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.)