Stereochemistry

            When you were a child, did you ever put your shoes on backwards?  Do you remember how uncomfortable it was?  You may have, but just not given it any thought or immediately switched them to the correct feet.  Your right shoe is superimposable on your right foot, and the same goes for your left foot and left shoe.  If you put a shoe on the wrong way, it feels awkward and uncomfortable.  This is because your shoes are non-superimposable on your opposite feet.  Now, place your right shoe in front of the mirror.  It looks like your left shoe, doesn’t it?  Shoes, as well as feet, are non-superimposable mirror images of each other. 

            The name of a non-superimposable mirror image is an enantiomer, one of the groups of stereoisomers.  They are not identical to their mirror images.  Enantio is Greek, meaning “opposite.”  Examples of enantiomers are (+)-Lactic Acid (S) and (-)-Lactic Acid (R).  (+)-Lactic Acid occurs in muscle tissue and sour milk, whereas the (-)-Lactic Acid occurs only in sour milk.  They are exact mirror images of each other, but have different structures and functions.  

Below is another example of enantiomers:

L-Alanine is one of the 20 amino acids coded by the genetic code.  D-Alanine occurs in some bacterial cell walls and in some peptide antibiotics.  Just like feet (or in this case, hands), they are non-superimposable mirror images.  

     The substances above have a property that allows them to bend light!  Dextrorotatory and Levorotatory are both terms that have to do with the Optical Activity of organic substances.  To be Optically Active, the organic solutions of the substances must rotate a plane of polarization through an angle.  Dextrorotatory rotates the light to the right (clockwise) and Levorotatory rotates the light to the left (counterclockwise).  A Racemic Mixture is a 50:50 mixture of two chiral enantiomers.  Racemates show no optical rotation because the dextrorotatory and levorotatory molecules are in equal amounts and therefore cancel each other out.

A molecule that is non-identical to its mirror image is said to be Chiral.  The Greek word for Cheir means “hand.”  A hand does not have a plane of symmetry.  Therefore, a molecule that has a plane of symmetry and is identical to its mirror image is said to be Achiral.  An example of an achiral object could be the Batman Logo.  If you cut the bat in half from top to bottom, it is an exact image on either side.   

                                                Achiral                                             Chiral

Rules for Configuration of Structures – Cahn-Ingold-Prelog Rules

·         These rules are named after the chemists who proposed the rules for configurations of chiral compounds. 

1)                  Find the chirality center, an atom that is bonded to four different substituents, of the molecule in question.  Look at the four atoms/substituents attached.  Rank the atoms according to atomic number.  Rank the atom with the highest atomic number the highest number (4).  The atom with the lowest atomic number is therefore ranked the lowest number (1) [The example below is depicted as highest as #1 and lowest as #4.  Therefore, you could do it either way].  For Example: I > Se > F > O > C > H.  Hydrogen is almost always ranked as the lowest priority substituent.

2)                  Re-orient the molecule so that the lowest priority is facing away from you.  Follow from highest to lowest priority (drawing an arrow sometimes helps).  If (the arrow is) clockwise, R is the correct configuration.  If counterclockwise, S is the correct configuration.

The other group of stereoisomers is called Diastereomers, which are not mirror images at all.  Take the shoe analogy explained above for example.  Suppose you have one of your shoes and some other random shoe.  They may be similar in shape and makeup, but they are not identical and are not mirror images.  Diastereomers have the same configuration at one or more chirality centers, but differ at other chirality centers.  The molecules below are Diastereomers.

Lastly, a Meso Compound is a compound that has chirality centers, but is achiral because it has a plain of symmetry.  An example of this is Tartaric Acid, or Tartrate.