preface to fifteenth edition

ORGANIC COMPOUNDS 1.49
Example of a chi-
FIGURE 1.18
ral plane.
FIGURE 1.19 Viewing angle as a means of designating the absolute configuration
of compounds with a chiral axis. (a) (R)-2-Butanol (sequence clockwise);
(b) (S)-2-butanol (sequence counterclockwise).
Absolute Configuration. The terms absolute stereochemistry and absolute configuration are used
to describe the three-dimensional arrangement of substituents around a chiral element. A general
system for designating absolute configuration is based upon the priority system and sequence rules.
Each group attached to a chiral center is assigned a number, with number one the highest-priority
group. For example, the groups attached to the chiral center of 2-butanol (see Fig. 1.19) are assigned
these priorities: 1 for OH, 2 for CH 2 CH 3 , 3 for CH 3 , and 4for H. The molecule is then viewed from
the side opposite the group of lowest priority (the hydrogen atom), and the arrangement of the
remaining groups is noted. If, in proceeding from the group of highest priority to the group of second
priority and thence to the third, the eye travels in a clockwise direction, the configuration is specified
R (from the Latin rectus, “right”); if the eye travels in a counterclockwise direction, the configuration
is specified S (from the Latin sinister, “left”). The complete name includes both configuration and
direction of optical rotation, as for example, (S)-()-2-butanol.
The relative configurations around the chiral centers of many compounds have been established.
One optically active compound is converted to another by a sequence of chemical reactions which
are stereospecific; that is, each reaction is known to proceed spatially in a specific way. The configuration
of one chiral compound can then be related to the configuration of the next in sequence. In
order to establish absolute configuration, one must carry out sufficient stereospecific reactions to
relate a new compound to another of known absolute configuration. Historically the configuration
of D-()-2,3-dihydroxypropanal has served as the standard to which all configuration has been
compared. The absolute configuration assigned to this compound has been confirmed by an X-ray
crystallographic technique.
1.1.5 Chemical Abstracts Indexing System
When compounds of complex structure are considered, the number of name possibilities grows
rapidly. To avoid having index entries for all possible names, Chemical Abstracts Service has developed
what might be called the principle of inversion. The indexing system employs inverted