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CHEM 261
SAMPLE LABORATORY EXAMINATION QUESTIONS
The difference between the water-solubility of an organic base and its corresponding salt is used in the organic chemistry laboratory to separate a mixture containing an organic base and an organic neutral compound. A mixture containing 0.60g of an unknown organic base and 0.60 g of an unknown neutral organic compound was dissolved in 30 mL of hexane ( B.P.68 oC, d = 0.659 g/mL). The possible unknowns are shown below. Questions A through F (inclusive) address this separation.
NH 2
OCH 3
N
H
NH C
O
CH 3
CH 3 CH 2 CH 2 C
O
NH 2
Acetanilide 4-Metoxyaniline Butanamide Diphenylamine M.P. = 113-115 oC M.P. = 62-64 oC M.P. = 114-115 oC M.P. = 66-67 oC
A) What would you add to the hexane solution to extract the organic base? Explain using a general chemical equation.
B) If you separate the aqueous layer (from part A) and then add 10 M sodium hydroxide solution, a white precipitate will form.
i) What is the precipitate?
ii) Support your answer with a general chemical equation.
C) The white precipitate formed (see question B) should be separated by:
i. gravity filtration ii. vacuum filtration using the water aspirator iii. vacuum filtration using the house vacuum iv. distillation v. hot gravity filtration
D) Describe, in three steps, the work-up procedure leading to the isolation of the neutral organic compound, after the organic base is removed.
i. ---------------------------------------------------------------------------------
ii. ---------------------------------------------------------------------------------
iii. ---------------------------------------------------------------------------------
E) What method of purification would you choose to purify the neutral organic compound?
F) Which two methods of identification would you choose to identify the neutral organic unknown? Assume that the pure standards of all compounds listed on (page one) are available. Explain in detail.
L) The following TLC Chromatogram was obtained from the ink of a purple pen using ethyl acetate and hexane as an eluent in a 1:1 ratio. The ink is composed of three compounds of about the same molecular weight; the three compounds are an alkene, an aldehyde, and an alcohol.
i) Determine the Rf value of each spot and indicate the class of the compound.
8 cm
7.9 cm
3cm
5cm
Blue
Teal
Red
Color of spot Rf (Show calculation) Class of compound
Blue
Teal
Red
ii) How could you verify that the blue spot on the TLC is a single compound not a mixture? Explain.
M) What would be the result of the following errors in TLC technique?
i) Too much of a sample is applied on the plate
ii) The solvent pool in the developing jar is too deep
iii) The developing solvent is too non-polar.
N) If silica gel is used to prepare a chromatography column:
i. ketones are eluted from the column faster than carboxylic acids ii. ketones are eluted from the column slower than carboxylic acids iii. ketones are eluted from the column at the same rate as alcohols iv. ketones are eluted from the column faster than alkanes v. ketones are eluted from the column mixed with carboxylic acid
O) A student was performing a mixed solvent recrystallization using ethanol/water as a solvent pair. The compound is very soluble in ethanol, and extremely insoluble in water at all temperatures. She noticed an oily substance formed in the solution. What can be done to rectify this problem?
i. nothing, an oil is normal ii. add few drops of water iii. continue heating to dissolve the oil iv. add few drops of ethanol v. put the hot solution in ice-water bath
P) Water was found to be a suitable solvent for recrystallization of benzoic acid. How should the solution of benzoic acid be prepared during the recrystallization?
i. by adding 10 ml of hot solvent, in small portions, to the solid and stirring until the solid dissolves ii. by adding 10 ml of the solvent at room temperature and stirring until the solid dissolves iii. by dissolving the solid in a minimum volume of the solvent at room temperature iv. by dissolving the solid in a minimum volume of the boiling solvent v. by dissolving the solid in a maximum volume of the boiling solvent
Q) Dichloromethane, hexane, and toluene are hydrophobic. Methanol, acetone, and acetic acid are hydrophilic. Which of the following solvent pairs are immiscible?
i. dichloromethane – hexane ii. methanol – acetone iii. acetone – acetic acid iv. hexane – toluene v. hexane – water
