last updated Friday, April 10, 2009

Meeting 4 (Condensed Key)

1.a. L-Asparagine exhibits a specific optical rotation of [a]_D= +35^o in 5 N HCl. In order to calculate the optical purity, the following formula is used.

a=a_obs/(l*c), l=10 cm = 1.0 dm, c=0.25 g/5 mL = 0.05

a=(+1.70^o)/(1*0.05) = +34.0^o

optical purity = +34.0^o/ +35.0^o *100 % = 97.1 %

b. If the solvent was changed, the optical rotation also would change in this case because a different species would be dominant in solution. If the amino acid was dissolved in 0.5 M NaOH, the ammonium functions and the carboxylic group would deprotonated while they are protonated under the conditions above.

2. a. The diammonium salt is reacted with two equivalents of potassium carbonate to release the free amine.

Both reactants are ionic, which means that they only dissolve reasonably well in a polar solvent like water.

b. The addition of 95% ethanol is necessary here to lower the polarity of the solution which ensures that the salicylic aldehyde also dissolves in the reaction mixture.

c. The addition of water after the reaction is completed increases the polarity of the solution, which in turn assists in the precipitation of the ligand. It is added while the reaction mixture is hot to lower the solubility slowly, which usually affords a more crystalline precipitate instead of oil which often times clogs up the filter paper.

d. The brown color originates usually from impurities from the oxidation reaction. The student can removed them by extraction with hot ethanol. Practically speaking, the students adds a small amount of ethanol to the crude ligand and refluxes the mixture for a couple minutes. This usually leads to the formation of a bright yellow powder ligand that is isolated by filtration afterwards.

e. The expected OH strectching mode is shifted due to a very strong intramolecular hydrogen bond, and is located below the CH(sp³) peak! Evidence of the intramolecular H-bond can be seen in the IR spectrum (shift to lower wavenumbers), in the ¹H-NMR spectrum (shift of the phenolic OH to d=13.6 ppm) and also in its low solubility in polar solvents like water and ethanol. The ligand also moves fairly high up the TLC plate with relatively low polarity solvents and dissolves fairly well in the non-polar solvents like hexane or heptane.

f. Dichloromethane is used as solvent in the UV-Vis spectroscopy. The concentration used is a function of the molar extinction coefficients of the peak in the range measured (200-500 nm?) and have to be located in the literature. A quartz cuvette (can be checked out from instructor) should be used here since the plastic cuvette are etched or dssolved by dichloromethane. The UV-Vis spectrometer is located in YH1102.