last updated Friday, January 16, 2015

Meeting 4 (Condensed Key)

1. a. 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.

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. Air is bubbled through the solution to provide the oxygen to oxidize Mn(II) salen to Mn(III) salen using a glass tube. 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.

d. The reflux is necessary to bring the igand and manganese(II) acetate gradually in solution. 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 drawn capillary spotter is used to spot the samples on the TLC plate (sample of the reaction mixture and the ligand). A mixture of ethyl acetate and hexane (1:4) is used as mobile phase.

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 down from 1630 cm- 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 more polar than the ligand due to the Mn-Cl bond, which is largely ionic usually.