CHEMISTRY 233, Section 71xx
Fall - 1996

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 (adj.)
describes a molecule that is different from its mirror image; enantiomers are chiral

achiral (adj.)
describes a molecule that is the same as its mirror image; achiral molecules have either planes or centers of symmetry

chiral center (or stereo or stereogenic center)
an atom attached to 4 different atoms or groups; most chiral molecules contain chiral centers

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 degree to which materials rotate ppl

optical activity
a substance that rotates ppl (eg. is chiral) is said to be "optically active"

specific rotation
amount that a substance rotates ppl expressed in a standard form; it accounts for variables such as concentration (c) and light path length (l)

[] = /c x l

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).

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

resolution
The process of separating enantiomers; basically involves converting the enantiomers into disastereomers which have different properties and 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 anion of acid). The salts have, among other things, different solubilities, and could be separated by fractional crystallization.

# of stereoisomers
a molecule with n stereogenic centers (and for which a meso isomer isn't possible) will have 2**n stereoisomers [2**n means 2 raised to the nth power]

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 ° has the effect of inverting the configuration (R -> S and vice versa). The projections ONLY can be rotated 180 ° about an axis perpendicular to the paper without altering the configuration.

question: (+) & (-)-carvone are enantiomers; why do they smell differently?