Reinitzer cholesterol, chiral nematics

Reinitzer discovered liquid crystallinity in 1888 the so-called fourth state of matter.4 Liquid crystalline molecules combine the properties of mobility of liquids and orientational order of crystals. This phenomenon results from the anisotropy in the molecules from which the liquid crystals are built. Different factors may govern this anisotropy, for example, the presence of polar and apolar parts in the molecule, the fact that it contains flexible and rigid parts, or often a combination of both. Liquid crystals may be thermotropic, being a state of matter in between the solid and the liquid phase, or they may be lyotropic, that is, ordering induced by the solvent. In the latter case the solvent usually solvates a certain part of the molecule while the other part of the molecule helps induce aggregation, leading to mesoscopic assemblies. The first thermotropic mesophase discovered was a chiral nematic or cholesteric phase (N )4 named after the fact that it was observed in a cholesterol derivative. In hindsight, one can conclude that this was not the simplest mesophase possible. In fact, this mesophase is chiral, since the molecules are ordered in... 

The first thermotropic liquid crystalline material (cholesteryl benzoate, 2), discovered in 1888 by the Austrian botanist Reinitzer, exhibits what is now known as the chiral nematic (N ) phase. Perhaps not sitrprisingly, this natural product, like many others based on cholesterol, has an asymmetric molecirlar stmcture and hence it is chiral and optically active. Cholesteryl benzoate (2) has 8 chiral centres giving a total of 256 stereoisomers however, only one is produced in nature. 

The best known of the chiral liquid crystal phases is the cholesteric phase or chiral nematic (N ). Here an asterisk is used to indicate a chiral phase. The cholesteric phase (Figure 2.10) was the first liquid crystal to be discovered by Reinitzer in 1888. Reinitzer observed pure cholesterol benzoate under the microscope and noticed two apparent melting points solid crystal to the phase that is now known as cholesteric and then a second melting point to the isotropic liquid phase. Cholesterol is a chiral molecule. 

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