Relative and Absolute Configurations
Optical activity refers to the ability of a molecule to rotate plane-polarized light: d- or (+)
molecules rotate light to the right; l- or (–) molecules rotate light to the left.
Racemic mixtures, with equal concentrations of two enantiomers, will not be optically active
because the two enantiomers’ rotations cancel each other out.
Meso compounds, with an internal plane of symmetry, will also be optically inactive because
the two sides of the molecule cancel each other out.
Diastereomers are non-mirror-image stereoisomers. They differ at some, but not all, chiral
centers. They have different chemical and physical properties.
Cis–trans isomers are a subtype of diastereomers in which groups differ in position about an
immovable bond (such as a double bond or in a cycloalkane).
Chiral centers have four different groups attached to the central carbon.
Relative configuration gives the stereochemistry of a compound in comparison to another
molecule.
Absolute configuration gives the stereochemistry of a compound without having to compare to
other molecules.
Absolute configuration uses the Cahn–Ingold–Prelog priority rules, in which priority is given
by looking at the atoms connected to the chiral carbon or double-bonded carbons; whichever
has the highest atomic number gets highest priority. If there is a tie, one moves outward from
the chiral carbon or double-bonded carbon until the tie is broken.
An alkene is (Z) if the highest-priority substituents are on the same side of the double bond and
(E) if on opposite sides.
A stereocenter’s configuration is determined by putting the lowest priority group in the back and
drawing a circle from group 1 to 2 to 3 in descending priority. If this circle is clockwise, the
stereocenter is (R); if it is counterclockwise, the stereocenter is (S).
Vertical lines in Fischer diagrams go into the plane of the page (dashes); horizontal lines come
out of the plane of the page (wedges).
Switching one pair of substituents in a Fischer diagram inverts the stereochemistry of the
chiral center. Switching two pairs retains the stereochemistry.
Rotating a Fischer diagram 90° inverts the stereochemistry of the chiral center. Rotating 180°
