STEREOCHEMISTRY

stereochemistry = chemistry in 3D

stereo isomers = isomers which differ only in their 3D structures; examples are cis & trans-2-butene and enantiomers

enantiomers = non-superimposable (different) mirror images; vast majority of chemical and physical properties are identical

chiral = describes a molecule that is different from its mirror image; enantiomers are chiral

achiral = describes a molecule that is the same as its mirror image; achiral molecules have either mirror planes of symmetry or centers of symmetry; (R)-(-)-lactic acid and water are examples of chiral and achiral molecules, respectively

chiral center (or stereo or stereogenic center) = an atom attached to 4 different atoms or groups; although the vast majority of chiral molecules contain chiral centers, a molecule can be chiral and not contain a chiral center (how about 2,3-hexadiene?)

plane polarized light = light whose electric field amplitude vector is in a single plane; the rotation of plane polarized light is about the only physical property that distinguishes enantiomers

polarimeter = instrument used to measure the amount (in deg) by which materials rotate ppl

optical activity = a substance that rotates ppl is chiral and is said to be "optically active"

specific rotation, [a] = amount (# of degs) that a substance rotates ppl expressed in a standard form; it accounts for variables such as concentration (c) and light path length (l); in the equation shown below, a is the measured rotation and [a] is the specific rotation

                         [a] = a/[c x l] Question: Suppose you prepare a solution of a molecule that is suspected to be chiral (should demonstrate optical activity) and find that the rotation (a)  is exactly 0 degs.  What else could be done to deminstrate that the molecule is chiral?  It is highly improbable that the specific rotation of a chiral molecule would be exactly 0 degs.  Hint - consider the definition of [a] given above and think how you might alter the experimental conditions to observe a rotation. 

absolute configuration = the precise (true) 3D arrangment of atoms in a molecule

Cahn-Ingold-Prelog R/S notation = specifies absolute configuration of a chiral center; there is no correspondence between R and + or S and -

diastereomers = stereoisomers that are not enantiomers; have different chemical and physical properties
 
 



The (2R,3R) and (2S,3S) stereoisomers are enantiomers (different mirror images). There are two pairs of diastereomers: (2R,3R) and (2R,3S); (2S,3S) and (2R,3S). Note that the absolute configurations of the two chiral centers in meso-tartaric acid are opposite; one is R and the other is S.

Question: 2,3-Dihydroxybutanoic acid has four stereoisomers. How many pairs of enantiomers and how many pairs of diastereomers are found within the stereoisomers?

meso isomer = an achiral molecule with 2 or more chiral centers and an internal plane of symmetry; the molecule is achiral; example is (2R,3S)-2,3-dibromobutane

racemic mixture = 1:1 mixture of enantiomers; has no optical activity.
 
resolution = the process of separating enantiomers; basically involves converting the enantiomers into disastereomers which have different properties and then can be separated. Takes advantage of the fact that diastereomers have different physical properties. For example, use a chiral amine (a base) to react with a racemic mixture of acids and form a mixture of diastereomeric salts (amine-H+ with conjugate base of acid). The salts have, among other things, different solubilities, and can be separated by fractional crystallization. One could also use a chiral chromatography column for the direct separation of the racemic mixture.

# of stereoisomers = a molecule with n stereogenic centers (and for which a meso isomer is not possible) will have 2n stereoisomers

Fischer projections = short-hand notation for displaying molecules with stereogenic centers. Horizontal bonds are out of paper toward viewer & vertical bonds are into the paper away from the viewer. Rotation of a Fischer projection by 90 deg has the effect of inverting the configuration (R to S and vice versa). They ONLY can be rotated 180 deg about an axis perpendicular to the paper without altering the configuration. If the projection is rotated in a way that lifts some groups out of the paper, the absolute configuration will be reversed.
 



Note: if you interchange two of the groups attached to a chiral center, the molecule is converted into the mirror image. An odd number of interchanges gives the mirror image whereas an even number leaves the absolute configuration unchanged.

Question: (+) & (-)-carvone are enantiomers; one smells like spearmint and the other like caraway. Why do they smell differently?
 
10/16/2000 9:54AM