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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.)

Bent-core liquid crystals, a class of mesogenic compounds with non-linear molecular structures, are well known for their unconventional mesophases, characterized by complex molecular (and supramolecular) ordering and often featuring biaxial and polar properties. In the nematic phase, their unique behavior is manifested in the formation of nano-sized biaxial clusters of layered molecules (cybotactic groups). While this prompted their consideration in the quest for nematic biaxiality, experimental evidence indicates that the cybotactic order is only short-ranged and that the nematic phase is macroscopically uniaxial. By combining atomic force microscopy, neutron reflectivity and wide-angle grazing-incidence X-ray scattering, here, we demonstrate that multilayer films of a bent-core nematic, deposited on silicon by a combined Langmuir–Blodgett and Langmuir–Schaefer approach, exhibit macroscopic in-plane ordering, with the long molecular axis tilted with respect to the sample surface and the short molecular axis (i.e., the apex bisector) aligned along the film compression direction. We thus propose the use of Langmuir films as an effective way to study and control the complex anchoring properties of bent-core liquid crystals. [ABSTRACT FROM AUTHOR]