Organic Chemistry

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Organic Chemistry - Ch 11 537 Daley & Daleyc) Spiro[4.3]octaned) trans-DecalinSample Solutionb) 1-Methylcyclohexanol has one plane of symmetry that bisects the —OH and —CH 3 groups on C-1 as well as C-1 and C-4.OHPlane of symmetryCH 3An enantiomer is one ofa pair ofnonsuperimposiblemirror imagemolecules.Molecular asymmetryis the result of amolecule’s shape.An organic molecule without a plane of symmetry or center ofsymmetry is asymmetric. A characteristic of all asymmetric moleculesis that they possess chirality. That is, for that molecule there exists anonsuperimposible mirror image molecule. All chiral, or asymmetric,molecules occur in pairs with a “right hand” and a “left hand”molecule. Each member of the pair is called an enantiomer. Forexample, there are two enantiomers of 2,3-pentadiene. To confirmthis, make a model of Figure 11.5 and then another of its mirrorimage. As you examine these two stereoisomers, note that they do nothave a plane of symmetry and that you cannot superimpose onemolecule upon the other. Their asymmetry is due to their shape, ormolecular asymmetry.HCH 3CCCHCH 3Figure 11.5. 2,3-Pentadiene is an example of a chiral molecule that does not have aplane or center of symmetry.A stereogenic center isa specific atom in amolecule that isresponsible for some orall of the molecularasymmetry.Although all chiral molecules have molecular asymmetry,many asymmetric molecules have a stereogenic center or, morespecifically, an asymmetric carbon atom. An asymmetric carbon atomis often called a stereocenter. An asymmetric carbon atom is bonded tofour nonequivalent substituents. This asymmetry is a propertyallowed by the tetrahedral structure of a carbon atom.www.ochem4free.com 5 July 2005
Organic Chemistry - Ch 11 538 Daley & DaleyA symmetricalmolecule has nostereogenic centers andhas an internal planeor center of symmetry.Examining the symmetric properties of the tetrahedral carbonatom will help you understand a stereogenic center. For example,make a molecular model of dibromomethane (Figure 11.6) which is acarbon atom with two pairs of equivalent substituents. Notice that themolecule contains two planes of symmetry. One bisects the carbonatom and lies between the two bromine atoms; the other also bisectsthe carbon atom, but it lies between the hydrogen atoms. The portionsof the molecule on either side of either plane of symmetry are mirrorreflections of each other. Construct the mirror image ofdibromomethane. Notice that the two are superimposible. That is,dibromomethane is symmetrical.HHCBrBrPlanes ofsymmetryBrHCHBrFigure 11.6. Planes of symmetry in dibromomethane.Now substitute chlorine for one of the bromines and fluorine forone of the hydrogens. The name of the new molecule isbromochlorofluoromethane (Figure 11.7).HFCClBrBrHCFClFigure 11.7. Two different three-dimensional representations of the same enantiomerof bromochlorofluoromethane. This molecule has no plane of symmetry.With four different substituents attached to one carbon atom,bromochlorofluoromethane contains no element of symmetrywhatsoever. The molecule is asymmetric. Because the molecule isasymmetric, a nonsuperimposible mirror image molecule ofbromochlorofluoromethane exists as its enantiomer.Convince yourself that the mirror image molecules ofbromochlorofluoromethane are not identical structures by trying tosuperimpose one model on the other. Orient the two enantiomers intoa mirror image position and slide them together placing the hydrogenwww.ochem4free.com 5 July 2005
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Organic Chemistry

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