5 Steps to a 5 AP Chemistry 2019

Organic Chemistry ❮ 273

HC

H

H

C

H

H

C

H

H

C

H

H

HC

H

H

CC

H

H

C H

H

H

HHC

H

H

C

C

H

C

H

H

H

H

H H C

H

H

H

HHC

Pentane 2-methylbutane 2,2-dimethylpropane

C

H

H

H

Figure 18.3 Structural isomers of C 5 H 12.

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.

Here is a practice problem. Name the following compound:

H

H HCHH HHHH

C C C H

C

C C C

C

H C

H H H

H

HH H

HH

H H

HH H

C C C

H HH

Answer: 5-ethyl-2,2-dimethylnonane

Solution:

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.

H

H

H

HHCHHHHC HH H

H H

H

H CCC H

H H H

CCCCC CCH

HCHHHHHH