last updated Tuesday, October 14, 2014

Meeting 4 (Condensed Key)

1. a. The ligand only dissolves poorly in ethanol at room temperature. The initial reflux serves the purpose to dissolve more of the ligand in ethanol.

b. The crushing of the manganese(II) acetate increases its surface area, which allows for the salt to dissolve better in 95 % ethanol faciliting the formation of the Mn(II) salen complex. .

c. The reflux is necessary to bring the ligand and the manganese(II) acetate in solution. The reaction would be extremely slow if it was carried out at room temperature.

d. The air is bubbled through the solution to provide the oxygen to oxidize Mn(II) salen to Mn(III) salen. The flow rate of the air stream has to be adjusted in a way that the the reaction proceeds smoothly but the ethanol is not evaporated away.

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

f. The student should spot the ligand, the reaction mixture and maybe a co-spot on the TLC plate. These compounds should be in a solution with about 5 mg/mL ethyl acetate. A mixture of ethyl acetate and hexane (1:4) is used as mobile phase here.

g. The removal of the ethyl acetate is necessary to obtained a more concentrated solution. 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 less 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.

h. The infrared spectrum of the catalyst should display a n(C=N), which is shifted to 1610 cm^-1. In addition, there should be two peaks around 550 cm^-1 due to the n(Mn-O) modes. The broad hydroxyl band observed in the ligand should not be observed anymore.

i. The catalyst is very dark in color, which means that optical rotation is not possible here. The high melting point of the catalyst indicates that the compound is not volatile enough for GC analysis. Thus, HPLC on a suitable chiral column should be used for the determination of the optical purity.