 3 and 4. If you number from left to right, the branching group is at the 5-position, while if you number from right to left, the branching group is at the 3-position. According to rule 3, the second option is correct. The branching group has one carbon, giving it the root of “meth” with the suffix of “yl”, according to rule 4. All together, there is a methyl group at the 3 position. Using the punctuation rules outlined in rule 6, the name of this molecule is “3-methylheptane”.
Applying rules 5 and 6:
The molecule below requires us to use all of the IUPAC rules. The first step is to find the longest continuous carbon chain. In the example below, the longest carbon chain is eight, making the root “oct”. As there are no multiple bonds, the root and suffix are “octane”.
This chain would generally be numbered to give the first substituent (ie the one closest to an end of the root) the lowest number possible. However, whether we number the chain from the left side or the right side, the first substituent is attached at carbon 3-. Thus, consider the next-closest branch. In both cases, the next-closest branch is attached at carbon 4-, so there is still a tie. The third-closest substituent would be attached at carbon 4- if we number from the left and at carbon 5- if we number from the right. Numbering from the left is correct because it gives the lower number to the substituents at the first point of difference (ie the third substituent is attached at carbon 4- instead of carbon 5-).
According to rules 4 and 5, there are three ethyl groups (i.e. triethyl) and 2 methyl groups (i.e. dimethyl). According to rule 6, the prefixes are not used to alphabetize the substituents, so the name is ``3,4,5-triethyl-4,6-dimethyloctane``.
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