University of Massachusetts Amherst Advanced Organic Chemistry Worksheet

Experimental procedurePut the metal jacket to be heated to approx. 80 °C. In a 100 mL round-bottom flask add
normal silica gel (2 g) and flash silica gel (3 g), followed by the stirring magnet, purified
benzaldehyde (0.51 mL), triethyl phosphonoacetate (1.03 mL) and then DBU (0.77 mL).
Place the flask on the metal jacket and the reaction mixture is heated at 80 °C while
mantaining a good stirring. In this regard, it may be necessary to undo the lumps formed
with a spatula. After 40 min, the reaction mixture is transferred to a filter plate funnel and
the solid is eluted with ethyl acetate (20 mL), collecting the crude product in another 100
mL round-bottom flask. Evaporate the solvent in vacuo to obtain the product (a mixture
of the E/Z isomers: 98/2). Record the weight of the product obtained and calculate the
yield. If possible, check the purity of the product by GC and analyze the NMR spectrum.
Questions:
1. Draw the mechanism of the reaction. Why the major product is the E isomer and not
the Z?
2. What is the role of silica gel?
3. In view of the mechanism of the reaction, what is the by-product that is formed and in
which step it is separated from the desired product?