Enantiomers are non-superimposable mirror images of one another. Not being able to superimpose one molecule on top of the other simply means that the two molecules are not equivalent or identical. For a compound to form an enantiomeric pair, it must have chiral molecules. Chiral molecules must not have an internal plane of symmetry, and they must have a stereocenter.
Each enantiomer exhibits what is called optical activity. Each isomer of the pair is capable of rotating plane polarized light. One isomer rotates this light to the right "x" number of degrees, and the other isomer of the pair rotates this light to the left for the same number of degrees. In fact, this is the only difference in the two isomers, their ability to rotate plane polarized light in opposite directions. All other physical properties are exactly the same. This makes it extremely difficult to separate the two isomers should they be mixed as often they are. If the enantiomers are crystalline salts like Pasteur's Tartrate salts, then the enantiomers will have a different appearance when observed under magnification and one can pick them out to separate them, but most enantiomeric pairs are not salts and therefore look the same. There is a way that we can separate such a mixture of enantiomers which is referred to as resolution to be disscussed in a later lesson.
Another characteristic that Enantiomers exhibit is configuration. Configuration is the spacial way that non-equivalent groups arrange themselves around a stereocenter carbon. One enantiomer will be configured right handedly (R) and the other will be configured left handedly (S).
Enantiomers are usually dipicted on a planar surface either as a 3-dimensional structural formula, or we can show the structure as a Fisher Projection. A Fisher projection is a 2-dimensional projection of a 3-dimensional chiral molecule. A Fisher projection consists of a long vertical line representing the longest contineous chain of the molecule with a series of horizontal line intersecting this vertical line along its length. At the point where the horizontal lines intersect the vertical line there will be a carbon atom. At the ends of the horizontal lines will be atoms or groups of atoms. In a Fisher projection atoms or groups that are projected in front of the plane where the vertical line extends will be at the ends of the horizontal lines.Those atoms along the vertical line will be projected behind the plane. For example 2-Bromobutane is a chiral molecule, and therefore, will be one of an enantiomeric pair with its mirror image. We can show this molecule in 3-D formula as is usually the case using solid wedges to show atoms in front of the plane and dotted wedges for atoms behind the plane with any atoms within the plane to be connected to a solid line(See Fig 1-a). Its mirror image could be projected as in Fig 1-b.
On the other hand we can dipict the same chiral molecule and its mirror image using a Fisher Projection (See Fig 2).
One has to be very careful about using Fisher Projections when you go and try to manipulate the projections by rotation out of plane. In fact, one cannot rotate Fisher projections of acyclic molecules out of plane without leading to incorrect conclusions concerning whether or not the mirror images are non-superimposable. That is because when you rotate out of plane (flip over) then you still have atoms that were at the end of horizontal lines, and therefore projected forward in front of the plane at the end of horizontal lines after flipping. However the use of simple molecular models will show that when you flip a molecule out of plane 180 degrees you will project the groups that were in front to the back of the plane. This is not indicated within a Fisher projection. Suffice to say that when dealing with Fisher projections, it is best not to manipulate out of plane. Any manipulations using Fisher Projections should be done within the plane either translation or rotation within the plane.
We can see that if we attempt to translate the Fisher projection in Fig 2-b over onto Fig 2-a we do not get them to match (See Fig 3-a below). If we attempt to rotate it 180 degrees within the plane we again fail to get them to match up. (See Fig 3-b below). Attempting to rotate 180 degrees out of plane (illegal for Fisher projections) we get the impression that we do get them to match up except the Hydrogen and Bromine would be projected to the back of the plane and would not match up.
Here is a few problems for you to consider. Construct the 3-D structure for the compounds shown as Fisher Projections in Fig 4 below.
Once you have written the 3-D structures check your results for the correct answers
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R. H. Logan, Instructor of Chemistry, Dallas County Community College District, North Lake College.
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All textual content copyrighted (c) 1997 R.H. Logan, Instructor of Chemistry, DCCCD All Rights reserved
The correct answers are given in the Fig 5 below:
Revised: 8/17/97
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