Page 300 - Academic Press Encyclopedia of Physical Science and Technology 3rd Organic Chemistry

P1: GRB Final Pages
Encyclopedia of Physical Science and Technology EN016B-738 July 31, 2001 14:0

Stereochemistry 81

20
typicalspecificrotationmightthusread[α] + 57.3 ± 0.2
D
◦
(95% EtOH, c = 2.3), denoting measurement at 20 Cat
the sodium D line (589 nm) in 95% ethanol at a con-
centration of 2.3 g/100 ml. (Monochromatic light of
this wavelength is easily generated in a sodium vapor
lamp and has thus classically been used for polarimetric
measurements.)
In 1822 Sir John Herschel found that mirror-image crys-
tals of quartz (discovered by R. J. Ha¨uy in 1801 and called FIGURE 1 Tetrahedral molecule Cabcd (the carbon atom at the
“enantiomorphs”) rotate the plane of polarization in oppo- center of the tetrahedron is conventionally not shown). [Reprinted
site directions. This provided the first correlation of optical with permission from Eliel, E. L., and Wilen, S. H. (1994). “Stere-
ochemistry of Organic Compounds,” Wiley, New York.]
rotation with enantiomorphism, in this case of crystals. In
1848LouisPasteurachievedseparationoftheenantiomor-
phous crystals he detected in the sodium–ammonium salt to differ in “configuration.” Thus they explained not only
of the (optically inactive) paratartaric acid (today called the stereoisomerism of such simple molecules as CHF-
racemic tartaric acid, see below) and thereby obtained two BrI or CH 3 CHOHCO 2 H, but also that of more complex
different substances, one of which rotated polarized light molecules such as tartaric acid (Fig. 2). The two tartaric
to the right, the other to the left, even in aqueous solu- acids, A and B in Fig. 2 (separated by Pasteur as sodium–
tion. Pasteur concluded that the enantiomorphous crys- ammonium salts), are enantiomers. Pasteur’s starting salt
tals were made up of molecules that themselves differed came from a 1:1 mixture of the two, called “racemic tar-
as object and (reflection) image on the molecular scale. taric acid.” A “racemate” is a mixture of equal quantities
Such mirror-image molecules are called “enantiomers” of corresponding enantiomers. Since the numerical values
and the separation which Pasteur accomplished is called for the optical rotations of the two enantiomers (negative,
“resolution.” Since the difference between enantiomers or −, for one; positive, or +, for the other) are equal and
resembles the difference between a right and a left hand opposite, the racemate displays no optical activity.
(which are also mirror images of each other, but other- In later work Pasteur discovered a fourth species of tar-
wise essentially identical in their dimensions), we call taric acid [if we count the enantiomers A and B and the
such molecules “chiral” (Greek “cheir” = hand) and we racemate (A + B) as three distinct species]. He was not
call the property of certain molecules to display enan- able to explain the nature of this optically inactive iso-
tiomerism “chirality” (terms coined by Lord Kelvin in mer, but the Le Bel–van’t Hoff theory led to assignment
1893). While every molecule has a mirror image, chiral- of the structure C (Fig. 2) to this stereoisomer. It has the
ity exists only if the image is nonsuperposable with the same connectivity (constitution) as A and B but differs
original molecule, just as a right shoe is not superposable in configuration. The reason for its lack of optical activ-
with a left one. (In contrast, socks worn on right and left ity is that it is superposable with its mirror image D; i.e.,
feet are superposable; they are “achiral.”) it is achiral. It is a stereoisomer of A and B, but not an
enantiomer. Such stereoisomers that are not mirror im-
ages of each other are called “diastereomers”;thusCisa
III. STEREOISOMERISM diastereomer of A and of B (and vice versa), whereas A
is an enantiomer of B (and vice versa). Compound C is
The understanding of molecular structure was not well called “meso-tartaric acid,” the prefix “meso” indicating
enough advanced in 1848 for Pasteur to explain enan- that it is the achiral member in a set of diastereomers that
tiomerism (chirality) in terms of atomic arrangement. The also contains chiral members.
basis for that was laid only a decade or so later when,
in separate publications, A. S. Couper, F. A. Kekul´e, J.
Loschmidt, and A. Crum Brown illuminated the struc- IV. STEREOISOMERISM OF ALKENES
ture of molecules in terms of the connectivity between AND CYCLANES
their constituent atoms. Then, in 1874, J. H. van’t Hoff
in the Netherlands and J. A. Le Bel in France simulta- So far it would appear that stereoisomerism is dependent
neously proposed the structural basis for chirality: When on three-dimensional structure, but van’t Hoff recognized
four different atoms or groups (jointly called “ligands”) that stereoisomers can also exist in two dimensions, as
are attached tetrahedrally to a given carbon atom, two in cis- and trans-disubstituted ethylenes (Fig. 3). The sub-
mirror-image arrangements are possible (Fig. 1) corre- stituents may be on the same side (cis) or on opposite sides
sponding to the two enantiomers. The enantiomers are said (trans) at the two ends of the (planar) double bond.