Liquid-crystalline solvents, effect reactions

The role of liquid-crystalline solvents in affecting the rates and specificities of solute reactions is not clear. In some cases, no detectable influence of solvent order has been reported (2 -6) while in others, seemingly quite similar, large effects are found (1,7-16). In the extreme, different laboratories have published conflicting claims for the same reaction performed in the same solvent (2,17-19). The need for care in performing these experiments and in analyzing results from them cannot be emphasized too strongly. 

In general, the greater the similarity in size and shape between the solute and liquid-crystalline solvent molecules, the easier it is for the solute to incorporate itself into the liquid-crystalline phase. If the result of a chemical reaction depends on the solvent order, then the largest effects can be expected for those solute molecules which fit best into the liquid-crystal structure. Since the structure of -heptadecyl phenyl ketone is identical to -butyl stearate except for the terminal phenyl and butoxy groups, it exhibits the largest solvent effect of the ketones studied [730]. 

All in all, liquid-crystalline media are not generally useful solvents for controlling the rates and stereochemistries of chemical reactions. In each case, careful consideration of the fine details regarding the structure of educts and activated complex, their preferred orientations in a liquid-crystalline solvent matrix, and the disruptive effects that each solute has on the solvent order has to be made. A mesophase effect can only be expected when substantial changes in the overall shape of the reactant molecule(s) occur during the activation process [734],... 

Analysis of natural abundance deuterium distribution in organic molecules, an important step in the study of kinetic isotope effects associated with enzyme-catalyzed reactions, by the use of chiral anisotropic media has been explored. An aspect of this analysis is the discrimination of the enantiotopic deuterons in prochiral molecules and the quantification of isotopic fractionation on methylene prostereogenic sites. Towards this an approach has been presented which is based on the use of natural abundance 2-dimensional NMR experiments on solutes oriented by chiral liquid crystalline solvents and the separation of the deuterium signals based on the quadrupolar interaction. The case of 1,1 -bis(phenylthio)hexane derived by cleavage from methyl linoleate of safflower has been used to illustrate the method with (D/H)pro-R and (D/H)pro-S measured at the same methylene position of a fatty acid chain. Enantiomers of water soluble materials can be observed using deuterium NMR spectroscopy in the lyotropic mesophase formed by glucopon/hexanol/buffered water. ... 

For a partieular phase type, the magnitude of the observed effects on reactivity correlates with the structural compatibility of the solute and solvent, to a first approximation. Since liquid crystals are expected to favour specific orientations or conformations of reactants, reactions involving solutes, transition states or products that are most closely related structurally to the liquid crystalline solvent often show the largest effect. However, examples also exist where the reactivity of small solutes that are not solvent-like, or of solutes whose reaction involves only minor changes in shape, is significantly altered in a liquid crystalline solvent. This is... 

Purify the off-white solid by crystallization from absolute ethanol in a small test tube using the standard technique of adding hot solvent until the solid dissolves. A small amount of impurity might not dissolve. If this is the case, use a Pasteur pipette to rapidly remove the hot solution away from the impurity and transfer the hot solution to another test tube. Cork the test tube and place it in a warm 25-mL Erlenmeyer flask. Allow the solution to cool slowly. Once the test tube has cooled and crystals have formed, place the test tube in an ice bath for at least 10 minutes to complete the crystallization process. Place 2 mL of absolute ethanol in another test tube and cool the solvent in the ice bath (this solvent will be used to aid the transfer of the product). Loosen the crystals in the test tube with a microspatula and pour the contents of the test tube into a Hirsch funnel under vacuum. Remove the remaining crystals from the test tube using the chilled ethanol and a spatula. Dry the colorless crystalline (plates) of frans,frans-l,4-diphenyl-l,3-butadiene on the Hirsch funnel for about 5 minutes to completely dry them. Save the filtrate from the crystallization for analysis by thin-layer chromatography. The ds,frans-l,4-di-phenyl-1,3-butadiene, which is also formed in the Wittig reaction, is a liquid, and crystallization effectively removes the isomer from the solid trans,trans product. 

The crystalline aluminosilicate-catalyzed aldol condensation of acetophenone to form dypnone has been reported (27). As shown in Table XXIII, hydrogen zeolites were the most effective catalysts for this conversion. Operation at low temperatures in the liquid phase is critical for this reaction, to avoid both coke formation and condensation with aromatic solvents. Catalyst aging was rapid, however. Only transient conversions of acetone to mesitylene were obtained over REX or H-mordenite at 315° owing to rapid intracrystalline self-condensation and coke formation. 

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