Organic Chemistry

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Organic Chemistry - Ch 11 539 Daley & Daleyand the fluorine together. As you move the two structures together tosuperimpose them, note that the bromine and the chlorine point inopposite directions. Next flip the molecules so that the bromine andchlorine are lined up, but now the hydrogen and fluorine point inopposite directions. No matter how you twist, slide or turn thestructures, you cannot obtain an overlap of the two isomers. You canmake any two of the atoms coincide, but then the other two do not.The structures are asymmetric because they are nonsuperimposible.Mirror planeHFCClBrClBrCHFMirror images of bromochlorofluoromethaneHFHCFCl BrCBr ClAttempting to superimpose the enantiomers of bromochlorofluoromethaneExercise 11.2Construct each of the following molecules and its mirror image. Thendetermine whether one is superimposible upon the other.a)H 3 CHCCH 3CH 2 Clb)HOCH 3c)OHd)CH 3CH 3e)HBrCCH 3f)CH 2 ClCH 3CH 3www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 540 Daley & DaleySample solutiona) The molecule is symmetrical; therefore the mirror images aresuperimposible. A plane of symmetry exists between the two methylgroups.CH 3CCH 3H CH2Cl11.2 Nomenclature of StereocentersThe Cahn-Ingold-Prelog convention is amethod of designatingthe placement of groupsabout a stereogeniccenter.After examining several pairs of enantiomers in Section 11.1,you should now know that the structure of each enantiomer within apair is different. However, based on the rules of nomenclature inChapter 2, they have the same name. Although the basic name of thetwo enantiomers is indeed the same, you need a way to differentiatebetween the two. The most widely accepted procedure, also adopted bythe IUPAC committee, is the Cahn-Ingold-Prelog convention. Thismethod designates each stereocenter with a letter, either (R) or (S). Todetermine whether to use an (R) or an (S) follows this three-stepprocedure.Step 1 Assign a priority number to each atom, or group of atoms,bonded to the stereocenter.Step 2 Redraw the structure using the Wedge and Dash method.Orient the lowest priority group away from you and the otherthree groups in the plane of the paper.Step 3 Then draw an arrow from the first priority group, past thesecond towards the third. If the arrow circles clockwise, call thestereocenter atom (R) (rectus, Latin for right-handed). If thearrow circles counterclockwise, call the stereocenter atom (S)(sinister, Latin for left-handed).To determine the priority numbers, look at the atomic numberof the four atoms—the higher the atomic number, the higher thepriority number. For example, bromochlorofluoromethane has a Br,Cl, F, and H attached to the carbon. Assign the highest prioritynumber, 1, to Br, as its atomic number is 35. The next prioritywww.ochem4free.com 5 July 2005
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Organic Chemistry

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