CH3 D isomer Enantiomers differ in spatial arrangement around an asymmetric carbon, resulting in molecules that are mirror images, like left and right hands. The two isomers are designated the L and D isomers from the Latin for left and right (levo and dextro). Enantiomers cannot be superimposed on each other.

DRAW IT There are three structural isomers of C5H12; draw the one not shown in (a). istry of vision involves a light-induced change of retinal, a chemical compound in the eye, from the cis isomer to the trans isomer (see Figure 50.17). Another example involves trans fats, which are discussed in Chapter 5. Enantiomers are isomers that are mirror images of each other and that differ in shape due to the presence of an asymmetric carbon, one that is attached to four different atoms or groups of atoms. (See the middle carbon in the ball-and-stick models shown in Figure 4.7c.) The four groups can be arranged in space around the asymmetric carbon in two different ways that are mirror images. Enantiomers are, in a way, left-handed and right-handed versions of the molecule. Just as your right hand wont t into a left-handed glove, a right-handed molecule wont t into the same space as the left-handed version. Drug Condition Effective Enantiomer Ineffective Enantiomer Ibuprofen

Pain; inflammation S-Ibuprofen R-Ibuprofen Albuterol Asthma R-Albuterol S-Albuterol (cid:2) Figure 4.8 The pharmacological importance of enantiomers. Ibuprofen and albuterol are examples of drugs whose enantiomers have different effects. (S and R are letters used in one system to distinguish between enantiomers.) Ibuprofen reduces inammation and pain. It is commonly sold as a mixture of the two enantiomers. The S enantiomer is 100 times more effective than the other. Albuterol is used to relax bronchial muscles, improving airow in asthma patients. Only R-albuterol is synthesized and sold as a drug; the S form counteracts the active R form. Usually, only one isomer is biologically active because only that form can bind to specic molecules in an organism.

The concept of enantiomers is important in the pharmaceutical industry because the two enantiomers of a drug may not be equally effective, as is the case for both ibuprofen and the asthma medication albuterol (Figure 4.8). Methamphetamine also occurs in two enantiomers that have very different effects. One enantiomer is the highly addictive stimulant drug known as crank, sold illegally in the street drug trade. The other has a much weaker effect and is even found as an ingredient in an over-the-counter vapor inhaler for treatment of nasal congestion! The differing effects of enantiomers in the body demonstrate that organisms are sensitive to even the most subtle variations in molecular architecture. Once again, we see that molecules have emergent properties that depend on the specic arrangement of their atoms. C O N C E P T C H E C K 4.2 1. 2. Which molecules in Figure 4.5 are isomers? For each