Chemistry Reference
In-Depth Information
observed; however, the sodium ammonium tartrate consisted of a mixture of left- and
right-handed crystals. By picking out the two forms from the mixture (using tweezers),
Pasteur was able to make solutions of only left-handed or only right-handed crystals and
showed that the solution of left-handed crystals gave the opposite optical rotation to that
from the right-handed crystals. This demonstrated that the building blocks of the crystals
in the two crystal forms were different from one another, since the crystal structure is lost
in solution. The conclusion that Pasteur drew was that tartaric acid molecules themselves
have a three-dimensional shape and can be left- or right-handed. This type of molecule
he classified as asymmetric, and he inferred that molecules that do not rotate the plane
of polarized light must be symmetric. We now know that naturally occurring tartaric acid
occurs as either the
R
,
R
or
S
,
S
molecular structures, as shown in Figure 3.1, and that Pas-
teur was fortunate to find an example where the crystallization process itself separates the
two enantiomers.
(a)
(b)
OH
O
OH
O
HO
HO
OH
OH
O
OH
O
OH
(
R,R
)-tartaric acid
(
S,S
)-tartaric acid
Figure 3.1
The naturally occurring chiral forms of tartaric acid: (a) R,R and (b) S,S. The
molecular models in the lower part of the figures are both set with the OH groups pointing
backward so that the mirror image relationship between the two forms can be readily seen.
This was the first example of classification based on molecular shape and gave some
indication of the physical properties of molecules that were classified as symmetric
compared with those that were labelled asymmetric. However, chirality is not the only
manifestation of molecular symmetry, and so a more complete classification of molecu-
lar shape has been developed; the system of point groups. To classify the symmetry of a
molecule we derive its point group, which carries much more geometric information than
Pasteur's symmetric or asymmetric designation.
The system of labels used for molecular point groups comes from the work of Schönflies
(1853-1928). The symbols are designed to carry information on the symmetry elements
present in the object being discussed. The symmetry of a molecule is governed by the
bonding geometries that are preferred by the particular atoms or chemical groups from
