CHAPTER SEVEN Stereochemistry Thomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if It Is rposable on its mirror image. Applying Thomson's term to chemistry, we sa that a molecule is chiral if its two mirror-image forms are not superposable in three dimen sions. The ork"chiral"is derived from the Greek word cheir meaning "hand, and it is entirely appropriate to speak of the"handedness"of molecules. The opposite of chiral is achiral. A molecule that is superposable on its mirror image is achiral. In organic chemistry, chirality most often occurs in molecules that contain a car- bon that is attached to four different groups. An example is bromochlorofluoromethane (BrCIFCH). is a known compound, and H been described in the chemi- described a method for the on of brClfCh that Bromochlorofluoromethane predominantly one enan tiome As shown in Figure 7. 1. the two mirror images of bromochlorofluoromethane cannot be uperposed on each other. Since the two mirror images of bromochlorofuoromethane a not superposable, BrCIFCH is chiral The two mirror images of bromochlorofuoromethane have the same constitution that is, the atoms are connected in the same order. But they differ in the arrangement of their atoms in space; they are stereoisomers. Stereoisomers that are related as an object and its nonsuperposable mirror image are classified as enantiomers. The word"enantiomer describes a particular relationship between two objects. One cannot look at a single mole- cule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, "ls that person a cousin? "Furthermore, just as an object has one, and only one, mirror image, a chiral molecule can have one, and only one, enantiomer. Notice in Figure 7.1c, where the two enantiomers of bromochlorofuoromethane are similarly oriented, that the difference between them corresponds to an interchange of the positions of bromine and chlorine. It will generally be true for species of the type C(w x, y, z), where w, x, y, and z are different atoms or groups, that an exchange of two of them converts a structure to its enantiomer but an exchange of three returns the orig inal structure. albeit in a different orientation. Consider next a molecule such as chlorodifluoromethane(Cif,Ch). in which two of the atoms attached to carbon are the same. Figure 7.2 on page 262 shows two molecular models of CIF2CH drawn so as to be mirror images. As is evident from these drawings, it is a sim- ple matter to merge the two models so that all the atoms match. Since mirror-image repl sentations of chlorodifluoromethane are superposable on each other, CIF2 CH is achiral The surest test for chirality is a careful examination of mirror-image forms for superposability. Working with models provides the best practice in dealing with mole cules as three-dimensional objects and is strongly recommended. 7.2 THE STEREOGENIC CENTER As we' ve just seen, molecules of the general type Back Forward Main MenuToc Study Guide ToC Student o MHHE WebsiteThomson (Lord Kelvin) coined a word for this property. He defined an object as chiral if it is not superposable on its mirror image. Applying Thomson’s term to chemistry, we say that a molecule is chiral if its two mirror-image forms are not superposable in three dimen￾sions. The work “chiral” is derived from the Greek word cheir, meaning “hand,” and it is entirely appropriate to speak of the “handedness” of molecules. The opposite of chiral is achiral. A molecule that is superposable on its mirror image is achiral. In organic chemistry, chirality most often occurs in molecules that contain a car￾bon that is attached to four different groups. An example is bromochlorofluoromethane (BrClFCH). As shown in Figure 7.1, the two mirror images of bromochlorofluoromethane cannot be superposed on each other. Since the two mirror images of bromochlorofluoromethane are not superposable, BrClFCH is chiral. The two mirror images of bromochlorofluoromethane have the same constitution. That is, the atoms are connected in the same order. But they differ in the arrangement of their atoms in space; they are stereoisomers. Stereoisomers that are related as an object and its nonsuperposable mirror image are classified as enantiomers. The word “enantiomer” describes a particular relationship between two objects. One cannot look at a single mole￾cule in isolation and ask if it is an enantiomer any more than one can look at an individual human being and ask, “Is that person a cousin?” Furthermore, just as an object has one, and only one, mirror image, a chiral molecule can have one, and only one, enantiomer. Notice in Figure 7.1c, where the two enantiomers of bromochlorofluoromethane are similarly oriented, that the difference between them corresponds to an interchange of the positions of bromine and chlorine. It will generally be true for species of the type C(w, x, y, z), where w, x, y, and z are different atoms or groups, that an exchange of two of them converts a structure to its enantiomer, but an exchange of three returns the orig￾inal structure, albeit in a different orientation. Consider next a molecule such as chlorodifluoromethane (ClF2CH), in which two of the atoms attached to carbon are the same. Figure 7.2 on page 262 shows two molecular models of ClF2CH drawn so as to be mirror images. As is evident from these drawings, it is a sim￾ple matter to merge the two models so that all the atoms match. Since mirror-image repre￾sentations of chlorodifluoromethane are superposable on each other, ClF2CH is achiral. The surest test for chirality is a careful examination of mirror-image forms for superposability. Working with models provides the best practice in dealing with mole￾cules as three-dimensional objects and is strongly recommended. 7.2 THE STEREOGENIC CENTER As we’ve just seen, molecules of the general type x z w C y Cl±C±Br H W W F Bromochlorofluoromethane 260 CHAPTER SEVEN Stereochemistry Bromochlorofluoromethane is a known compound, and samples selectively enriched in each enantiomer have been described in the chemi￾cal literature. In 1989 two chemists at Polytechnic Uni￾versity (Brooklyn, New York) described a method for the preparation of BrClFCH that is predominantly one enan￾tiomer. Back Forward Main Menu TOC Study Guide TOC Student OLC MHHE Website