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Students in the second-semester organic chemistry laboratory perform a Wittig reaction between butylidenetriphenylphosphorane (an ylide) and benzaldehyde and determine the relative percentages of the cis and trans isomers of the 1-phenyl-1-pentene product. Because of the highly reactive nature of this unstabilized ylide, students are introduced to working under anhydrous and inert atmosphere conditions. Students prepare the ylide by treatment of butyltriphenylphosphonium bromide with THF solutions of the lithium, sodium, or potassium salt of hexamethydisilazide (HMDS), either alone or in the presence of added LiBr, NaBr, or KBr. After reaction with the benzaldehyde, [superscript 1]H NMR spectroscopy and gas chromatography are used to determine the relative percentages of the cis and trans isomers of the product. By combining their results, students observe that in the absence of lithium the percentage of cis isomer is approximately 90%, whereas in the presence of lithium the percentage of cis isomer is much lower (29-59% depending on the conditions). A discussion led by the instructor develops the mechanistic rationale for understanding these unexpected results. (Contains 1 table, 2 figures and 1 scheme.)

In previous research, we have found that the incorporation of three lateral substituents on bent-core oxadiazole-based liquid crystals has allowed for the presence of a nematic phase that supercooled to room temperature. X-ray diffraction (XRD) studies of these derivatives have confirmed that the nematic phase consists of cybotactic clusters. Moreover, the wide-angle XRD pattern showed a splitting indicative of local biaxial ordering within this phase. In this paper, we report the phase behaviour of two series of compounds that contain up to four lateral methyl groups. In one series, the arrangement of the lateral groups is symmetrical, and in the other case, the groups are not symmetrically distributed. All of the non-symmetric derivatives supercool to room temperature in the nematic phase, while symmetric derivatives tend to crystallise well above room temperature. XRD analysis of all of these compounds indicates the presence of cybotactic order. However, a splitting of the wide-angle XRD pattern is observed only in derivatives with lateral groups on the outer benzene rings, regardless of their supercooling behaviour, indicating the crucial influence of these substituents on the molecular packing. [ABSTRACT FROM AUTHOR]

A series of diphenylacetylene derivatives were synthesized to serve as ligands for metal alkyne complexes. The target compounds, alkyl 4′-(4-phenylethynylbenzoyloxy)-biphenyl-4-carboxylates, were prepared using a three-step synthetic procedure. Both the diphenylacetylene derivatives as well as their immediate precursors, alkyl 4′-(4-iodobenzoyloxy)-biphenyl-4-carboxylates, exhibit liquid crystalline behavior. The diphenylacetylene derivatives possess large liquid crystalline phase ranges and high clearing temperatures. Synthetic details and the phase behavior of these compounds are discussed. [ABSTRACT FROM AUTHOR]

The possibility of biaxial orientational order in nematic liquid crystals is a subject of intense current interest. We explore the tendencies toward local and global biaxial ordering in the recently synthesized trimethylated oxadiazole-based bent-core mesogens with a pronounced asymmetric (bow-type) shape of molecules. The combination of x-ray diffraction and optical studies suggests that the biaxial order is expressed differently at the short- and long-range scales. Locally, at the scale of a few molecules, x-ray-diffraction data demonstrate biaxial packing. However, above the mesoscopic scale, the global orientational order in all three compounds is uniaxial, as evidenced by uniform homeotropic alignment of the nematic phase which is optically tested over the entire temperature range and by the observations of topological defects induced by individual and aggregated colloidal spheres in the nematic bulk. [ABSTRACT FROM AUTHOR]