Chemists draw enantiomers using either perspective formulas or Fischer projections.
Perspective formulas show two of the bonds to the asymmetric carbon in the plane of the paper, one bond as a solid wedge protruding out of the paper, and the fourth bond as a hatched wedge extending behind the paper. You can draw the first enantiomer by putting the four groups bonded to the asymmetric carbon in any order. Draw the second enantiomer by drawing the mirror image of the first enantiomer.
A shortcut—called a Fischer projection—for showing the three-dimensional arrangement of groups bonded to an asymmetric carbon was devised in the late 1800s by Emil Fischer. A Fischer projecton represents an asymmetric carbon as the point of intersection of two perpendicular lines; horizontal lines represent the bonds that project out of the plane of the paper toward the viewer, and vertical lines represent the bonds that extend back from the plane of the paper away from the viewer. The carbon chain always is drawn vertically with C-1 at the top of the chain.
To draw enantiomers using a Fischer projection, draw the first enantiomer by arranging the four atoms or groups bonded to the asymmetric carbon in any order. Draw the second enantiomer by interchanging two of the atoms or groups. It does not matter which two you interchange. (Make models to convince yourself that this is true.) It is best to interchange the groups on the two horizontal bonds because the enantiomers then look like mirror images on paper.
Note that interchanging two atoms or groups gives you the enantiomer—whether you are drawing perspective formulas or Fischer projections. Interchanging two atoms or groups a second time, brings you back to the original molecule.
A stereocenter (or stereogenic center) is an atom at which the interchange of two groups produces a stereoisomer. Therefore, both asymmetric carbons—where the interchange of two groups produces an enantiomer and the carbons where the interchange of two groups converts a cis isomer to a trans isomer (or a Z isomer to an E isomer)—are stereocenters.
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