last updated Wednesday, October 07, 2015

Meeting 4 (Condensed Key)

1. a. The crushing of the manganese(II) acetate increases its surface area, which allows for the salt to dissolve faster in 95 % ethanol faciliting the formation of the Mn(II) salen complex, which is important to reduce the formation of MnO₂ caused by the oxidation of Mn(II) in the absence of the ligand.

b..The ligand and manganese(II) acetate only dissolves poorly in 95 % ethanol at room temperature. The initial reflux serves the purpose getting both in solution and to form the Mn(II) salen complex.

c. The oxygen in air is used as oxidant. The air is bubbled through the solution to oxidize Mn(II) salen complex to Mn(III) salen complex. A glass tube has to be immersed in the suspension for an efficient uptake of oxygen.

d. The reflux is necessary to bring the ligand and manganese(II) acetate gradually in solution to form the Mn(II) salen complex. The student should observe that (i) the solution boils (ii) a reflux ring is observed in the low third of the condenser.

e. The color will change from bright yellow to dark-brown and the entire ligand should dissolve as the reaction proceeds.

f. A mixture of ethyl acetate and hexane (1:4) is used as mobile phase in the TLC. The incomplete reaction shows two spots in the lane for the reaction mixture, the ligand (R_f= ~0.8) and the precursor to the catalyst (R_f= ~0.1).

g. The drying agent is brown because some of the catalyst absorbed on drying agent. The student should treat the drying agent with some ethyl acetate to extract the catalyst.

h. The addition of high-boiling petroleum ether (hbPE, =mixture of hydrocarbons) lowers the polarity of the solution, which facilitates the precipitation of the catalyst as the ethyl acetate is slowly removed from the mixture using the rotary evaporator. The weakly polar ligand and the aldehyde remain in solution. If the solvent was completely removed, the catalyst, the ligand and the aldehyde would be comingled in the solid.