Stereoisomers

Two molecules may have exactly the same atoms arranged in exactly the same sequence yet differ with respect to the spatial orientation of a key functional group. Such molecules are called stereoisomers of each other. Depending upon the direction in

■ Figure 2.10 Various functional groups of organic molecules. The general symbol for a functional group is "R."

0 II

O ||

CH3—C — CH3

CH3—C—OH

Ketone

Organic acid

II

H

CH3—C—H

CH3—C—OH |

H

Aldehyde

Alcohol

■ Figure 2.11 Categories of organic molecules based on functional groups. Acids, alcohols, and other types of organic molecules are characterized by specific functional groups.

Chemical Composition of the Body 31

■ Figure 2.12 The carboxyl group of an organic acid. This group can ionize to yield a free proton, which is a hydrogen ion (H+). This process is shown for lactic acid, with the double arrows indicating that the reaction is reversible.

Severe birth defects often resulted when pregnant women used the sedative thalidomide in the early 1960s to alleviate morning sickness. The drug available at the time contained a mixture of both right-handed (D) and left-handed (L) forms. This tragic circumstance emphasizes the clinical importance of stereoisomers. It has since been learned that the L-stereoisomer is a potent tranquilizer, but the right-handed version causes disruption of fetal development and the resulting birth defects. Interestingly, thalidomide is now being used in the treatment of people with AIDS, leprosy, and cachexia (prolonged ill health and malnutrition).

Test Yourself Before You Continue

1. List the components of an atom and explain how they are organized. Explain why different atoms are able to form characteristic numbers of chemical bonds.

2. Describe the nature of nonpolar and polar covalent bonds, ionic bonds, and hydrogen bonds. Why are ions and polar molecules soluble in water?

3. Define the terms acidic, basic, acid, and base. Also define pH and describe the relationship between pH and the H+ concentration of a solution.

4. Using chemical equations, explain how bicarbonate ion and carbonic acid function as a buffer pair.

5. Explain how carbon atoms can bond with each other and with atoms of hydrogen, oxygen, and nitrogen.

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