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80 Stereochemistry

resolved substance, the excess of one enantiomer over more of its constituting atoms within the repulsive
the other as a percentage of the total. regime of the van der Waals potential. Also called
Enantiomers Stereoisomers that are mirror images of “nonbonded (repulsive) interaction.”
each other.
Enantiomorphs Macroscopic objects (e.g., crystals)
whose mirror images are not superposable with the I. INTRODUCTION
original entity.
Factorization The analysis, where possible, of chirality This article deals with the stereochemistry of organic com-
in terms of individual chiral elements, such as chiral pounds,althoughmanyofthegeneralprinciplesalsoapply
centers, chiral axes, and chiral planes. to organometallic and inorganic compounds.
Gauche Conformation of a segment X–C–C–Y in which The term “stereochemistry” is derived from the Greek
◦
the torsion angle (which see) is ±60 or near ±60 ◦ “stereos” meaning solid—it refers to chemistry in three
(also “synclinal”). X and Y are said to be gauche (to dimensions. Since nearly all organic molecules are three
each other). dimensional (with the exception of some olefins and aro-
Kinetic resolution Separation of enantiomers by virtue matics to be discussed later), stereochemistry cannot be
of their unequal rates of reaction with a nonracemic considered a branch of chemistry. Rather it is an aspect of
chiral reagent. all chemistry, or, to put it differently, a point of view which
Optical activity The property of chiral assemblies of has become increasingly important and, as will be shown,
molecules or chiral crystals of rotating the plane of essential for the understanding of chemical structure and
polarized light. function.
Optical purity The ratio of the observed specific rotation
of a substance to the specific rotation of the enantiomer-
ically and chemically pure substance. II. HISTORY. OPTICAL ACTIVITY
Optical rotation Rotation of the plane of polarized light
produced by the presence of chiral molecules or chiral Historically, stereochemical thinking developed from the
crystals in the light path. observationsofJ.B.Biotin1812–1815thatquartzcrystals
Racemic mixture (racemate) A composite of equal as well as solutions of certain organic substances rotate
amounts of opposite enantiomers. the plane of polarized light; this phenomenon is called
Racemization Conversion of individual enantiomers into “optical rotation.” When the beam of light coming to-
a racemic mixture. ward the observer rotates the plane to the right (clock-
Resolution Separation of enantiomers from a racemic wise) we call the rotation positive (+); when the rota-
mixture. tion is to the left or counterclockwise, it is negative (−).
Staggered conformation Conformation of a nonlinear The angle of rotation α is proportional to the concentra-
array of four atoms (A–B–C–D) in which the torsion tion c (conventionally expressed in grams per milliliter,
angle (which see) is near 60 . Ligands A and D are said i.e., density, for solids and pure liquids and gases) and
◦
to be gauche or synclinal to each other. the thickness of the layer—usually the length of the cell
Stereochemistry Chemistry in three dimensions; topo- in which the liquid or solution is contained (convention-
graphical aspects of chemistry. ally expressed in decimeters): α = [α] ·l · c (“Biot’slaw”).
Stereogenic center An atom where exchange of two of The proportionality constant [α] is called “specific rota-
3
its ligands changes the configuration. Such an atom is tion”:[α] = α/l (dm) · c(g/cm ). For solutions, where the

usually, but not necessarily, a chiral center. concentration c is conventionally expressed in g/100 ml,

Stereoisomers Isomers of the same constitution but dif- [α] = 100α/l (dm) · c (g/100 ml). The constant [α] (con-
fering in spatial arrangement of their constituting ventionally reported without units) is used for the char-
atoms. acterization of “optically active” substances (i.e., which
Torsion angle In a nonlinear array of four atoms A–B– display optical rotation). It varies with temperature and
C–D, the angle between the plane containing A, B, and with the wavelength of the light used in the observation,
C and the plane containing B, C, and D. which need to be indicated (as subscripts and superscripts,
t
Torsional strain Excess potential energy of a molecule respectively): thus, [α] , where t is the temperature in
λ
caused by incomplete staggering of pertinent bonds ◦ C and λ is the wavelength in nanometers (nm). Unfor-
(e.g., deviation of the torsion angle in H–C–C–H from tunately, since most substances are prone to solvation
the normal 60 ). Also called “Pitzer strain.” and intermolecular association in solution or in the liq-
◦
van der Waals or compression strain Excess potential uid state, and since such association varies with concen-
energy of a molecule caused by approach of two or tration and solvent, these items must also be recorded. A