1. Find the longest continuous chain of carbons in the molecule.
This chain is used as the parent structure. In the example, the longest continuous chain contains seven carbons. Therefore the parent hydrocarbon structure is heptane.
2. Number the carbons in the main chain in sequence.
In doing this start at the end that will give the groups attached to the chain the smallest numbers. This has already been done in the preceding example. In this instance the numbers go from right to left, which places the substituent groups at carbon atoms 2, 3, and 4. If the chain were numbered from left to right, the groups would be at positions 4, 5, and 6. These are higher numbers and therefore violate the rule.
3. Add numbers to the names of the substituent groups to identify their positions on the chain.
These numbers become prefixes to the name of the parent alkane. In this example the substituents and positions are 2‑methyl, 3‑methyl, and 4‑ethyl.
4. Use prefixes to indicate the appearance of a group more than once in the structure.
Common prefixes are di (twice), tri (three times), tetra (four), and penta (five). This example has two methyl substituents. Thus the word dimethyl will be part of the complete name.
5. List the names of alkyl substituents in alphabetical order.
For purposes of alphabetizing, the prefixes di, tri, and so on are ignored. In this example, the 4‑ethyl group is listed before the 2‑methyl and 3‑methyl groups (which are combined as 2,3‑dimethyl in the name).
6. Use proper punctuation.
This is very important in writing the names of organic compounds in the IUPAC system. Commas are used to separate numbers. Hyphens are used to separate numbers and words. The name of the alkane is written as one word. The demonstration compound, then, would be written as dimethylheptane, not dimethyl heptane.
According to the IUPAC rules, the name of this compound is 4‑ethyl‑2,3‑dimethylheptane.
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