Cholesterol cholesteryl benzoate

Thermotropic cholesterics were officially discovered2 in 1888 by the Austrian botanist Friederich Reinitzer, while studying the melting of cholesterol esters cholesteryl benzoate first melted to give a cloudy liquid that, at higher temperature, turned into an ordinary clear liquid. The cloudy liquid (the mesophase) was a thermotropic cholesteric liquid crystal. These phases... 

A second category of liquid crystals is the type produced when certain substances, notably the esters of cholesterol, are heated. These systems are referred to as thermotropic liquid crystals and, although not formed by surfactants, their properties will be described here for purposes of comparison. The formation of a cloudy liquid when cholesteryl benzoate is heated to temperatures between 145 and 179°C was first noted in 1888 by the Austrian botanist Reinitzer. The name liquid crystal was applied to this cloudy intermediate phase because of the presence of areas with crystal-like molecular stmcture within this solution. 

In 1888 the Austrian botanist and chemist Friedrich Reinitzer, interested in the chemical function of cholesterol in plants, noticed that the cholesterol derivative cholesteryl benzoate had two distinct melting points. At 145.5°C (293.9°F) the solid compound melted to form a turbid fluid, and this fluid stayed turbid until 178.5°C (353.3°F), at which temperature the turbidity disappeared and the liquid became clear. On cooling the liquid, he found that this sequence was reversed. He concluded that he had discovered a new state of matter occupying a niche between the crystalline solid and liquid states the liquid crystalline state. More than a century after Reinitzer s discovery, liquid crystals are an important class of advanced materials, being used for applications ranging from clock and calculator displays to temperature sensors. 

The liquid-crystalline mesophases of cholesteryl benzoate and p-phenylbenzoate provide useful stationary phases for the gas chromatography of steroids, giving separations largely on the basis of molecular shape. These steroidal materials have distinct advantages over the more conventional stationary phases for certain applications, including separations of the isomeric androstane-3,17-diols or pregnane-3,20-diols, and of cholest-4-ene from the 5-ene. The dicho-lesteryl esters of some dicarboxylic acids are reported to exhibit liquid-crystalline properties. The temperature-dependence of the Kerr effect has been measured for a series of liquid-crystalline esters of cholesterol. 

Ordinarily a crystalline solid melts sharply at a single, well-defined temperature to produce a Uquid phase that is amorphous and isotropic. A different behavior is exhibited by a class of organic compounds known as liquid crystals. The oldest examples are cholesterol derivatives, e.g., cholesteryl benzoate. This substance, for instance, does not have a sharp transition to amorphous Uquid at 145.5°C, but changes to a cloudy liquid, which becomes clear and isotropic only at 178.5°C. This cloudy intermediate state that possesses an ordered stmcture with some resemblance to a crystaUine soUd, while still in the liquid state, is called a mesophase or mesomorphic phase from the Greek mesos, meaning in between or intermediate. 

Engaged with the constitution of cholesterol, for cholesteryl benzoate Reinitzer reported two phase transitions in the eourse of melting. Melting starts with the formation of a cloudy liquid, which transforms on further heating into a clear liquid. This type of liquid crystal is addressed as thermotropic. In addition, there are lyotropic liquid crystals that change their behavior as both a function of concentration in a solvent and of temperature. 

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. 

We mentioned already cholesteryl benzoate as one of the first discovered liquid crystals, although the chemical structure of cholesterol was unknown at the time. From this compound the name cholesteric phase was derived. p-Azoxyanisole (PAA), which belongs to the nOAOB homologous series (cf. Table I), was one of the first synthesized liquid crystals with known chemical structure. It is a typical representative of a rodlike compound. The first six members of this series form the nematic phase,... 

In 1888, the Austrian botanical physiologist Reinitzer (1888), while working at the German University of Prague, extracted cholesterol from carrots in order to establish its chemical formula. Reinitzer examined the physico-chemical properties of various derivatives of cholesterol. A number of workers previously observed some distinct color effects on cooling cholesterol derivatives just above the solidification temperature. Reinitzer himself found the same phenomenon in cholesteryl benzoate. However, the colors, near the solidification of cholesteryl benzoate were not the most peculiar feature and he finally detected that cholesteryl benzoate, does not melt like other compounds, as it indicated two melting points. At 145.5 °C, it melted into a cloudy liquid and at 178.5 °C, it melted again and the cloudy liquid suddenly became clear. Furthermore, the phenomenon was reversible. 

Cholesteryl para-substituted benzoates give mesophases with transition temperatures and thermodynamic parameters which depend upon the para-substituent. " Crystal and mesophase structures of cholesteryl myristate appear to show some similarities in molecular packing. " X-Ray studies show that cholesteryl 17-bromoheptadecanoate crystals contain alternating regions with cholesterol and hydrocarbon-chain packing. ""... 

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