How to Identify and Order the Substituents in a Branched Alkane - dummies

How to Identify and Order the Substituents in a Branched Alkane

By Arthur Winter

When naming a branched alkane, after you locate and number the parent (longest) chain, you need to locate and identify all the substituents that stick off of the parent chain, and then order the substituents alphabetically in front of the parent chain.

Locate and identify the substituents in a branched alkane

For example, say you have a heptane molecule with two substituents coming off the parent chain — one substituent at carbon number three, and one substituent at carbon number four, as shown here.

The locations of substituents on a parent chain.
The locations of substituents on a parent chain.

Substituents are named in the same manner as the parent chains, except that instead of ending with the suffix –ane they end with the suffix –yl, which indicates that the group is a substituent off the main chain. For example, the one-carbon substituent at carbon number three is a methyl substituent (not a methane substituent). A two-carbon substituent would be ethyl, a three-carbon substituent would be propyl, a four-carbon substituent would be butyl, and so on.

Some complex substituents have common names rather than systematic ones. These simply must be memorized. The most important common substituents are the isopropyl group (a three-carbon group that looks like a snake’s tongue), the tert-butyl (or t-butyl) group and the sec-butyl group, all shown here.

The common names of some substituents.
The common names of some substituents.

In the example, the substituent at carbon number four is, in fact, an isopropyl group. Now you have the name of the parent chain and the names of all the substituents. The one-carbon substituent at carbon three is a methyl group, and the three-carbon substituent at carbon four is an isopropyl group. Now you just have to put it all together!

Order the substituents in a branched alkane

The next step is to order the substituents alphabetically in front of the parent name, using numbers to indicate the location of the substituents. Because i comes before m in the alphabet, the isopropyl group is placed in front of the methyl group in the name of the molecule: 4-isopropyl-3-methylheptane. Note that dashes are used to separate the numbers from the substituents, and that there is no space between the last substituent and the name of the parent chain.

Of course, there’s always a stick to throw into the spokes. One quirk involving the common names of tert-butyl and sec-butyl substituents comes when placing them in alphabetical order, as the tert and sec portions of the name are ignored. In other words, tert-butyl would be ordered as if it started with the letter b, the same as with sec-butyl. Isopropyl, however, is alphabetized normally, under the letter i.