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TLC Analysis
In this experiment you may use Thin Layer Chromatography (TLC) to determine the
composition of your sample. You will be given two silica-gel TLC plates where four standards
(aspirin, ibuprofen, acetaminophen and caffeine), your sample (you had to have kept a small
amount of it) and the extraction fractions will be separately spotted. The standard compounds will
be available as solutions of ca. 1 g of each substance dissolved in 20 mL of a 1:1 mixture of DCM
and ethanol. You have to dissolve on your own your sample and the extraction fractions in the
same solvent mixture at a concentration similar to that of the standards. The purpose of the first
plate (standards plus sample) is to determine which standards are present in the sample by means
of your comparing the various retention factors (R
f
). The second plate (sample plus extraction
fractions) will serve you to assess your extraction skills.
It is possible to use several developing solvents (eluents) for this experiment but ethyl
acetate containing 0.5% glacial acetic acid is preferred. The small amount of acid supplies protons
and suppresses ionization of aspirin and ibuprofen, allowing them to travel upward on the plates in
their protonated form. Without the acid these compounds do not move.
Two methods of visualization will be used to observe the positions of the spots on the developed
TLC plates. First, the plates will be observed while under illumination from a short-wavelength
ultraviolet lamp. Under these conditions, some of the spots will appear as dark areas on the plate,
whereas others will fluoresce brightly. This difference in appearance under UV illumination will
help you to distinguish the substances from one another. You will find it very convenient to lightly
outline the spots with a pencil and place a small “x” on the fluorescent spots. Iodine vapors will be
used as a second method of visualization. Not all the spots will be revealed but some will turn
yellow, tan or brown. The difference in the behaviors of the various spots can be used to further
differentiate among them.
Please, read the provided laboratory manual for further guidance as to the correct
performance of the TLC experiment.
Report
Pre-lab
1. Make a table showing the physical properties (molecular mass, m.p., b.p. solubility,
flammability [for solvents only], and toxicity/hazards) of aspirin, ibuprofen, naproxen,
acetaminophen (synonym: N-acetyl-4-hydroxyaniline), caffeine, acetone, methylene chloride,
and ethyl acetate.
2. Classify the following compounds as acidic, basic, or neutral: acetaminophen, aspirin,
ibuprofen, naproxen, and caffeine. Identify the functional groups present in each molecule and
determine which ones affect their acid-base properties.
3. Could you use NaOH to separate aspirin and acetaminophen by acid-base extraction? Briefly
justify your answer.
4. Study the structure of caffeine and decide whether all nitrogen atoms are basic or not. Why?