5 Steps to a 5 AP Chemistry

270  Step 4. Review the Knowledge You Need to Score High

5. The last substituent group becomes a part of the base name as a prefix.

Studying Figures 18.1 and 18.2 may help you learn the naming of substituted alkanes.

Structural Isomerism

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 C 5 H 12. 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.

CH 3 CH 2 CH 2 CH CH 2 CH 3

CH 2 CH 3

CH 3

CH

CH 3 CH 2 CH 2 CH CH 2 CH 3

CH 2

ethyl group

CH 3

CH 3

CH

765432 1 heptane

methyl group

4-ethyl-2-methylheptane

Figure 18.1 Naming an alkane.

CH 3 CH 2 CH 2 CH 2 C CH 2 CH 2 CH 3

CH 2

CH 2

CH 3

CH 3

CH 3

CH 2

CH 2

C

CH 3 CH 2 CH 2 CH 2 C CH 2 CH 2 CH 3

CH 2

CH 2

CH 3

CH 3

CH3

CH 2

CH 2

C

ethyl

96543218 7

5-ethyl-3,3-dimethyl-5-propylnonane

methyl

methyl

nonane

propyl

Figure 18.2 Naming of another alkane.