preface to fifteenth edition

1.48 SECTION 1
FIGURE 1.15 Isomers of cyclopropane-1,2-dicarboxylic acid. (a)
Trans isomer; (b) meso isomer.
interactions between large substituent groups inhibit free rotation about a sigma bond. In some cases
these compounds can be separated into pairs of enantiomers.
A chiral axis is present in chiral biaryl derivatives. When bulky groups are located at the ortho
positions of each aromatic ring in biphenyl, free rotation about the single bond connecting the two
rings is inhibited because of torsional strain associated with twisting rotation about the central single
bond. Interconversion of enantiomers is prevented (see Fig. 1.16).
For compounds possessing a chiral axis, the structure can be regarded as an elongated tetrahedron
to be viewed along the axis. In deciding upon the absolute configuration it does not matter from
which end it is viewed; the nearer pair of ligands receives the first two positions in the order of
precedence (see Fig. 1.17). For the meaning of (S), see the discussion under Absolute Configuration
on p. 1.49.
A chiral plane is exemplified by the plane containing the benzene ring and the bromine and
oxygen atoms in the chiral compound shown in Fig. 1.18. Rotation of the benzene ring around the
oxygen-to-ring single bonds is inhibited when x is small (although no critical size can be reasonably
established).
FIGURE 1.16 Isomers of biphenyl compounds with bulky groups attached
at the ortho positions.
FIGURE 1.17
Example of a chiral axis.