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Priority Rules: How to Name Compounds with 2 or More Functional Groups

Learn how to name compounds with multiple functional groups using priority rules in Dr. B's Chemistry Confidential blog.

When a molecule has several types of functional groups, only one can be the parent. The priorities of functional groups we will cover in this section are:

  1. Alcohols (highest)
  2. Alkenes
  3. Alkanes (lowest)

A molecule is an alkane (by default) only if it has no other functional groups.

If a molecule contains only a double bond, the basic rules change.

  1. The parent must include the alkene whether it is the longest chain or not.
  2. The alkene must get the lowest possible number and all alkyl substituents get whatever number results for this.
  3. The corresponding alkane name is changed from -ane to -ene and the number of the alkene goes before the whole parent name (Old IUPAC) or directly before the -ene (New IUPAC).

If the molecule is an alcohol and an alkene, the hydroxyl is the most important and highest priority of all. The rules change as follows:

  1. The parent must include the alcohol. The corresponding alkane name is changed from -e to -ol.
  2. If ther is also a double bond, change the parent name from -anol to -enol.
  3. The alcohol must get the lowest number possible because it has highest priority. Any alkyls or double bonds get whatever number results from this. The number for the alcohol goes directly before the -ol. The double bond number goes before the entire parent name (Old IUPAC) or right before the -en (new IUPAC.)

In the molecule below, the alcohol defines the parent and the numbering can only go in the direction to give the hydroxyl the lowest possible number (2). The alkene(4) and methyl (5) positions simply result from this assignment.

We learn the IUPAC nomenclature of more functional groups in Unit 3 (alkynes and alkyl halides) and Unit 14 in my second book (aromatic molecules, carbonyl molecules and amines). We discuss all the priorities in Unit 14. We always build on the basic IUPAC rules we learned here. In each case you get more detailed rules, sample problems that show you the process and practice problems with answers.




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