Structural Isomerism for AP Chemistry

By — McGraw-Hill Professional
Updated on Feb 2, 2011

Practice problems for these concepts can be found at:

Compounds that have the same molecular formulas but different structural formulas are called isomers. With hydrocarbons, this applies to a different arrangement of the carbon atoms. Isomers such as these are called structural isomers. Figure 18.3 shows the structural isomers of C5H12. Note that there are the same number of carbons and hydrogens in each structure. Only the way the carbons are bonded is different.

In writing structural isomers, or any other organic compounds, remember that carbon forms four bonds. One of the most common mistakes that a chemistry student makes is writing an organic structure with a carbon atom having fewer or more than four bonds.

Figure 18.1 Naming an alkane.

Figure 18.2 Naming of another alkane.

Figure 18.3 Structural isomers of C5H12.

Here is a practice problem. Name the following compound:

Structural Isomerism

Answer: 5-ethyl-2,2-dimethylnonane


First, pick the longest chain. This is bold-faced in the diagram below. The carbons are attached by single bonds, so this is an alkane. Because the longest chain has nine carbons, it is a nonane.

Structural Isomerism

Next, the longest chain should be numbered from one end to the other with the lowest number(s) going to the branches. For the above example the numbering of the chain (bold-face carbon atoms) would be:

Once these numbers have been assigned, do not alter them later.

All carbon atoms that are not part of the nine-atom main chain are branches. Branches have -yl endings. It may help you to circle the carbon atoms belonging in the branches. In the above example, there are three branches. Two consist of only one carbon and are called methyl groups. The remaining branch has two carbons, so it is an ethyl group. The branches are arranged alphabetically. If there is more than one of a particular type, use a prefix (di-, tri, tetra-, etc.). The two methyl groups are designated dimethyl. The position of each branch is indicated with a number already determined for the main chain. Each branch must get its own number, even if it is identical to one already used.

In the above example this gives: 5-ethyl-2,2-dimethylnonane

  1. ethyl before methyl (alphabetical—prefixes are ignored)
  2. two methyl groups = dimethyl
  3. three branches = three numbers

Numbers are separated from other numbers by commas, and numbers are separated from letters by a hyphen.

Another type of isomerism is optical isomerism. These molecules are capable of rotating light to either the left or right and are said to be optically active. The presence of an asymmetric or chiral carbon (a carbon atom with four different groups attached to it) will make a compound optically active.

Practice problems for these concepts can be found at:

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