Enantiomers bromochlorofluoromethane

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 enantiomer... 

CFlBrClF (bromochlorofluoromethane) dipole moment, 99ff enantiomers, 79 symmetry elements, 79ff CF12F2 (difluoromethane) cartesian axes, 89 symmetry elements, 77, 83 CF13F (methyl fluoride) dipole moment, 116 symmetry elements, 74, 83 C Fl3F (methyl fluoride) vibration-rotation band, 178... 

The minor images of bromochlorofluoromethane have the sane constitution. That is, the atoms are connected in the sane order. But they differ in the anangement of then-atoms in space they are stereoisomers. Stereoisomers that are related as an object and its nonsuperimposable minor image are classified as enantiomers. The word enantiomer describes a paiticulai- relationship between two objects. One cannot look at a single molecule 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 Fuithennore, just as an object has one, and only one, minor image, a chiral molecule can have one, and only one, enantiomer. 

Let s return to bromochlorofluoromethane as a simple example of a chiral molecule. The two enantiomers of BrCIFCH are shown as ball-and-stick models, as wedge-and-dash drawings, and as Fischer projections in Figure 7.6. Fischer projections are always generated the same way the molecule is oriented so that the vertical bonds at the chirality center are directed away from you and the horizontal bonds point toward you. A projection of the bonds onto the page is a cross. The chirality center lies at the center of the cross but is not explicitly shown. 

FIGURE 7.6 Ball-and-spoke models (/eft), wedge-and-dash drawings (center), and Fischer projections (right) of the R and S enantiomers of bromochlorofluoromethane. 

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 original structure, albeit in a different orientation. 

Though this method has given satisfactory results in many cases, it has the disadvantage of depending on functional groups (acid function). However, beside salt formation in some cases formation of clathrates of alkaloids plays a decisive role in racemate resolution processes. As an example the enantiomeric enrichment of bromochlorofluoromethane (CHClBrF) was achieved successfully by means of brucine Typically enough measurement of the enantiomeric excess (ee) of this interesting anaesthetic was achieved by differentiation of the enantiomers in a cavity of the cyclophane type (cf. section 2.6). 

PRACTICE PROBLEM 5.9 Write the enantiomeric forms of bromochlorofluoromethane and assign each enantiomer... 

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