Liquid Crystals as Solvents in Chemical Reactions

The special combination of orientational order and mobility possessed by liquid crystals, the wide variation in these properties in different liquid crystalline types, and the fact that bulk orientation can be easily controlled by a number of different methods have led to numerous applications in which liquid crystals are employed as anisotropic solvents for the study of various physicochemical properties of molecules. This chapter deals with the use of liquid crystals as solvents or supports in spectroscopic applications, in gas chromatography, and for chemical reaction. In each case, the emphasis is on studies in which the solute is of primary interest, and in which calamitic liquid crystals are employed as solvents to control orientation or mobility. Spectroscopic studies in which some property of the mesophase is of primary interest, whether they involve a study of probe molecules or the liquid crystal itself, are treated in earlier chapters and will not be dealt with here. In many cases, studies of this type are intimately re-... 

Liquid crystals possess physical properties which lie somewhere between those of solids and liquids cf. Section 3.1 and [725]. The rigidity which is present in a solid matrix is absent in liquid crystals, thus permitting molecular motion as well as conformational flexibility of the dissolved solute molecules. At the same time, due to the order in the liquid-crystalline phase, the randomness in motion and conformational flexibility of the dissolved solute molecules is to some extent restricted. If the structures of the solute and solvent molecules are compatible, then solute molecules can be incorporated into the liquid-crystalline phase without disrupting its order. Thus, the reactivity of substrate molecules incorporated into liquid crystals without destroying their order should be different from that in isotropic solvents. Apart from the first report on the influence of liquid crystals on chemical reactions by Svedberg in 1916 [726], the use of liquid crystals as... 

Cholesteric liquid crystals are optically active nematic phases as a result of their gradual twist in orientational alignment. Therefore, cholesteric liquid-crystalline solvents are expected to induce enantioselectivity in chemical reactions see reference [713] for a review on photoasymmetric induction by chiral mesophases. The existing results are not very promising. So far, the maximum photoasymmetric induction reported has... 

Furthermore, we proceeded to covalently attach mesogenic groups to the ionic liquid to obtain a liquid crystalline ionic liquid with anisotropic conductivity. Alkylated pyridinium and ammonium salts are well known to form smectic liquid crystals [174, 175]. Furthermore, at the time when ionic liquid came into the limehght, smectic liquid crystal of long-chain alkylated imidazolium salts had been reported by the groups of Prof. Bruce and Prof. Seddon in the United Kingdom [176-178], and they pointed out the potential of anisotropic solvent for chemical reactions. However, there was no report about their use as an ion conductor. 

The synthesis of sodium amide, NaNH2 (or sodamide ), by passing ammonia over heated sodium metal, was first reported almost two centuries ago. A number of studies have since been made of its properties, but no crystal structure has been reported. Sodamide is used as a strong base in organic chemistry (often in liquid ammonia solution). In contrast, sodium bis(trimethylsilyl)amide NaN(SiMe3)2 (or sodium hex-amethyldisilazide , NaHMDS), whose crystal structure is discussed later, is widely used for deprotonation reactions or base catalysed reactions due to its solubility in a wide range of non-polar solvents. An overview of some of the types of chemical reactions in which NaHMDS is used is presented in Scheme 2.3. 

For production volumes that lie below about 10001 a , distUlative separation is preferably performed batchwise. For small product quantities, batch distillation has the advantage of lower investment costs, since the individual fractions can be separated one by one in the same plant. It is highly flexible, since it can easily be combined with other process steps. If the distillation vessel is designed as a stirred tank, then other reaction steps such as dissolution of sohds, chemical reactions, distUlative solvent change, liquid-liquid extraction, evaporative crystallization, crystallization with cooling, and precipitation can be carried out in the distillation appartus. 

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