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Alkanes and the Lowest Combination of Numbers Rule

Master the basics of organic chemistry nomenclature with Dr. B's Chemistry Confidential blog. Learn the lowest combination of numbers rule for alkanes.

No matter how big the alkane, you can name it systematically, uniquely and unambiguously using IUPAC nomenclature. These are the foundation for all organic chemistry nomenclature. We’ll cover the basics here but go to Unit 3 for more details and some great sample and practice problems.

The basic rules are:

  1. Find the parent
  2. Find the substituents (i.e. branches) and identify them. (We circle them for clarity)
  3. Put the substituent names (in alphabetical order) in front of the parent name.
  4. Number to give the lowest combination of numbers.

First find the parent which is the longest chain. It may go left to right but is may snake through the molecule. In the molecule below the longest chain is 9 carbons long so the parent is nonane.

Once you identify the parent, circle the substituents and identify them. If you have more than one of each substituent then use the prefix di-, tri, tetra-. Here there are 2 methyls so we call them dimethyl. We call this molecule dimethylnonane (no space between the last substituent and parent).

But where are the methyls? There are many dimethylnonanes. Number substituents to give the lowest combination of numbers. The easiest way to do this is to number in both directions of the parent and compare the 2 possibilities to look for the 1st point of difference in the numbers. The 2 competing schemes are 3,6 vs. 4,7. 3 beats 4 at the 1st number (i.e. the 1st point of difference) so 3,6 wins.

In the molecule above the parent (i.e the longest chain) is 10 carbons long so it’s a decane. It has an ethyl, an isopropyl and a methyl in alphabetical order. But where are the substitutents? Numbering from the left we get 4,6,9 and from the right we get 2,5,7. 2 beats 4 at the 1st number (1st point of difference) and wins. This is a considerably larger molecule but we have named it easily.

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