Organic Chemistry

Organic Chemistry - Ch 11 552 Daley & DaleyTo this point, you have used only the wedge-and-dashstructures to indicate a three-dimensional perspective when drawingmolecules. As the complexity of the molecules you study increases,these drawings will become more difficult to understand and moretime consuming to draw. Similarities and differences between complexstereoisomers are difficult to see with the wedge-and-dashstereochemical drawings.Around 1900, as Emil Fischer studied the stereochemistry ofsugars that contain up to seven stereocenters each, he begandeveloping a simple way to represent these complex molecules so hecould draw them more rapidly. Later, Victor Meyer worked to simplifyFischer's method of drawing molecules. This method of representingmolecules is called Fischer projections. Fischer projections alsofacilitate the comparison of stereoisomers by emphasizing thedifferences in the complex stereochemistry of enantiomers with morethan one stereocenter.Fischer projections use perpendicular lines to represent astereocenter. The point where the lines intersect represents theasymmetric carbon. The horizontal lines represent the bonds thatproject forward, and the vertical lines represent the bonds that projectbackward. For example, to convert a wedge-and-dash representationof (R)-bromochlorofluoromethane to a Fischer projection begin bymaking a model to help yourself visualize the conversion. Rotate themodel until the atoms projecting back in the wedge-and-dashrepresentation, the hydrogen and the fluorine, are in the verticalpositions, as in Figure 11.8, and the atoms projecting forward, thebromine and the chlorine, are in the horizontal positions. (The centerdrawing shows why this representation is sometimes called the “bowtie” representation.) Replace the wedges and dashes with a pair ofperpendicular lines.HFCClBrBrHCFWedge-and-dash projectionsClHBr ClFFischer projectionFigure 11.8. Converting a wedge-and-dash structure to a Fischer projection. TheFischer projection uses perpendicular lines to represent a stereogenic carbon atom.The horizontal lines project toward the viewer, and the vertical lines project awayfrom the viewer.When working with a Fischer projection, you may rotate theprojection 180 o in the plane of the paper because the vertical (dashed)lines are still vertical and the horizontal (wedge) lines are stillhorizontal.www.ochem4free.com 5 July 2005