last updated Wednesday, January 14, 2015

Meeting 3 (Condensed Key)

1.a. In order to determine the equilibrium constant for the reactions, the following three equations have to be considered (assuming that the reactants are reacted in a 1:1 ratio).

(i) The first reaction is used as is and the second reaction is reversed leading to an equilibrium constant of K_a1r=10^6.70. After adding the two equations, one obtains the equilibrium constant for the deprotonation reaction of K_R=K_DA*K_a1r = 10^-9.94*10^6.70=10-^3.24= 5.75*10^-4. Thus, only a very small fraction of the mono-protonated salt would be deprotonated by the bicarbonate ion.

(ii) The first reaction is used as is and the third reaction is reversed leading to an equilibrium constant of K_a2r=10^10.33. After adding the two equations, one obtains the equilibrium constant of K_R=K_DA*K_a2r = 10^-9.94*10^10.33=10^0.39= 2.45. Much more of the mono-protonated salt would be deprotonated by the carbonate ion (about 4266 times more to be exact) than in the bicarbonate case. A stronger base (i.e., hydroxide) would probably be better for this step but any excess of the base would also lead to a deprotonation of the phenol group of the aldehyde, which has a similar pK_a-value compared to the mono-protonated ammonium salt..

b. The reaction in a. is carried out in water because both reactants are ionic. Afterwards, 95 % ethanol has to be added to lower the polarity of the solution, which increases the solubility of salicylic aldehyde in the solvent mixture.

c. The reflux is required to keep the aldehyde in solution and to eliminate the water from the intermediate. At low temperatures, the rate of the ligand formation is very low.

d. The solution turns bright yellow and a bright yellow precipitate is formed immediately.

e. The extraction in the solvent mixture (ethyl acetate:hexane=1:1) is necessary to remove the unreacted diamine and other polar compounds. The dry ligand dissolves in either solvent. Unfortunately, the presence of water in the crude ligand after the precipitation from the reaction mixture causes problems because the water and the ethanol dissolve in ethyl acetate as well. Both of them increase the polarity of the solution and therefore lower the solubility of the weakly polar ligand significantly. In case of the hexane, there seems to be a wetting problem due to the large difference in polarity. The addition of the hexane to the ethyl acetate reduces the solubility of water in the solvent and the polarity of the solvent.

f. A small amount of the ligand canbe dissolved in a small amount of a low boiling solvent like diethyl ether, hexane or ethyl acetate. This solution applied to the ATR crystal. The solvent is allowed to evaporate leaving behind a thin film that canbe measured.

g. The signal at d=166 ppm is due to the imine carbon atom, which is located at higher d-values compared to alkenes and lower d-values compared to aldehydes. The signal at d= 159 ppm is due to the phenolic carbon atom that is shifted downfield due to the attached hydroxyl function.

h. The concentration of the solution is 0.50 % (=0.0500/10.00 mL). Thus, the specific optical rotation for the sample is [a]= -302.0^o (=-1.53^o/(0.005 * 1)). This results in an optical purity of 95.9 % (= -302.0^o/-315^o * 100 %). Since the value is only slightly below the desired value of 99+ %, the ligand is probably not completely dry. However, the student needs to look at this result and compare it with the data for his tartrate salt.

i. The optical purity of the ligand could be determined using HPLC on a a suitable chiral HPLC column. Alternatively, ¹H-NMR could be used after conversion into diastereomers or in conjunction with chemical shift reagents.