preface to fifteenth edition

ORGANIC COMPOUNDS 1.47
Specific rotation is the number of degrees of rotation observed if a 1-dm tube is used and the
compound being examined is present to the extent of 1 g per 100 mL. The density for a pure liquid
replaces the solution concentration.
observed rotation (degrees)
Specific rotation [] length (dm) (g/100 mL)
The temperature of the measurement is indicated by a superscript and the wavelength of the light
20
employed by a subscript written after the bracket; for example, [] 590 implies that the measurement
was made at 20C using 590-nm radiation.
Optically Inactive Chiral Compounds. Although chirality is a necessary prerequisite for optical
activity, chiral compounds are not necessarily optically active. With an equal mixture of two enantiomers,
no net optical rotation is observed. Such a mixture of enantiomers is said to be racemic
and is designated as () and not as dl. Racemic mixtures usually have melting points higher than
the melting point of either pure enantiomer.
A second type of optically inactive chiral compounds, meso compounds, will be discussed in the
next section.
Multiple Chiral Centers. The number of stereoisomers increases rapidly with an increase in the
number of chiral centers in a molecule. A molecule possessing two chiral atoms should have four
optical isomers, that is, four structures consisting of two pairs of enantiomers. However, if a compound
has two chiral centers but both centers have the same four substituents attached, the total
number of isomers is three rather than four. One isomer of such a compound is not chiral because
it is identical with its mirror image; it has an internal mirror plane. This is an example of a diastereomer.
The achiral structure is denoted as a meso compound. Diastereomers have different physical
and chemical properties from the optically active enantiomers. Recognition of a plane of symmetry
is usually the easiest way to detect a meso compound. The stereoisomers of tartaric acid are examples
of compounds with multiple chiral centers (see Fig. 1.14), and one of its isomers is a meso compound.
FIGURE 1.14
Isomers of tartaric acid.
When the asymmetric carbon atoms in a chiral compound are part of a ring, the isomerism is
more complex than in acyclic compounds. A cyclic compound which has two different asymmetric
2
carbons with different sets of substituent groups attached has a total of 2 4 optical isomers: an
enantiometric pair of cis isomers and an enantiometric pair of trans isomers. However, when the
two asymmetric centers have the same set of substituent groups attached, the cis isomer is a meso
compound and only the trans isomer is chiral. (See Fig. 1.15.)
Torsional Asymmetry. Rotation about single bonds of most acyclic compounds is relatively free
at ordinary temperatures. There are, however, some examples of compounds in which nonbonded