Stereochemistry Vocabulary

Chiral carbon atom

A carbon atom that is bonded to four different groups.

Resolving agent

A chiral compound (or chiral material on a chromatographic column) used for separating enantiomers.

Specific rotation

A measure of a compound's ability to rotate the plane of polarized light (Formula) where c is the concentration in g/mL and l is length of sample cell (path length) in decimeters.

Fischer projection

A method for drawing an asymmetric carbon atom as a cross. The carbon chain is kept along the vertical, with the IUPAC numbering from top to bottom. Vertical bonds project away from the viewer, and horizontal bonds project towards the viewer.

Racemic mixture

A mixture of equal quantities of enantiomers, such that the mixture is optically inactive.

Chiral probe

A molecule or an object that is chiral and can use its own chirality to differentiate between mirror images.

2^n rule

A molecule with n chiral carbon atoms might have as many as 2^n stereoisomers.

Enantiomers

A pair of nonsuperimposable mirror-image molecules: mirror-image isomers.

Internal mirror planes

A plane of symmetry through the middle of a molecule, dividing the molecule into two mirror-image halves. A molecule with an internal mirror plane of symmetry cannot be chiral.

Meso compound

An achiral compound that contains chirality centers (usually asymmetric carbon atoms).

Stereocenter

An atom that gives rise to stereoisomers when its groups are interchanged. Asymmetric carbon atoms and double-bonded carbons in cis-trans alkenes are the most common stereocenters.

Leftorium

An imaginary store that sells the enantiomers of everyday chiral objects such as scissors, rifles, can openers, etc.

Polarimeter

An instrument that measures the rotation of plane-polarized light by an optically active compound.

Optically active

Capable of rotating the plane of polarized light.

Allenes

Compounds having two C=C double bonds that meet at a single carbon atom, C=C=C. The two outer carbon atoms are trigonal planar, with their planes perpendicular to each other. Many substituted allenes are chiral.

Optical isomers

Compounds with identical properties except for the direction in which they rotate polarized light.

D-L configurations

D has the same relative configuration as (+)-glyceraldehyde. L has the same relative configuration as (-)-glyceraldehyde. Fischer-Rosanoff

Isomers

Different compounds with the same molecular formula.

Chiral

Different from its mirror image.

Superimposable

Identical in all respects. The three-dimensional positions of all atoms coincide when the molecules are placed on top of each other.

Constitutional isomers

Isomers that differ in the order in which their atoms are bonded together.

Cis-trans isomers

Isomers that differ in their geometric arrangement on a ring or double bond; cis-trans isomers are a subclass of diastereomers.

Stereoisomers

Isomers whose atoms are bonded together in the same order but differ in how the atoms are oriented in space.

Plane-polarized light

Light composed of waves that vibrate in only one plane.

Achiral

Not chiral.

Trans

On opposite sides of a ring or double bond.

Cis

On the same side of a ring or double bond.

Dextrorotatory

Rotating the plane of polarized light clockwise.

Levoratory

Rotating the plane of polarized light counterclockwise.

Optical activity

Rotation on the plane of polarized light.

Diastereomers

Stereoisomers that are not mirror images.

Conformers

Structures that differ only by rotations about single bonds.

Chirality center

The IUPAC term for an atom holding a set of ligands in a spatial arrangement that is not superimposable on its mirror image. Asymmetric carbon atoms are the most common.

Cahn-Ingold-Prelog convention

The accepted method for designating the absolute configuration of a chirality center (usually an asymmetric carbon) as either (R) or (S).

Absolute configuration

The detailed stereochemical picture of a molecule, including how the atoms are arranged in space. Alternatively, the (R) or (S) configuration at each asymmetric carbon atom.

Enantiomeric excess

The excess of one enantiomer in a mixture of enantiomers expressed as a percentage of the mixture.

Relative configuration

The experimentally determined relationship between the configurations of two molecules, even though the absolute configuration of either may not be known.

Resolution

The process of separating a racemic mixture into the pure enantiomers.

Optical purity

The specific rotation of a mixture of two enantiomers, expressed as a percentage of the specific rotation of one of the pure enantiomers.

Stereochemistry

The study of the three-dimensional structure of molecules.

Configurations

The two possible spatial arrangements around a chirality center or other stereocenter.