6 Principles of Stereochemistry - Sapling Learning

1 22666 This chapter and the one that follows deal with stereoisomers and their properties. Stereoiso-mersare compounds that have the same atomic connectivity but a different arrangement ofatoms in space. Recall that E and Zisomers of an alkene (Sec. ) are stereoisomers. In thischapter we ll learn about other types of study of stereoisomers and the chemical effects of stereoisomerism is called few ideas of Stereochemistry were introduced in Sec. This chapter delvesmore generally into Stereochemistry by concentrating on the basic definitions and l l s e e h ow s t e r e o c h e m i s t r y p l a y e d a key r o l e i n t h e d e t e r m i n a t i o n o f t h e g e o m e t r y o ftetravalent carbon. Chapter 7 continues the discussion of Stereochemistry by considering boththe stereochemical aspects of cyclic compounds and the application of stereochemical princi-ples to chemical use of molecular models during the study of this chapter is essential.

2 Models will helpyou develop the ability to visualize three-dimensional structures and will make the two-di-mensional pictures on the page come to life. If you use models now, your reliance on themwill gradually , CHIRALITY, AND SYMMETRYA. Enantiomers and ChiralityAny molecule indeed, any object has a mirror image. Some molecules are congruenttotheir mirror images. This means that all atoms and bonds in a molecule can be simultaneouslysuperimposed with identical atoms and bonds in its mirror image. An example of such a mol-ecule is ethanol, or ethyl alcohol, H3 CLCH2 LOH (Fig. ). Construct a model of ethanoland another model of its mirror image, and use the following procedure to show that these twomodels are congruent. For simplicity, use a single colored ball to represent the methyl groupand a single ball of another color to represent the hydroxy (LOH) group. Place the twocentral carbons side by side and align the methyl and hydroxy groups, as shown in Fig.

3 12/5/08 2:54 PM Page ENANTIOMERS, CHIRALITY, AND SYMMETRY227 The hydrogens should then align as well. The congruence of an ethanol molecule and its mir-ror image shows that they are molecules, such as 2-butanol, are notcongruent to their mirror images (Fig. ,p. 228).Build a model of 2-butanol and a second model of its mirror image. If you align the carbonwith the asterisk and any two of its attached groups, the other two groups do not align. Hence,a 2-butanol molecule and its mirror image are noncongruent and are therefore different these two molecules have identical connectivities, then by definition they arestereoisomers. Molecules that are noncongruent mirror images are called ,the two 2-butanol stereoisomers are must not only be mirror images; they must also be noncongruentmirror im-ages. Thus, ethanol (Fig. ) has no enantiomer because an ethanol molecule and its mirrorimage are (or other objects) that can exist as enantiomers are said to be chiral(pronouncedk - ru l); they possess the property of chirality,or handedness.

4 (Chiralcomes from the Greekword for hand.) Enantiomeric molecules have the same relationship as the right and left2-butanolH3 CCH"OHLC2H5L*mirror planeall groups aligntherefore moleculesare congruent180 HCH3 OHCHHCH3 OHCHCH3 HOHHCHCH3 OHCHF igure mirror-image ethanol molecules for congruence. One mirror image is shown with yellowbonds to distinguish it from the other. Aligning the central carbons, the CH3groups, and the OH groups on the dif-ferent molecules causes the hydrogens to align as well. Notice that this alignment requires rotating one of themolecules in 12/5/08 2:54 PM Page 227228 CHAPTER 6 Principles OF Stereochemistry hands the relationship of an object and its noncongruent mirror image. Thus, 2-butanol is achiral molecule. Molecules (or other objects) that are not chiral are said to be achiral with-out chirality. Ethanol is an achiral molecule.

5 Both chiral and achiral objects are matters ofeveryday acquaintance. A foot or a hand is chiral; the helical thread of a screw gives it chiral-ity. Achiral objects include a ball and a soda of ChiralityChiral molecules occur widely throughout all of nature. For example, glucose, an important sugarand energy source, is chiral; the enantiomer of naturally occurring glucose cannot be utilized as afood source. All sugars, proteins, and nucleic acids are chiral and occur naturally in only one enan-tiomeric form. Chirality is important in medicine as well. Over half of the organic compounds used asdrugs are chiral, and in most cases only one enantiomer has the desired physiological activity. In rarecases, the inactive enantiomer is toxic (see the story of the drug thalidomide in Sec. ). The safetyand effectiveness of synthetically prepared chiral drug molecules have become issues of increasingconcern for both pharmaceutical manufacturers and the Food and Drug Administration (FDA).

