Meeting 4 Problem set - CONDENSED KEY

1. Online Quiz (no key, for questions and clarifications please consult the instructor please)

2.a. Tetrabutylammonium hydrogensulfate (TBHS, N(C₄H₉)₄HSO₄) is used as phase-transfer catalyst in this reaction. The tetrabutylammonium cation forms an ion-pair with the hypochlorite ion of the bleach (TBA^+...OCl^-). Due to its size and neutrality, this ion pair is more soluble in the organic layer than NaOCl.  The cation also transports the byproduct, the chloride ion, into the aqueous layer (TBA^+...Cl^-). Without the use of the catalyst, the reaction would not take place (or will be extremely slow and many side reactions would occur).

b. Ethyl acetate is used as solvent in the reaction because it dissolves the somewhat polar benzoin better than hexane or diethyl ether. The reaction would progress very slowly if the benzoin would not dissolve in the organic or the aqueous layer.

c. An elevated temperature would cause the ethyl acetate to evaporate and potentially get in contact with the hotplate resulting in a fire. In addition, benzil would undergo rearrangement reactions under basic conditions at elevated temperatures (i.e., benzilic acid rearrangement).

d. Water is very polar and therefore absorbs very strongly on the polar stationary phase (alumina). This will cause a significant interference in the separation of less polar compounds because neither benzil nor benzoin will absorb well (if at all) on the polar stationary phase, resulting in a very fast elution and very poor (or no) separation.

e. The compounds to be separated are weakly polar. Thus, they interact with a polar stationary phase like silica or alumina resulting in a separation of these compounds using the proper mobile phase (in the lab ethyl acetate is used to keep it simple).

f. The crude should contain primarily benzil that is contaminated with some benzoin. Since benzil is less polar than benzoin, a polar solvent is used as solvent for the recrystallization. Benzil is less soluble in cold 95 % ethanol than benzoin causing the benzil to precipitate first. However, the recrystallization only works well if there is not too much benzoin present.

g. Ice-cold 95 % ethanol is used to rinse the crystals. Usually, 1-2 mL of solvent is usually sufficient to remove the mother liquor from the crystals. Any larger amounts will result in a loss of product.

h. The product is dissolved in isopropanol. The concentration should be 1 mg/mL. The student should submit 1.5 mL of solution.

i.  Rule 20 states

A common problem is that the student gets confused about which layer contains the target compound. In most cases, the organic compound is dissolved in the organic layer due to its low polarity. In some cases, the polarity of the compound is increased due to protonation or deprotonation. In either way, all layers (properly labeled) should be kept until the product is isolated and characterized.