"not entirely true. The definition of chirality is only that molecule has a non-superimposable mirror image. That's it."
I absolute agree, and after reflecting on this question after I answered, this is a more precise answer. Whether a compound has a stereocenter or not is irrelevant. A stereocenter need not be a carbon center, but this is not important for the MCAT. The definition of enantiomers are two molecules that are stereoisomers that are non-superimposable images of each other. This immediately disqualifies meso compounds.
The confusion arises in the way in which we evaluate chirality based on looking at a molecule on the page--it's an issue of our rules for evaluating stereoisomerism. We look at a written molecule, whether it be a Fischer projection, or a Lewis structure, and look at whether a carbon has four different substituents.
So, it really depends on whether you are approaching the question from a definitional or pragmatic aspect. For the purist, well, either two molecules are superimposable mirror images or not. For the pragmatist: 1. Determine the number of stereocenters; 2. If there is more than one, determine if there is a plane of symmetry.
This can be done in Fischer projections more easily. If you think that you are dealing with a meso compound in a Fischer projection, then you can rotate the molecule 180 degrees (like a record, not like a pancake!). If the molecule matches its mirror image after this transformation, then you have a meso compound.
In regard to the D/L issue, I researched it more fully and confirmed what I already knew. The first thing to note is that capital D/L are different from d/l. Lower case d/l correspond to the optical designations: +/-. + means the same thing as d: dextrorotary, and - means the same thing as l: levorotary. Capital D/L are used, as I have seen them, in reference to amino acids and sugars. They are relative configurations, based on convention. The important point for the MCAT is how to determine whether a saccharide is D or L (the rule is fairly simple if you look at a Fischer projection); virtually all of the biologically relevant amino acids are L; virtually all of the biologically relevant carbohydrates are D. Be aware that +/- can ONLY be determined by polarimetry. +/- CANNOT be determined by looking at a structure and DO NOT correspond to D/L.
Further, the epimer question is superfluous. If you're an organic chemist, then go nuts over this definition. If you're studying for the MCATs, then I would know the difference between a stereoisomer, diastereomer, enantiomer, and meso compound. Whether a diastereomer differs in 1 or 6 stereocenters is irrelevant.
Best,
Homestar