6 60 align central carbon,OH, andLC2H5 Lmirror planethese groups fail to aligntherefore, molecules are noncongruentC2H5 OHCHCH3C2H5 OHCHCH3C2H5 CHOHCH3C2H5 HCOHCH3 Figure mirror-image 2-butanol molecules for congruence. As in Fig. , the bonds of one mirrorimage are yellow. When the central carbon and any two of the groups attached to it (OH and C2H5in this figure)are aligned, the remaining groups do 12/5/08 2:54 PM Page ENANTIOMERS, CHIRALITY, AND SYMMETRY229B. Asymmetric Carbon and StereocentersMany chiral molecules contain one or more asymmetric carbon atoms. An asymmetric car-bon atomis a carbon to which four different groupsare bound. Thus, 2-butanol (see Fig. ),a chiral molecule, contains an asymmetric carbon atom; this is the carbon that bears the fourdifferent groups LCH3, LC2H5, LH, and LOH. In contrast, none of the carbons of ethanol,an achiral molecule, is asymmetric.

7 A molecule that contains only one asymmetric carbon ischiral. No generalization can be made, however, for molecules with more than one asymmet-ric carbon. Although many molecules with two or more asymmetric carbons are indeed chiral,not all of them are (Sec. ). Moreover, an asymmetric carbon atom (or other asymmetricatom) is not a necessarycondition for chirality; some chiral molecules have no asymmetriccarbons at all (Sec. ). Despite these caveats, it is important to recognize asymmetric carbonatoms because so many chiral organic compounds contain Problem the asymmetric carbon(s) in 4-methyloctane:SolutionThe asymmetric carbon is marked with an asterisk:This is an asymmetric carbon because it bears four different groups: H, CH3, CH3CH2CH2, andCH2CH2CH2CH3. Notice that the propyl and butyl groups are notdifferent at the point of attach-ment both have CH2groups at that point, as well as at the next carbon removed.

8 The differenceis found at the ends of the groups. The point is that two groups are different even when the differ-ence is remote from the carbon in asymmetric carbon atom is another type of stereocenter,or stereogenic atom. Recall(Sec. ) that a stereocenteris an atom at which the interchange of two groups gives astereoisomer. In Fig. , for example, interchanging the methyl and ethyl groups in one enan-tiomer of 2-butanol gives the other enantiomer. If this point is unclear from Fig. , use mod-els to demonstrate this to yourself. To do this, you need to build two models. First construct amodel of either enantiomer, and then construct a model of its mirror image. Then show thatthe interchange of anytwo groups on one model gives the other all carbon stereocenters are asymmetric carbons. Recall (Sec. ) that the carbonsinvolved in the double bonds of Eand Zisomers are also stereocenters.

9 These carbons are notasymmetric carbons, though, because they are not connected to four different groups. In otherwords, the term stereocenteris not associated solely with chiral molecules. All asymmetricatoms are stereocenters, but not all stereocenters are asymmetric Chirality and SymmetryWhat causes chirality? Chiral molecules lack certain types of symmetry. The symmetry of anyobject (including a molecule) can be described by certain symmetry elements,which are lines,points, or planes that relate equivalent parts of an object. A very important symmetry elementis a plane of symmetry,sometimes called an internal mirror is a plane thatCH3CH2CH2 CHCH2CH2CH2CH3"CH3*CH3CH2CH2 CHCH2CH2CH2CH3"CH3 STUDY GUIDE LINK AsymmetricCarbons in RingsSTUDY GUIDE LINK andAsymmetric 12/5/08 2:54 PM Page 229230 CHAPTER 6 Principles OF Stereochemistry divides an object into halves that are exact mirror images.

10 For example, the mug in Fig. hasa plane of symmetry. Similarly, the ethanol molecule shown in Fig. also has a plane ofsymmetry. A molecule or other object that has a plane of symmetry is achiral. Thus, the ethanolmolecule and the mug in Fig. are achiral. Chiral molecules and other chiral objects do nothave planes of symmetry. The chiral molecule 2-butanol, analyzed in Fig. , has no plane ofsymmetry. A human hand, also a chiral object, has no plane of important symmetry element is the center of symmetry, sometimes also called apoint of molecule has a center of symmetry if you can reproduce it by first form-ing its mirror image and then rotating this mirror image by 180 about an axis perpendicularto the mirror (Fig. ). More descriptively, a center of symmetry is a point through whichanyline contacts exactly equivalent parts of the object at the same distance in both directions(Fig.)