Chirality centers rotating molecules around

Chiral Smectic. In much the same way as a chiral compound forms the chiral nematic phase instead of the nematic phase, a compound with a chiral center forms a chiral smectic C phase rather than a smectic C phase. In a chiral smectic CHquid crystal, the angle the director is tilted away from the normal to the layers is constant, but the direction of the tilt rotates around the layer normal in going from one layer to the next. This is shown in Figure 10. The distance over which the director rotates completely around the layer normal is called the pitch, and can be as small as 250 nm and as large as desired. If the molecule contains a permanent dipole moment transverse to the long molecular axis, then the chiral smectic phase is ferroelectric. Therefore a device utilizing this phase can be intrinsically bistable, paving the way for important appHcations. 

S) atoms. In both cases, the reference system does not change the three-dimensional arrangement of the groups around the chiral center. For example, if the sense of rotation had been chosen by convention to go from the small to the large atom (Schemes 11c and d), the corresponding symbols would have been reversed, but the absolute configuration of the molecule would have remained the same. 

The molecule is rotated around the axis between the chiral center and the sub-stitnent of the lowest priority (see diagram below). The direction of rotation follows the order of priorities of the three substituents. If the rotation has clockwise orientation, the molecule has (R) absolute configuration and if the rotation is anticlockwise, the molecule will have absolute configuration (S). 

Figure 5.5 A demonstration of chirality of a generalized molecule containing one chirality center, (a) The four different groups around the carbon atom in in and IV are arbitrary, (b) III is rotated and placed in front of a mirror. Ill and IV are found to be related as an object and its mirror image, (c) III and IV are not superposable therefore, the molecules that they represent are chiral and are enantiomers.

Substituted [2.2]paracyclophane compounds exhibit planar chirality [3, 4]. In contrast to conformationally flexible chiral molecules with an axis of rotation and a rotatable bond around a stereogenic center, the planar chirahty of [2.2]paracyclo-phane compounds possesses a conformationally stable chiral space because of the fixed aromatic rings. From this structural viewpoint, various methods for the optical resolution of planar chiral [2.2]paracyclophanes have been developed since the 1990s [79-92]. In comparison with the establishment of the optical resolution of monosubstituted [2.2]paracyclophanes, there was more scope for the development of optical resolution of pseudo-ortfio-disubstituted [2.2]paracyclophanes [85-92]. Recently, a simple optical resolution method for racemic pseudo-ort/to-dibromo[2.2] paracyclophane rac-38 was reported [93]. As shown in Scheme 11, treatment of rac-38 with a slight excess of n-BuLi and (lR,25,5R)-(-)-menthyl S)-p-toluenesulflnate provided a mixture of diastereomers (Rp,5)-42 and (5p,5)-42. These were readily separated by conventional Si02 column chromatography to afford (Rp,S)-42 and (5p,5)-42 (each in 39 % isolated yield). 

Molecules that do not possess an asymmetric center may still have nonsuperimposable mirror images and exist as enantiomers. These molecules contain a chiral plane or chiral axis and are dissymmetric with respect to either that plane or axis. The structures of the enantiomers of the sedative-hypnotic methaqualone are presented in Fig. 4. In this molecule there is a chiral axis between the nitrogen atom (N-1) and phenyl ring (C-1). The dissymmetry of the two forms of the molecule is a result of hindered rotation around this axis, which is due to steric interactions between methyl groups (M-1 and M-2). Other axially dissymmetric molecules include allene, biaryls, alkylidenecyclohexanes, and spiranes. Planar dissymmetric molecules are exemplified by molecules such as tra s-cycloalkenes. 

Fig. 2 Examples of chiral molecules with and without asymmetric center, a The sp hybridized carbon bearing four different substituents is by far the most common asymmetric center, b The C=C=C allene arrangement forms a chiral axis. The l-chloro-3-bromoallene is chiral, c Atropoisomerism occurs when the free rotation around a o bond is hindered, d Steric hindrances create a chiral plane in hehcenes...

Atropic molecules without any materialized asymmetrical center (carbon or hetereo-atom), can have a chiral conformation and optical activity. Limitations of free rotation around bonds by intramolecular links or steric hindrances engage such molecules into a helical conformation, and their mirror plane passes through bonds and not through atoms. Organic chemistry gave the first synthetic examples of such molecules. 

---